INTERACTIONS BETWEEN SCHISTOCERCA GREGARIA (FORSKAL) AND LOCUSTA MIGRATORIA MIGRATORIOIDES (REICH & FARMAIRE) IN RELATION TO PHASE POLYMORPHISM
BY
ABDOULAYE NIASSY
INTERACTIONS BETWEEN SCHISTOCERCA GREGARIA (FORSKAL)AND 
LOCUSTA M1GRATORIA MXGRATORIOIDES 
(REICH & FARMAIRE) IN RELATION TO PHASE 
POLYMORPHISM
A Thesis
presented to the Department of Crop Science of the
Faculty of Agriculture, University of Ghana, Legon 
in fulfilment of the requirements 
for the degree of "‘Doctor of Philosophy in 
Crop Science (Entomology)
BY
ABDOULAYE NIASSY
B.Sc. (AGRICULTURE), M.Sc. (ENTOMOLOGY)
OKLAHOMA STATE UNIVERSITY, STILLWATER
OKLAHOMA, USA
Crop Science Department,
Faculty of Agriculture
Univ. of Ghana
Legon September, 1996
Q 3527*9
H 51
17-}£S'&S ^O o j v
DECLARATION
ii
I hereby declare that the work herein now submitted as a 
thesis for the Doctor of Philosophy Degree in crop Science 
(Entomology) is the result of my own investigations and has not 
been submitted for a similar degree in any other University.
Dr. 
ICIPE
Professor Ahmed Hassanali 
ICIPE Supervisor
Dr.
ICIPE Supervisor
Professor^jjll. Ayertey 
University Supervisor
Dr. D. Obeng-Ofori
University Supervisor
DEDICATION
To my father, Baboucar Niassy, my mother Fatoumata 
Badji, my adopted mother, Sire Badji, whose love, care, 
and encouragement have helped me strive to this level. 
They all never lived to see me through. I also dedicate 
this work to all my family members for their patience and 
endurance while I was away for these studies; to my step 
mother Seynabou Badji who took care of the family after 
mother passed away; to my elder brother Mamadou Lamine 
Niassy, my elder sister Hady Djikoher Niassy, my cousins 
and friends Daouda Diatta, Estella Djitome Kassa Mane 
(Diatta's wife), Augustin Niassy, and Saliou Djiba; to my 
uncles El-Hadj Ibrahima Badji, Djissambou Amanga Niassy, 
Vincent Niassy; and to all my aunts, especially to Famata 
Adjouberay Kangouye Niassy and Marie Aloumbeuteu Niassy, 
for their care and encouragement.
iii
ABSTRACT
iv
Cross pheromone-mediating releaser effects between 
the desert locust, Schistocerca gregaria gregaria, and 
the African migratory locust, Locusta migratoria 
migratorioides, were investigated in olfactometer 
bioassays. These were compared with responses of 
gregarious individuals of the two locust species to their 
own air-borne volatiles.
Similar to previous reports, nympha1 and adult stages 
of S. gregaria responded strongly to their own volatiles; 
immature and mature adults responded to mature adult, but 
not to nymphal volatiles; nymphs did not respond to 
mature adult volatiles. The responses of both nymphal 
and adult stages to their respective volatiles were dose- 
dependent .
In L. migratoria, nymphal and adult stages also 
responded strongly and in a dose-dependent fashion to 
their own volatiles. Immature adults responded to 
volatiles of mature adults, but both immature and mature 
adults did not respond sufficiently to nymphal volatiles. 
Nymphs also responded to volatiles of mature adults, but
not to those of immature adults.
The two locust species cross-responded to each 
other's volatiles in a dose-dependent fashion. Both
nymphal and mature adult stages of S. gregaria were less
responsive to the volatile emissions of the corresponding 
stages of L. migratoria. On the other hand, volatiles 
from nymphal and mature adults of S. gregaria evoked 
strong aggregation responses in corresponding nymphal and 
mature adult stages of L. migratoria. S. gregaria 
immature adults were more indifferent to volatiles from 
nymphs of L . migratoria, but immature adults of L. 
migratoria were actually repelled by conspecific nymphal 
volatiles; they further responded poorly to volatiles of 
nymphal S. gregaria.
These results confirm previous findings that in S. 
gregaria, different pheromone systems mediate grouping 
behaviour in different stages of the locusts. They also 
suggest that there is an overlap in the pheromone systems 
mediating grouping behaviour in S. gregaria and L. 
migratoria.
The changes in the phase characteristics (primer 
effects) of nymphal and adult solitarious desert locusts 
reared mixed with gregarious migratory locusts and the 
converse, were investigated. Body colour changes, the 
number of instars and stage duration, pheromone titres 
(as measured by the amounts of phenylacetonitrile 
produced by males), morphometries, and haemolymph 
pigments composition (as measured by the absorbance ratio 
at 460 and 680 nm) in test insects were determined. In 
cage bioassays, significant changes occurred in the phase
V
characteristics of solitarious nymphs and immature adults 
of S. gregaria which were reared with gregarious nymphs 
or immature adults of L. migratoria with respect to all 
parameters monitored (though at differerent rates). 
Similarly, solitarious immature adults of L. migratoria 
which were reared with immature adults of S. gregaria, 
changed significantly in their phase characteristics. 
Significant changes in phase characteristics also 
occurred in solitarious S. gregaria exposed to volatiles 
of L. migratoria. These findings confirm previous 
reports that interactions between certain groups of 
acridids are able to provide the necessary stimuli to 
initiate locust gregarization (shift from solitarious to 
gregarious phase).
In another experiment, the effects of gregarious 
fift’n-instar nymphs and mature adults of L. migratoria on 
the sexual maturation of newly moulted gregarious 
immature males and females of the desert locust, S. 
gregaria, and vice-versa, were investigated by monitoring 
colour changes and copulation in males, and basal oocyte- 
length in females. Maturation in S. gregaria was 
signifcantly accelerated by gregarious fifth-instar 
nymphs of gregarious L. migratoria; mature adults did not 
produce consistent effects. Fifth-instar nymphs and 
mature adults of S. gregaria significantly delayed 
maturation of newly fledged L. migratoria.
vi
Gas-chromatographic (GC) and GC-mass spectrometric 
analyses of volatiles of similar stages of S. gregaria 
and L. migratoria showed quantitative and qualitative 
differences. In particular phenylacetonitrile was found 
to be present in the volatiles of nymphal and mature 
adult L. migratoria migratorioides.
The implications of these results are discussed in 
relation to the behavioural ecologies of the two locust 
species.
vii
ACKNOWLEDGEMENTS
I wish to acknowledge the following:
a) My employer, the "Minister of Agriculture", 
Senegal, and the "Minister for State Modernisation" for 
granting me a study leave to concentrate on this project.
b) The Director of the ICIPE, Dr. Hans Herren, and 
the Head of Capacity Building Unit, Dr. V.O. Musewe, for 
their efforts, in securing me from the International Fund 
for Agricultural Development (IFAD), a fellowship, thus 
offering me a valuable opportunity to conduct this 
important research project towards the scientific level 
of Ph.D, I have always been striving for.
c) My supervisors at the ICIPE: Dr. B. Torto, major 
Supervisor, for his continuous effort in the follow-up, 
experimental designs, and in checking the validity of the 
results. I also thank him for training me in the 
techniques of analytical chemistry needed in my studies, 
and for his effort in reviewing my write up; Professor 
Ahmed Hassanali (Head of the Chemical Ecology Department, 
Coordinator of the Locust Semiochemical Project, and 
Deputy Director General of the ICIPE), for his 
continuous, prompt and pinpointed pieces of advice 
especially in the overall conceptualization of this 
research project, and for his pertinent and objective 
critique of the results; Dr. P.G.N. Njagi, at the
viii
Chemical Ecology Department, Locust Semiochemical 
Project, also for his assistance in many of the volatile 
analyses, and for his valuable critique and contributions 
in this write-up.
d) My University Supervisors: Prof. J.N. Ayertey 
(former Head, Crop Science Department, Head of the 
Entomology Laboratory at the University of Ghana, Legon; 
and ARPPIS Regional Coordinator for West Africa) who 
followed my progress through valuable discussions, and 
encouragement; Dr. D .Obeng-Ofori who, when at the ICIPE 
got me started in the aggregation bioassay experiments, 
and while at the University followed my work with 
particular attention.
e) Colleagues from the African Regional Postgraduate 
Programme in Insect Science(ARPPIS), and the technicians 
at the Chemical Ecology Department. Mr. David Munyinyi, 
Biomathematician at the ICIPE for his assistance in the 
massive data analysis. Prof. S. El-Bashir, former 
Supervisor, and Locust Research Programme Leader at the 
ICIPE, for his assistance and encouragement.
f) My former work supervisor at the Dakar Training 
Center, Senegal, my friend, Dr. William A. Overholt 
(Project Leader ICIPE / WAU Project, ICIPE) for being the 
first to appreciate my qualifications and for his effort 
to get me into Oklahoma State University in 1980, and for 
having encouraged me to get into the ARPPIS.
ix
g) My former Oklahoma State University Supervisor, 
Prof. Don. C. Peters;|Dr. Walter Knaussenburger,
USAID Washington; Prof. Ellis Huddleston, New Mexico 
State University; Dr. Mbaye Ndoye, Organisation of the 
African Unity; Dr.Ba Daoule, former Director of the IPM 
project, Coordinator of the UCTR/PV, and Permanent 
Secretary of the Sahelian Pesticide Committee (CSP) at 
the Permanent Inter State Committee for Drought Control 
in the Sahel (CILSS). All for their encouragement in my 
undertaking these studies.
h) Colleagues from the Environmental Impacts of 
Locust Control(LOCUSTOX) Project: Dr. Eloi Dieme, Mrs. 
Jude Andreasen, Dr. James Everts, Mr. Harold Van Der 
Valk, Mr. Samba Thioub, and Mme Aminata Samb, for their 
encouragement and assistance; and Colleagues from the 
National Crop Protection Directorate: Mr. Faustin Diatta 
(Director), Mr. Seni Dieme(Head of the Quaranteen 
Division) , Abdoulaye Danfa, Alpha Omar Diallo, Aliou 
Badji, and Moussa Konate.
i) The Senegalese Community in Nairobi: Mr. and Mrs 
Abdoulaye Sene, Mr. and Mrs. Ibrahima Coly, Dr. Amadou 
Diaite, Mr. Aladji Diack, Dr. and Mrs. Cheikh Mbacke.
j) Dr. Melaku Girma, and all the Ethiopian Community 
in Nairobi.
X
xi
COPYRIGHT
All rights reserved. No part of this thesis may be 
reproduced, stored in any retrieval system or transmitted 
in any form or by any means: electronic, mechanical, 
photocopying, recording or otherwise, without prior 
written permission of the author or University.
TABLE OF CONTENTS
TITLE PAGE
Title page .................................................1
Declaration................................................ii
Dedication................................................ii i
Abstract................................................... iv
Acknowledgements......................................... viii
Copyright................................................... xi
Table of contents......................................... xii
List of tables.......................................... xviii
List of figures........................................... xix
List of plates........................................... xxiv
CHAPTER ONE
1.1 General introduction................................... 1
1.2 Objectives of the study................................. 6
CHAPTER TWO
2.0 Literature review................................... 9
2.1 Interactions in insects........................... 9
2.1.1 Negative interactions..............................10
2.1.2 Positive interactions..............................15
2.1.3 Neutral interactions............................... 16
2.2 Phase transformation in locusts.................. 17
xii
2.2.1 Interactions with other locust and grasshoppers.19
2.2.2 Environmental conditions......................... 21
2.2.3 Density ..........................................  24
2.2.4 Chemical stimuli...................................2 8
2.2.5 Dietary factors.................................... 31
2.3 Methods of characterizing phase in locusts...... 33
2.3.1 Morphometries...................................... 33
2.3.2 Body colour.........................................35
2.3.3 Eye stripes and shading........................... 36
2.3.4 Haemolymph pigments............................... 37
2.3.5 Pheromones..........................................38
2.4 Developmental life cycle..........................38
2.4.1 Egg development.................................... 40
2.4.2 Nymphal development ............................. 41
2.4.3 Adult sexual maturation...........................42
2.4.4 Rainfall, humidity, and temperature complex 43
2.4.5 Chemical stimuli......... ......................... 44
2.4.6 Interactions between species......................46
2.4.7 Host and nonhost plants........................... 47
2.5 Methods of characterizing sexual maturation 48
2.5.1 Integumental colour.........................   48
2.5.2 Copulation..........................................49
2.5.3 Pheromone emission................................. 49
2.5.4 Length of basal oocytes and oviposition...........49
xiii
CHAPTER THREE
3 General materials and methods..................... 52
3.1 Insects.............................................. 52
3.2. Bioassays............................................54
3.2.1 Single chamber olfactometer....................... 54
3.3.2 Cage assays..........................................56
3.3 Collections of volatiles........................... 58
3.4 Analysis of volatiles.............................. 60
3.5 Integumental colour grading........................61
3.6 Statistical analyses................................67
CHAPTER FOUR
4 Aggregation responses of S. gregaria and L.
migratoria migratorioides to their airborne 
volatiles........................................... 68
4.1 Introduction........................................ 68
4.2 Materials and methods..............................69
4.2.1 Olfactometer assays................................ 69
4.3 Data analyses....................................... 72
4.4 Results..............................................74
4.4.1 Responses of 5. gregaria to volatiles of L.
migratoria migratorioides......................... 74
4.4.2 Responses of L. m. migratorioides to volatiles
of S. gregaria..................................... .
xiv
4.4.3 Responses of S. gregaria or L. migratoria to
choices of volatiles of both species............ 80
4.4.4. Sex responses to volatiles in L. migratoria 
migratorioides......................................82
4.4.5. Intra and inter-stage responses in
L. migratoria migratorioides...................... 85
4.5 Discussion...........................................87
CHAPTER FIVE
5.0 Primer effects of gregarious population of 
one species on solitary individuals of the
other........................... .................. 92
5.1 Introduction........  92
5.2 Materials and methods.............................93
5.2.1 Effects of the presence of gregarious L.m
migratorioides and two grasshopper species 
on the gregarization of solitarious S. 
gregaria........................................... 94
5.2.2 Effects of gregarious S. gregaria on the 
gregarization of solitarious L.
migratoria........................................104
5.2.3 Effects of volatiles of gregarious
L. migratoria on the gregarization of 
solitarious S. gregaria......................... 105
XV
5.3 Data analysis,  106
xvi
5.4 Results.......................................... ■*-07
5.4.1 Effects of the presence of gregarious
L. m. migratorioides and two grasshopper 
species on the gregarization of solitarious 
S. gregaria .....................................107
5.4.2 Effects of gregarious S. gregaria on the 
gregarization of solitarious L. migratoria...134
5.4.3 Effects of volatiles of gregarious
L. migratoria on the gregarization of 
solitarious S.gregaria.........................136
5.5 Discussion....................................... 144
CHAPTER SIX ?_*
•I
6 Inter-and intra-specific effects of volatiles 
of S. gregaria and L. m. migratorioides 
on their sexual maturation times............... 149
6.1 Introduction......................................149
6.2 Materials and methods........................... 151
6.2.1 Effects of gregarious L. m. migratorioides 
on sexual maturation of gregarious S. 
gregaria.......................................... 151
6.2.2 Effects of gregarious S. gregaria on 
sexual maturation of gregarious L. m. 
migratorioides................................... 154
6.2.3 Data analysis.....................................
jV
vH
O
6.4 Results........................................... 158
6.4.1 Effects of gregarious L. m. migratorioides
on sexual maturation of gregarious
S. gregaria....................................... 158
6.4.2 Effects of gregarious S. gregaria on
sexual maturation of gregarious L.m. 
migratorioides....................................162
6.5 Discussion........................................ 169
CHAPTER SEVEN
7.1 General Discussion............................... 172
7.2 Recommendations.................................. 181
REFERENCES.............................................182
APPENDICES.............................................2 02
xvii
xviii
LIST OF TABLES
Table 1,
Table 2.
The E/F and F/C ratios of S. gregaria 
and L. migratoria (Meinzingen, 1993).. .34
Aggregation responses of immature adults 
of S. gregaria (SG) and L. migratoria 
migratorioides (LM) to nymphal and mature 
adult conspecific and interspecific 
volatiles in the olfactometer............. .76
Table 3. Responses of S. gregaria (SG) and
L. migratoria migratorioides (LM) to 
volatiles emanating simultaneously from 
10 locusts placed in olfactometer choice 
tests...................................... . 83
Table 4. Sex responses of fifth-instar nymphs and 
mature adults of L. m. migratorioides to 
volatiles in the olfactometer.................84
Table 5. Cross-stage aggregation responses of L. m. 
migratorioides to volatiles in the 
ol f actometer.....................................8 6
Table 6. Time(days) taken by test S. gregaria(SG) 
and L. migratoria migratorioides(LM) 
to reach gregarious colour stage and the 
corresponding percent shift with respect 
to colour when reared mixed or exposed to 
volatiles.......................................Ill
Table 7. Developmental time(days) of solitarious
S. gregaria(SSG) reared with P. viridipes 
(PV)„ [ (PV/SSG)] and E. plorans(EP) ,
[(EP/SSG)] as compared to those from 
gregarious[(GSG)/GSG)] and solitarious 
[ (ISSG/cage) ] controls........................ 129
LIST OF FIGURES
XXX
Figure 1.
Figure 2.
Figure 3. 
Figure 4 . 
Figure 5,
Figure 6.
Figure 7.
Figure 8.
Breeding (recession) habitats and
invasion areas of S. gregaria and
L. migratoria migratorioides...................2
Life cycle of gregarious desert locust 
and migratory locust(Steedman, 1988)
(solitary S. gregaria and sometimes L. 
migratoria have a sixth-
instar) ......................................... 39
Diagrams of standard cages for rearing 
crowded (A) and isolated locusts (B)..........53
Diagram of the olfactometric aggregation 
bioassay system................................ 55
Dose-aggregation response curves for 
fifth-instar nymphs of S. gregaria 
(FSG) to volatiles of fifth-instar 
conspecifics[(FSG/FSG)] and to those of 
L. migratoria(FLM),[(FSG/FLM)] in the 
olfactometer (bars are SE).................... 75
Dose-aggregation response curves for
mature adults of S. gregaria (ASG)
to volatiles of mature adult
conspecifics[(ASG/ASG)] and to
those of L. migratoria(ALM),[(ASG/ALM)]
in the olfactometer (bars are SE)............78
Dose-aggregation response curves for 
fifth-instar nymphs of L. migratoria 
(FLM) to volatiles of fifth-instar 
conspecifics[(FLM/FLM)] and to those 
of S. gregaria (FSG),[(FLM/FSG)] 
in the olfactometer(bars are SE).... .79
Dose-aggregation response curves for 
mature adults of L. migratoria (ALM) 
to volatiles of mature adult 
conspecifics[(ALM/ALM)] and to 
those of S. gregaria(ASG),
[(ALM/ASG)] in the olfactometer 
(bars are SE)..........................
Figure 9. Black colour pattern used in grading 
nymphal colour changes in test S. 
gregaria......................... 98
XX
Figure 10. The standard morphometric chart for
locust phase determination............... 101
Figure 11. Developmental times of solitarious 
test S. gregaria[SSG) reared with 
gregarious L. migratoria(GLM) ,
[ (SSG/GLM)] compared to gregarious
[(GSG)] and solitarious[(SSG)]
controls................................... 113
Figure 12. Nymphal instar and stage durations of
solitarious test S. gregaria(SSG) reared 
with gregarious L. migratori a (GLM) , 
[(SSG/GLM)] and that of those which it 
was reared with [(GLM)]................... 114
Figure 13. Adult E/F and F/C ratios of solitarious 
S. gregaria(SSG) reared with gregarious 
L. migratoria{GLM) from nymphal stage 
[(SSG/GLM)] compared to gregarious[(GSG)] 
and solitarious controls [ (SSG) ]......... 116
Figure 14. Haemo lymph pigment ratios (at 460 over 
that at 680 nm wavelength) of test 
S. gregaria[(SSG/GLM)] compared to 
gregarious[(GSG)] and solitary[(SSG)] 
controls................................... 118
Figure 15. Gas chromatograms showing the production
of phenylacetonitrile (PAN) in adult males 
of S. gregaria reared with gregarious 
L. migratoria at 10-12, 15-20, 25-30, 
and 35-40 days after fledging (30m,
0.2mm ID methyl silicone column) ........ 119
Figure 16. Gas chromatograms showing peaks of
phenylacetonitrile (PAN) produced by 
gregarious locusts from control cages 
at 10-12, 15-20, 25-30, and 35-40 days 
after fledging (30m, 0.2mm ID methyl 
silicone column)........................... 120
Figure 17. Phenylacetonitrile titres (pg) produced
by test S. gregaria(SSG) when reared with 
gregarious L. migratoria(GLM)at nymphal 
stage[(SSG/GLM)] and immature adult 
[(ISG/GLM)] as compared to titres of 
conspecifics from gregarious 
control [ (GSG) ]..............................122
xxi
Figure
Figure
Figure
Figure
Figure
Figure
Figure
18. Gas chromatograms showing the production 
of phenylacetonitrile (PAN) in gregarious 
mature adults of L. migratoria at 10-12,
15-17, 20-22, and 25-27 days after 
fledging (30m, 0.2mm ID SPB-1 methyl 
silicone column)........................... 123
19. Gas chromatograms showing A) peaks of 
phenylacetonitrile (PAN) produced by
S. gregaria reared from immature solitary 
adult stage with gregarious adults of L. 
migratoria ; B) produced by L. migratoria 
reared from immature solitary adult stage 
with gregarious adults of S. gregaria; and 
Oproduced by adults of S. gregaria reared 
from immature solitary adult stage with 
fifth-instar nymphs of L. migratoria(50m,
0.2mm ID Carbowax column)...................126
20. Adult E/F and F/C ratios of solitarious
S. gregaria (SSG) reared with P. viridipes 
(PV) at nymphal stage[(SSG/PV)] compared to 
gregarious[(GSG)] and solitarious[(SSG)] 
controls......................................131
21. Adult E/F and F/C ratios of solitarious 
test S. gregaria (SSG) reared with E. 
plorans(EP) at nymphal stage[(SSG/EP)] 
compared to gregarious[(GSG)] and 
solitarious [ (SSG) ] controls................ 133
22. Production(in pg) of phenylacetonitrile 
by solitary immature L. migratoria(ILM) 
when reared with gregarious immature 
adults of S. gregaria(ISG) ,[(ILM/ISG)] 
compared to those reared with gregarious 
conspecific controls [ (ILM/ILM) ]............135
23. Nymphal instar and stage duration of 
test solitarious S. gregaria[(SSG)) 
exposed to volatiles of gregarious 
source L. migratoria[(GLM)] from
nymphal stage stage [ (SSG:GLM) ]............. 137
24. Nymphal instar and stage durations of 
solitarious test S. gregaria(SSG) exposed 
to volatiles of gregarious L. migratoria 
(GLM),[(SSG:GLM)] from nymphal stage 
compared to source gregarious L. 
migratoria [(GLM)]..........................139
xxii
Figure
Figure
Figure
Figure
Figure
Figure
25. Nymphal instar and stage duration 
of test solitary S. gregaria(SSG) 
reared with gregarious L. migratoria 
(GLM),[(SSG/GLM)] or exposed to its 
volatiles [ (SSG: GLM) ]........................ 140
26. E/F and F/C ratios of solitary
S. gregaria(SSG) exposed to volatiles 
of gregarious L. migratoria(GLM) from 
nymphal stage [ (SSG:GLM) ]....................141
27. Stage durations of solitarious S. 
gregaria(SSG) a) reared with[(SSG/GLM)] 
or b) exposed to volatiles of [(SSG:GLM)] 
gregarious L. migratoria(GLM) c)reared 
with P. viridipes (PV) ,[ (SSG/PV) ] and
d) reared with E. plorans(EP) , [(SSG/EP)]
compared to gregarious conspecifics
from the control [ (GSG) ].....................143
28. Time (days) taken by maturing adult 
males of S. gregaria (ISG) to attain 
stage colour III when exposed to 
volatiles of live conspecific 
fift'n-instar nymphs and mature 
adults[(ISG/SG)]; conspecific 
immature adults[(ISG/ISG)] ;
and to those of similar stages
of L. migratoria [ (ISG/LM) ]................. 159
29. Amounts(pg) of phenylacetonitrile 
produced by maturing males of
S. gregaria(ISG) exposed to volatiles
from live conspecific fifth-instar
nymphs(FSG) , [(ISG/FSG)]; immature
adults[(ISG/ISG)] and mature adults
(MSG) , [(ISG/MSG)] ; and to those of
fifth-instar(FLM),[ (ISG/FLM)] and
mature adults(MLM), [ (ISG/MLM)] of
L. migratoria................................ 160
30. Time taken by maturing adult females 
of S. gregaria(ISG) to attain mature 
oocyte length when exposed to volatiles 
of conspecific(SG) fifth-instar nymphs 
and mature adults[ (ISG/SG)]; conspecific 
immature adults[(ISG/ISG)]; and to those 
of similar stages of L. migratoria(LM), 
[(ISG/LM) ].................................. 163
xxiii
Figure 31. Time(days) taken by maturing adult 
males of L. migratoria(ILM) to 
attain stage colour III when exposed 
to volatiles of live conspecific(LM) 
fifth-instar nymphs and mature adults, 
[(ILM/LM)]; conspecific immature 
adults,[(ILM/ILM)]; and to those of 
similar stages of S. gregaria(SG) ,
[(ILM/SG)].............................. 164
Figure 32. Amounts(pg) of phenylacetonitrile 
produced by maturing males of L. 
migratoria(ILM) exposed to volatiles 
Of live conspecific fifth-instar 
nymphs(FLM),[(ILM/FLM)] immature adults 
[(ILM/ILM)], and mature adults(MLM)
[(ILM/MLM)]; and to those of fifth-instar 
(FSG),[(ILM/FSG)] and mature adults(MSG), 
[(ILM/MSG)] of S. gregaria...................165
Figure 33. Time taken by maturing adult females
of L. migratoria(ILM) to attain mature 
oocyte length when exposed to volatiles 
of conspecific(LM) fifth-instar and 
mature adults,[(ILM/LM)]; conspecific 
immature adults[(ILM/ILM)]; and to those 
of similar stages of S. gregaria (SG) ,
[(ILM/ISG)]....................................167
xxiv
LIST OF PLATES
A. Two chamber bioassay cages, a) source
chamber, b) recipient (test) chamber................. 57
B. Volatile collection system(B-l) showing
connected to it a)three-neck round bottomed 
flask containing mature S. gregaria b)quick- 
fit trapping chambers, and the medical air 
source cylinders; quickfit chambers(c) with 
connected charcoal traps(d) (B—2)....................59
C. Immature adults of S. gregaria(C-l) and L. 
migratoria(C-2)(note the pink and blackish
body colours of the respective insects)............ 62
D. Gregarious(D—1) and solitarious(D-2) mature
adults of S. gregaria................................ 63
E. Gregarious(E—1) and solitarious(E-2) adults
of L. migratoria migratorioides.....................64
F. Gregarious fifth-instar(F-l) and solitarious 
fourth-instar(F-2) nymphs of L. migratoria 
migratorioides........................................ 65
G. Olfactometer tests showing locusts (1,2,..) 
orienting themselves towards or sitting on
the treated side of the arena....................... 73
H. One chamber bioassay cage used for mixed rearing 
experiments and described in Chapter III, and 
showing a)glass door, b)egg laying tube
insertion holes.......................................95
I. Gregarious fifth-instar and fourth-instar 
nymphs of S. gregaria from gregarious
(I — 1) and solitarious(1-2) control cages 
respectively.......................................... 99
J. Solitarious S. gregaria{A) reared with
gregarious L . migratoria migratorioides(B)....... 109
K. Adult male S. gregaria{A) reared with crowded
L. migratoria(B) and showing yellow colour 
typical of mature gregarious locust............... n o
L. Solitarious S. gregaria(A) reared with
P. viridipes(B)(L-l), and E. plorans(B)
(L-2) from nymphal stage............................ 128
CHAPTER ONE
1 General Introduction
Locusts are short horned grasshoppers, a large group 
in the order Orthoptera. There are six major locust 
species in Africa:
- the desert locust, Schistocerca gregaria gregaria (F),
- the African migratory locust, Locusta migratoria
migratorioides (R & F),
- the Red locust, Nomadacris septemfasciata (Serville),
- the Brown locust, Locvstana pardalina (Walker),
- the Moroccan locust, Dociostaurus maroccanus 
(Thur.berg! , and
- the Tree locust, Anacridium melanorhodon(Walker)
The first four species are major agricultural pests in 
Africa, but the desert locust is regarded as the single 
most serious pest due to its polyphagous feeding 
behaviour, and its migratory habits (Popov et al. 1 9 9 1 ;  
Steedman, 1988; Meinzingen, 1993). While the desert 
locust is widely distributed in Africa, the Middle-East, 
and parts of Southwest Asia, the African migratory locust 
is restricted to Africa where it breeds in three 
ecological zones including the Sahelian, Sudanese and 
Guinean zones(Zolotarevsky, 1938; Showier, 1995) (Fig.1) .
1
2Figure 1. Breeding (recession) habitats and 
invasion areas of S. gregaria and 
L. migratoria migratorioides
S. gregaria and L. migratoria both show density 
dependent phase polymorphism which refers to the ability 
of these insects to exist reversibly, depending on 
density, in two extreme forms or phases: solitaria and 
gregaria (Uvarov, 1921; 1923), with intermediate phases 
known as "transiens". The two locust phases differ in 
behavioural, morphological, and biochemical features, and 
in fecundity (Uvarov, 1921; 1923) . Between plagues, the 
two locust species live as solitary, scattered (less than 
100 individuals/ha) , and harmless individuals in locust 
affected countries, in areas commonly known as "recession 
areas" (Fig. 1). When weather conditions(rainfall) 
become suitable, these individuals concentrate to breed 
successfully into large numbers, and become gregarious 
(Roffey and Popov, 1968) . Gregarious hoppers do not have 
developped wings to fly, but they march in dense groups. 
Gregarious adult stages, however are winged, and 
therefore, they can swarm over long distances (Descamps, 
1961; Duranton and Lecoq, 1990; Steedman, 1988; and 
Meinzingen, 1993). A typical swarm of S. gregaria can be 
of 300 km2 in size containing about 40 million 
individuals, and can travel up to 1000 km a week 
(Steedman, 1988; Meinzingen, 1993). Swarms of the 
African migratory locusts are smaller, seldom of similar 
size, and are usually restricted to their breeding areas 
(Steedman, 1988).
3
Locust hopper bands and swarms are very voracious 
and are, therefore, very destructive. Swarms of the 
desert locust can cause losses of thousands of tons of 
crop such as cereals, and vegetables. Meinzingen(1993) 
reported that damage caused by the desert locust stems 
from the fact that a locust consumes 1.5 - 3 g of 
vegetation daily, the equivalent of its own weight. Thus 
a medium density swarm of 50 million individuals per km2 
can consume 100 tons, and a swarm of 1000 Km; can eat
100.000 tons, enough to feed 500,000 people a year 
(Meinzingen, 1993). However, damage varies with the 
locust stage and as Meinzingen (1993) had indicated, 8% 
of the total damage by locusts is due to hoppers, 69% to 
immature and 23% to mature adults. Direct yield losses 
from damage due to the desert locust during the plagues 
of 1954 to 1958, were estimated to vary from 15 million 
US Dollars in Morocco, 55,000 tons of cereals in Sudan,
16.000 tons of millet in Senegal, and 167,000 tons of 
cereals in Ethiopia (Steedman, 1988; Popov et al., 1991; 
Meinzingen, 1993). About 76,000 to 900,000 tons of 
sorghum were lost in the Sahelian Africa during the 
desert locust plagues from 1986 to 1993, with an 
estimated maximum cost of control in 1988 of about 102.7 
million US Dollars (Herok and Krall, 1995).
The gregarious migratory locust is also polyphagous, 
but to a lesser extent compared to the desert locust, and
4
is restricted to graminaceous wild and cultivated plants 
such as millet, sorghum, maize, rice, sugar cane, and 
bamboo (Meinzingen, 1993) . No yield loss assessment 
exists for this species.
Locust control mainly relies on the use of 
pesticides. Despite the fact that large scale locust 
control involves intensive surveys and mapping in order 
to optimize pesticide use and reduce environmental 
degradation, the risk of casualities and extensive 
contamination still exist in locust affected countries 
(Everts, 1990), because of the fragile ecosystems due to 
recurrent droughts. Alternative methods including 
monitoring locust breeding zones for rains and hatchings 
using remote sensing techniques, the development of 
gregarization models (Popov et al., 1991), and the use of 
biological control agents such as entomopathogenic fungi 
and protozoans (Lomer and Prior, 1992; Thomas et al., 
1995), have been proposed. More recently, an integrated 
pest management approach (El-Bashir, 1991; Joffe, 1995) 
has been proposed using different, and environmentally 
sound, packages. It is in this context that the 
International Center of Insect Physiology and Ecology 
(ICIPE), launched the Desert Locust Semiochemical Project 
in 1990 to investigate the semiochemicals that mediate 
aggregation behaviour, sexual maturation, oviposition, 
and gregarization of this insect. By the end of 1993,
5
significant progress had been made by this project in the 
characterization of these pheromones, and the mechanisms 
underlying some aspects of phase transformation.
However, the project did not include investigations on 
the role played by other locust species and grasshoppers 
on the phase dynamics in the desert locust.
To fill this gap, the present study was designed to 
investigate the interactions between the desert locust,
S. gregaria and the African migratory locust, L. 
migratoria migratotioides with regard to releaser effects 
of semiochemicals, and primer effects of all the phase 
mediating factors.
2 Objectives
2.1 General objectives
The main objective of this work is to establish 
whether there are interactions between S. gregaria and L. 
migratoria, and to investigate releaser effects of 
semiochemicals and primer effects of the interactions 
(all phase mediationg factors) on both locust species.
6
2.2 Specific objectives
To investigate :
1) phase associated releaser effects between and within
stages of S. gregsria and L. migratoria;
2) phase associated primer effects of crowded 
populations of one species on isolated individuals 
of the other species in relation to:
a) the rate of phase transformation in 
solitarious individuals of the two locust 
species;
b) the influence of different stages of 
crowded locusts on phase change;
c) the role played by volatiles in these 
interactions; and
3) cross-effects of crowded live locusts of one species 
on the rate of sexual maturation of the other.
The procedures used to achieve these objectives are 
presented in this thesis which is divided into seven 
chapters. The first chapter contains the general 
introduction, the second, the review of literature. A 
general description of the methods and materials used in 
experimentation follows. Chapters four, five and six 
deal with studies on locust aggregation (releaser 
effects), gregarization (primer effects), and maturation
7
(primer effects), respectively. The last chapter 
contains two sections comprising a general discussion of 
the results, and recommendations for future studies. A 
list of references, and appendices are also provided at 
the end.
8
CHAPTER TWO
2 Literature review
2.1 Interactions in insects
Insect species may exhibit diverse types of 
interactions (Odum, 1971; Price, 1984) . Odum (1971) 
reported that between any two species there could be as 
many as nine types of interactions including neutralism 
(neither of the interacting species is benefited), 
competition (direct interference, and resource use), 
amensalism (one is inhibited, the other not affected), 
parasitism, predation, commensalism, protocooperation 
(non obligatory, but beneficial for both species), and 
mutualism (both species benefit from their interactions 
and have become totally dependent on each other for their 
survival) . He added that these can be grouped into 
negative types of interactions such as predation, 
parasitism, antibiosis, and interspecific competition; 
and positive types of interactions including 
commensalism, cooperation and mutualism (Odum, 1971). To 
this could be added neutral interactions. Saini and 
Hassanali (1991) pointed out that in any multitrophic 
network, a large number of theoretical combinations are 
possible. For example, between any two levels, six
9
theoretical categories of interactions are possible 
depending upon the benefit or harm associated with the 
signal emitted by a member of interacting pairs (Saini 
and Hassanali, 1991). Due to the complexity of these 
ecological relationships, this review is focussed only on 
examples of recent studies on one and two stage negative, 
positive and neutral types of interactions.
10
interactions between two populations which result in 
negative effects of growth and survival of one of the 
interacting populations. Where the two populations have 
had a common evolutionary history in relatively stable 
ecosystem, the negative effects tend to be relatively 
small. Thus natural selection tends to ultimately reduce 
the detrimental effects and the interactions altogether 
(Odum, 1971). Severe negative interactions, when coupled 
with drastic changes in the ecosystem may lead to the 
extinction of one species and the resurgence of the 
other. This is probably how pest epidemics (i.e locust or 
grasshopper invasions) are born, especially when natural 
ennemies of a species are affected. Predator or 
parasitoid-prey relationships are the most widely studied 
interactions because they are of special interest and
provide an understanding of the mechanisms of host 
location by pest and predators and/or parasitoids (Saini 
and Hassanali, 1991). According to Price (1984) , this 
type is the central issue in understanding both the 
adaptative strategies for getting food and for avoidance 
of becoming food. While a predator consumes more than 
one prey for its survival, a parasitoid survives on a 
single prey (Price, 1984). Models of predator and/or 
parasitoid-prey interactions which are developed 
independently by different authors are used in ecological 
studies especially in those related to biological control 
(Price, 1984) . There are as many as three levels of 
interactions of this type: between the natural enemy
species and the host species (carnivore-herbivore); 
between species of natural enemies (carnivore-carnivore); 
and between species or host plants, natural enemies, and 
the herbivore species (plant-carnivore-herbivore).
Carnivore-herbivore
The example reported by Ngi-Song et al.(1995), in 
which they determined the suitability of the herbivore as 
food for the carnivore helps to illustrate this level of 
interaction. These workers found that the exotic Cotesia 
flavipes Cameron preferentially oviposited on larvae of 
the pyralid species Chilo partellus Strand, Chilo
11
orichalcociliellus Strand, Busseola fusca ('Fuller), and 
Sesamia calamistis (Hampson). However, the local 
endoparasitoid, Cotesia sesamiae Cameron, preferred S. 
calamistis, probably because of some evolutionary 
adaptation whereby this parasitoid has learned to use 
this lepidopteran species as a host. They also found 
that both Chilo species and S. calamistis were suitable 
for development of C. flavipes with Chilo being the 
preferred hosts, and that C. sesamiae preferred 5. 
calamistis to the two Chilo species. B. fusca was not a 
suitable host for the two parasitoids because of its 
ability to encapsulate parasitoid eggs in the haemolymph, 
a form of evolutionary resistance by B. fusca to 
parasitization by this wasp.
Carnivore-carnivore
This level of interaction is well illustrated by the 
work of Heinz and Nelson (1996) who studied the 
interactions between the natural enemies of the whitefly 
Bemisia argentifolii (Grenadius) (ex B. tabaci) , and found 
that there were no detrimental competitive interactions 
among the three species of natural enemies studied which 
would have negatively affected the level of predation 
and/or parasitism.
12
Herbivore-herbivore
13
Coexistence among insects involves spatial 
interactions which could be viewed to some extent as 
competition for space resource. The interacting species 
may learn to coexist for ever or will show competitive 
periodic displacement (Odum, 1971). This aspect had been 
studied by Brown et al.(1995) on two aphid species on 
apple, and found that in such an ecosystem, the Spirea 
aphid, Aphis spirascola Patch and the apple aphid, Aphis 
pomi De Geer, were similar in their distribution and 
abundance in late summer when apple is a less suitable 
host. No significant positive and negative interactions 
occurred between the two species. A. spirascola 
reproduced faster to yield more generations through 
spring to the point that, by the end of the summer, it 
was the only species present.
PI ant-ca m i  vore-herbi vore
Recent work on this tritrophic level of interactions 
include the comprehensive studies by Wiedenmann et al. 
(1992) involving stemborers such as Diatrea saccharalis 
Dyar, their host plants, and parasitoids such as Cotesia 
flavipes. These studies have revealed some interesting 
aspects of these interactions especially with regard not
only to the role played by the plant in host location by 
parasitoids, but also the suitability of the host for
survival of the parasitoids.
Abundance of the natural enemies may be favoured by 
host plant density. Brust et al.(1986) showed that the 
activity of four most important Carabid predator species 
of the genera Pterostichus, Amphasia, Pterosticus, 
Ambacidus, and Harpalus was more important in no tillage 
than in tillage corn ecosystem. Thus in no tillage 
system host plants are more abundant favouring rapid 
build-up of pest species and consequently, higher 
populations of their natural enemies. Perfecto et 
a1. (1996) studied the effects of plant diversity and 
density, on the migration of two ground beetles of the 
genera Harpalus and Evarthrus. They found that the 
emigration rate was higher at low plant density, with 
negative interactions between the insect migration, plant 
density, and plant diversity. Thus, emigration was 
higher in dense polyculture than in dense monoculture. 
Rupasas et al. (1996) studied in a 4-armed air flow 
olfactometer the chemical attraction of fourteen Oryza 
sativa Savannah cultivars and one wild Oryza species, to 
the mirid and coccinellid predators of the genera 
Cyrtorhinus, and Micraspis, respectively, of the brown 
planthopper, Nilaparvata sp. They found that odours from 
the plant attracted significantly Cyrtorhinus species.
14
The host plant can, in many forms, resist attack by 
herbivores, thus enhancing the actions of carnivores. The 
effects of plant resistance and natural enemies on sawfly 
were compared by Stein and Price (1995), who found a 
strong relationship between oviposition preference and 
larval performance of two tenthredinid sawflies, Euura 
sp., and Pontiana sp. Plant resistance to attacks, in 
addition to natural enemies, played an important role in 
the reduction of the sawfly populations.
2.1.2 Positive interactions
Positive association between two species populations 
are widespread in nature. Positive interactions may be 
considered as having evolved from commensalism to 
protocooperation and to mutualism (Odum, 1971). Thus 
mutualism is the ultimate type of positive interaction, 
and is discussed in this Section. Symbiotic or 
mutualistic trophic interactions occur between various 
species of insects. For example, aphids excrete honeydew 
consisting mainly of excess ingested sap which is a 
favourite food for ants (Borror et al., 1976). The ants 
in turn protect the aphids against predation. Termites, 
Nasutitermes corniger (Motschulsky) obtain nitrogen-rich 
nutrients and additional nest defence from various 
species of ants which shelter in their nests during the
15
wet season (Jaffe et al., 1995). The ants also prey on 
some live and dead termite workers.
Other forms of somewhat positive trophic 
interactions include facultative carnivory and/or 
cannibalism among species, e.g., orthopterans such as 
Chrotogonus spp., Aulacara spp., Eyprepocnemis spp., and 
S. gregaria (Duranton and Lecoq, 1990; Whitman et al., 
1994). In S. gregaria, and L. migratoria migratorioides, 
cannibalism is common among fifth-instar nymphs and 
adults, in Cataloipus cymbiferus Krauss, and Aulacara 
spp., it is common among adults. It has been suggested 
that this provides a source of necessary proteins to 
insects that feed on protein-deficient plants (Whitman et 
al., 1994). This feeding habit has also been documented 
in many other insect species of the orders Isoptera and 
Heteroptera (Whitman et al., 1994).
2.1.3 Neutral interactions
Neutral interactions occur when neither of the 
interacting species are benefited. However, there may 
never be total neutral interactions unless species are 
geographically far apart. There are situations in which 
non related species (in habit and niche) are forced to 
coexist and with time tend to share some characteristics 
of survival value such as colour mimicry or semiochemical
16
signals, without directly affecting each other's life. 
Such types of interactions are common in grasshoppers, 
and solitarious or gregarious locusts coexisting in 
certain breeding areas {Johnston and Buxton, 1949; 
Steedman, 1988; Showier, 1995). Johnston and Buxton 
(1949) reported that mixed bands of S. gregaria and L. 
migratoria migratorioides, in the Red Sea coastal 
breeding zones, exhibited marching behaviour 
characteristic of their gregarious phase. It was not 
known whether there was change in colour and 
morphometries associated with phase change, nor whethe 
they might share the same chemical stimuli. El-3ashir 
and Abdel-Rahman (1991) later reported some changes in 
colour and morphometries in mixed bands of the species 
These are discussed in more detail in the following 
Sections.
2.2 Phase transformation
Phase transformation refers to the ability of 
locusts to exist reversibly in two extreme phases: 
solitarious and gregarious. The forms between the two 
extremes are known as transiens (Uvarov, 1921; 1923). 
Uvarov (1921; 1923) pointed out that all gregarious 
acridids belong to polymorphic species, and are true 
locusts. However, some grasshoppers, e.g., Oedaleus
senegalensis (Krauss), and Gastrimargus africanus 
Saussure show colour polymorphism and marching behaviour 
like locusts, but do not swarm nor transform into 
different phases (Rowell, 1970; Launois and Launois- 
Luong, 1989). In the family Pyrgomorphidae, some species 
in the genera Zonocerus, Phymateus, and Poikylocerus, 
produce aggregation and defence pheromones (Whitman,
1990). However, they do not exhibit reversible phase 
change, nor form bands and adult swarms like locusts.
Phase transformation is an important trait of 
locusts which facilitates their survival through 
migration and recolonization of old habitats and 
colonization of new areas. It is affected by many 
factors including, environmental (Steedman, 1988; Popov 
et al., 1991; Zolotarevsky, 1938), density (Pasquier,
1950; Duranton et Lecoq, 1990, Popov et al., 1991; Deng 
et al., 1996), chemical stimuli (Nolte, 1963; Gillett, 
1975a; Obeng Ofori et al, 1993; Torto et al. 1994; Deng 
et al., 1996), diet (Jackson et al., 1978), and host and 
nonhost plants (Steedman, 1988) . Interactions with other 
locusts have also been implicated (Johnston and Buxton, 
1949; El Bashir and Abdel-Rahman, 1991).
18
2.2.1 Interactions with other locusts and grasshoppers
The coexistence of locusts and grasshoppers may be 
regarded as a neutral type of interaction whereas those 
between locusts are either mutual inhibitory competition 
or protocooperation (in accordance with the 
classification by Odum (1971) . Experiments under 
laboratory conditions have demonstrated that locusts 
affect phase polymorphism and development of one another 
when they are crowded in cages while grasshoppers do not 
(Gillett, 1968; Ba-Angood, 1976). Gillett (1968) studied 
the grouping behaviour of the desert locust, S. gregaria, 
reared singly with a non-swarming grasshopper species, 
Humbe tenuicornis (Sc’naum) . Isolated locusts and those 
reared crowded with H. tenuicornis were unable to form 
groups like gregarious populations. She concluded that 
the grasshopper had no significant effect on the phase 
status of the desert locust.
In a similar experiment (Ba-Angood, 1976), crowded 
individuals of S. gregaria, and individuals of S. 
gregaria kept among groups of three grasshopper species, 
Kraussaria spp.r Oedaleus spp., and Aiolopus spp., had no 
significant effect on phase characters such as the number 
of eye stripes and morphometries. On the other hand, 
locust species have been reported to affect phase 
polymorphism and development of one another when they
19
occur crowded in the same ecological zones in the field 
(El-Bashir and Abdel-Rahman, 1991). Johnston and Buxton 
(1949) documented the occurrence of mixed populations of 
S. gregaria and L. migratoria migratorioides whose adults 
laid eggs together in some breeding areas in the Sudan. 
They further noted that, within the mixed hopper bands, 
hatchlings of the two locust species could also be 
differentiated by their host plant species, and in 
addition, hatchlings of L. m. migratorioides were better 
swimmers than those of S. gregaria. Although effect of 
the presence of either locust species on the phase 
characteristics of the other was not investigated, their 
observations suggest some mutual interactions in addition 
to overlap of the ecological niches of the two locust 
species. Gillett (1983) investigated the effects of the 
fecal volatiles of either L. migratoria and S. gregaria 
on each other's gregarization, and found that faeces of 
the latter affected the melanization and behaviour of the 
former lending support to Nolte's (1973) observations 
that these two species produce a common aggregation 
pheromone (locustol) . However, the faeces of L. 
migratoria did not seem to produce the same effects 
(Gillett, 1983).
El-Bashir and Abdel-Rahman (1991) reported 
observations of mixed populations of the desert locust 
and the migratory locust in the Sudan. They observed
20
that solitarious hoppers of the desert locust which 
interacted with dense populations of gregarious stages of 
the migratory locust tended to change into transiens. 
There were increased encounters between hoppers of both 
species as they marched, roosted, and sheltered under 
Zygophyllum simplex L. They fed together near water 
courses where the vegetation was greener and soil 
moisture was conducive to egg-laying. They concluded 
that the prevailing edaphic factors appeared to have 
stimulated marching of mixed bands, colour and 
morphological changes of solitarious S. gregaria towards 
the transient phase. Such observations were confirmed by 
Torto in 1995 (personnal communication) in the locust 
breeding areas along the Red Sea coast in the Sudan. The 
observations by these authors did not involve analyses of 
parameters such as locust body colour, morphometries, 
marching behaviour, number of eye stripes, and pheromone 
production. Therefore, detailed studies on the 
interactions between the desert locust and the migratory 
locust are lacking.
2.2.2 Environmental conditions
Environmental factors, especially rainfall play a 
significant role in the phase transformation of both the 
desert locust and the African migratory locust
21
(Zolotarevsky, 1938; Ellis and Carlisle, 1965; Carlisle 
and Ellis, 1965; Popov et al., 1991). Adequate rainfall 
supports the growth of annual plants and shrubs which 
when fed on by immature solitary adults hasten their 
sexual maturation. This is followed by congregation of 
individuals at suitable sites for mating and egg laying. 
Uvarov (1966), Rowell (1970), and Fuzeau-Braesch (1972) 
have shown that in locusts, environmentally regulated 
phenotypic colour polymorphism such as homochromy, green 
brown polymorphism, and density dependent polymorphism 
may be found. Faure (1942), Zolotarevsky (1938), and 
Albrecht (1964) showed that solitary L. migratoria were 
green under high humidity, but in low humidity they were 
near brown in colour. Furthermore, they observed that 
under low humidity, solitary hoppers adjusted their 
colour from whitish-cream, yellow, brown, grey, or black 
to match the colour of the background environment they 
were in. Zolotarevsky (1938) studied the effects of 
humidity on the ratios of lengths of the elytron to the 
femur (E/F), and the length of the femur to the width of 
the head capsule (F/C) of the migratory locust. He found 
that dry conditions (RH 5 45%) tended to favour 
gregarious characters as compared to wet conditions (RH £ 
70%) . Increasing humidity coupled with increasing 
daylength and temperature favoured solitarious
22
morphometric ratios in L. migratoria (Albrecht and Lauga, 
1978; 1979).
Temperature has an effect on nymphal displacement 
behaviour in L. migratoria (Descamps, 1961). When 
temperatures are high (above 30”C) nymphs tend to find 
shelter by hiding under shrubs, trees or roost on top of 
grasses (Descamps, 1961: Steedman, 1988). At very low 
temperatures (below 26°C), they tend to find cover under 
dead leaves, and shrubs. This facilitates grouping of 
solitarious nymphs especially in patchy vegetation, and 
may lead to gregarization of nymphs if they are in 
contact with each other for sufficiently long periods 
Oouaichi et al., 1995). Colour patterns of locust 
cuticle also vary with temperature. Field and laboratory 
observations (Stower, 1959; Dudley, 1964), showed that at 
low temperatures the black patterns were predominant on 
locust hoppers while at high temperatures they were 
fewer. Morphometries of adults were found to change 
significantly with temperature. At a temperature of 
about 30CC and relative humidity of about 90%, locusts 
tended to shift toward gregarious E/F ratios. On the 
other hand, temperatures greater than 30°C, and at the 
same relative humidity of 90%, E/F ratios of individuals 
were characteristic of solitarious phase. F/C ratios 
only changed at relatively lower humidity (50%) . Stower 
(1959), and Meinzingen (1993) showed that at high
23
temperature (above 30°C) gregarious desert locusts showed 
morphometries which were similar to those of solitarious 
locusts reared at temperatures below 30°C. Thus there is 
ecological relative fitness of locusts to their 
environment as manifested by the changes in morphometric 
and colour, in relation to changing condtions.
Habitat patchiness also plays an important role in 
phase dynamics, especially during the dry season when 
host plants are found in patches throughout the habitat. 
It is more evident in the case of the migratory locust in 
the breeding areas of the Niger (Lean, 1931;
Zolotarevsky, 1938; Steedman, 1988) where individuals are 
forced to concentrate on green patches of vegetation 
resulting from previous floods. Grouping of solitary 
adults is then facilitated, and if conditions are 
conducive, synchronous egg-laying occurs leading to 
synchronous egg development and hatching (Roffey and 
Popov, 1968) .
2.2.3 Density
Gregarization is mainly a density-driven process 
(Pasquier, 1950). Under favourable environmental 
conditions (e.g. sufficient rainfall and vegetation) 
adult locusts mature faster, copulate, and gravid females 
concentrate at suitable sites for egg-laying. The
24
attraction of such females to egg laying sites is 
favoured by adequate soil moisture and froth volatiles 
from deposited eggs (Saini et al., 1995). Gregarized 
females lay eggs which yield gregarious nymphs, a 
phenomenon known as "maternal inheritance" (Hunter-Jones 
1958; Injeyan and Tobe, 1981; Bouaichi et al., 1995;
Islam et al., 1995). Furthermore, grouping of nymphs 
that hatched together favoured prolonged contact and 
emission of aggregation pheromones (Deng et al., 1996), 
which could keep nymphal groups together. In addition, 
at high densities tactile, visual, olfactory, and 
acoustic interactions play a role in maintaining the 
gregarious phase. Nymphs hatching together form small 
cohesive groups at the early stage, feed from plant to 
plant, and keep merging with other small groups in their 
paths into larger groups (Steedman, 1988; Meinzingen,
1993). This progressive increase in density was 
referred to by Pasquier (1950) as "densation". Roffey 
and Popov (1968) and Duranton and Lecoq (1990) described 
three stages (multiplication, concentration, and 
gregarization! in the process leading to the build up of 
locust populations in the field, and which also influence 
phase shifts. They observed that in the Tamesna 
mountains breeding area in the Niger, the multiplication 
factor (ratio between parental and actual density) was 
estimated at 16 - 20 fold. Popov et al. (1991) suggested
25
that the actual multiplication factor in such situations 
could be about 100 - 200 fold. Population build up could 
also be facilitated by active immigration, e.g. through 
attraction (Pedgley, 1981), or passive immigration e.g. 
by random effects of wind (Roffey, 1969; Popov, 1969; 
Shaeffer, 1972). Hunter-Jones (1958) studied the 
inheritance of phase characters in hoppers and adults of 
L. migratoria, and S. gregaria. By monitoring hopper 
colour patterns, weight of hatchlings, number of nymphal 
instars, and adult colour and morphometries, he found 
that hoppers reared in isolation appeared green similar 
to solitarious locusts. Morphometric ratios (E/F and 
F/C) also corresponded to those of solitarious insects.
He correlated weights of hoppers of S. gregaria with 
colour or adult morphometries and found that, the green 
hatchlings (solitary) had relatively higher morphometries 
and lower weight compared to their black and yellow 
counterparts (gregarious). Nolte (1973) showed that 
crowding at high densities in the field and in the 
laboratory increased melanin deposition, chiasma 
frequencies in L. pardalina, and L. migratoria while 
transient morphometries occurred in S. gregaria.
Crowding of the grasshopper Paracinema tricolor Thunberg 
had no effect on chiasma frequency (Nolte, 1973).
Deng et al. (1996) studied the effects of shifting 
S. gregaria from crowded to isolated conditions and vice-
26
versa on the emissions of adult aggregation pheromones in 
the laboratory, and compared this with changes in 
morphometries. They demonstrated the extreme sensitivity 
of the desert locust to crowding, such that even pairing 
two solitarious locusts provided each other with the 
necessary stimuli to cause their gregarization. Further, 
they showed that adult males of the parental generation 
resulting from shifting crowd-reared hoppers, immature or 
mature adults to isolated conditions did not produce 
phenylacetonitrile similar to solitary-reared adults. 
Conversely, adults of the parental generation resulting 
from shifting solitary-reared hoppers, fledglings, or 
mature adults to crowded conditions produced pheromone at 
levels which were similar to those of control adults from 
the crowd-reared colony. In contrast, they showed that 
morphometric changes were slow and required several 
generations to show significant variations, confirming 
earlier findings by Chapman (1979) . Also, the F/C ratio 
(length of the femur to the width of the head capsule) 
was shown to be more sensitive to treatment effects than 
the E/F ratio (length of the elytron to the length of the 
femur).
27
2.2,4 Chemical stimuli
Nolte (1963) was the first researcher to demonstrate 
the mediation of a chemical stimulus in the gregarization 
of locusts. He reported the presence of a chemical factor 
in the air-borne volatiles of crowded locusts which acted 
as a "primer" pheromone facilitating the grouping of 
locusts reared in isolation. Gillett (1968) confirmed 
the existence of such a chemical stimulus in the air­
borne volatiles of crowded locusts. Nolte referred to 
this as a "gregarization pheromone" which was later 
identified as 5-ethyl guaiacol in the nymphal faeces and 
was called "Locustol" (Nolte, 1970). Locustol evoked 
such gregarious characteristics in crowded locust 
populations as increase in chiasma frequency, melanin 
pigmentation, and changes in adult morphometric ratios 
towards those of phase gregaria. More detailed studies 
by Nolte (1973) suggested that three locust species, S. 
gregaria, L. migratoria, and L. pardalina produced, and 
utilized locustol for gregarization. The red locust, 
Nomadacris septemfasciata (Serv), the Australian locust, 
Chortoicetes terminifera Walk., and a grasshopper 
Paracinema tricolor, did not demonstrate evidence of the 
production of this pheromone (Nolte, 1973) .
Studies by Gillett (1975) showed that the 
aggregation pheromone mediated grouping behaviour in
28
gregarious nymphs and it was not produced by isolated 
nymphs in quantities that could alter each other's 
behaviour. She discussed two sets of pheromones, a 
gregarizing pheromone which emanated from faeces of 
crowded nymphs, and a solitarizing pheromone, only 
detectable in the faeces of crowded adults (Gillett and 
Phillips, 1977). A more detailed comparative study on 
the pheromonal systems of S. gregaria and L. migratoria 
migratorioides was conducted by Fuzeau-Braesch et 
al. (1988) . They detected four volatile aromatic 
compounds in the environment surrounding S. gregaria, L. 
migratoria migratorioides, and Locusta migratoria 
cinearensis (Fabricius) of which three were identified as 
phenol, guaiacol, and veratrole. Phenol, guaiacol, and 
the blend of the three compounds were found to act as 
cohesion pheromones for the first two locust species, 
while L. m. cinearensis did not produce detectable 
amounts of these pheromones.
Recently, detailed work relating to releaser 
pheromones mediating aggregation behaviour in the desert 
locust revealed that a complex of pheromones produced by 
different locust stages and sexes, and in their faeces 
are involved (Obeng-Ofori et al. 1993; 1994a;1994b; Torto 
et al., 1994; 1996). The pheromone systems mediating 
aggregation behaviour in nymphal and adult stages of S. 
gregaria were charaterized by Torto et al.(1994; 1996)
29
using a more efficient volatile collection technique.
They identified six electrophysiologically active 
compounds in the volatiles of adult males of the desert 
locust comprising anisole, benzaldehyde, veratrole, 
guaiacol, phenylacetonitrile, and phenol. Veratrole, 
guaiacol, and phenol which were previously detected by 
Fuzeau-Braesh et a1. (198 8) , were also present, but in 
minor amounts in the atmosphere surrounding fifth-instar 
nymphs and adults of S. gregaria. Of the six compounds, 
only four compounds phenylacetonitrile, benzaldehyde, 
guaiacol, and phenol were found to elicit aggregation 
responses (Torto et al., 1994). Phenylacetonitrile was 
found to be a major and key component since it elicited 
aggregation responses in adults similar to the crude 
extracts. Furthermore, it was shown that only gregarious 
mature adult male locusts produced the aggregation 
pheromone, and that solitary locusts responded to it 
(Torto et al., 1994; Njagi et al., 1996). It was 
suggested that, male aggregation pheromone may play a 
role in the 'arrestment' and recruitment of solitary 
individuals into gregarizing or gregarious groups in the 
early stages of locust outbreaks (Njagi et al., 1996).
Subsequently, it has been shown that production of 
phenylacetonitrile by gregarious mature adult male desert 
locusts follows a sigmoidal pattern, starting in the 
first 10 days after fledging, peaking between 15 and 20
30
days, and leveling off by the 35th day after which it 
starts to drop to reach very low levels in very old 
adults (Torto et al., 1994; Deng et al., 1996; Assad et 
al., 1996). Deng et al.(1996) showed that pheromone 
titres (as measured by the amounts of phenylacetonitrile) 
are a more sensitive measure than morphometries in 
determining the onset of phase change in S. gregaria.
The nymphal pheromone system of gregarious S. 
gregaria was characterized as consisting of three sets of 
corrpounds: aliphatic C.;,Ci-C:o aldehydes and their 
corresponding acids produced by the insects themselves, 
augmented by faecal phenols, guaiacol and phenol (Torto 
et al., 1996) .
2.2.5 Dietary factors
Locusts use their host plants for food and/or 
shelter. The effects of diet from such plants as 
alfalfa, hedge mustard, Johnson grass, and the mixture of 
the three, on the lesser migratory grasshopper,
Melanoplus mexicanus mexicanus (Saussure) were studied by 
Barnes (1955). He monitored many parameters of which 
nymphal development and weight, and adult body dimension 
ratios (morphometric) showed significant diet-dependent 
variations. Adults reared on hedge mustard and mixed 
diet showed traits towards the migratory phase, those
31
reared on Johnson grass were intermediate whereas those 
reared on alfalfa showed little or no development toward 
the migratory phase. Toye (1973) studied the effects of 
five different food plants on the development of the 
desert locust, S. gregaria and found that hoppers fed on 
Agropyron and Poa grasses showed the highest 
developmental rate of nymphs (29 - 32 days). The 
heaviest nymphs and adults had morphometries typical of 
the gregarious phase. When fed on lime, privet, and 
spinach they produced morphometrically smaller and less 
viable males which had some abnormalities in nymphal 
development (five instars with a long fourth and without 
the fifth instar). Jackson et al.(1978) investigated the 
effects of natural food plants on the phase of the desert 
locust, S. gregaria using cultivated plants Pennisetvm 
typhoides(Burm.f.) and Sorghum sp. and the desert host 
plant Dipterygium glaucum Oecn. Tribulus spp., 
Chrosozophora oblongifolia(Del.), Panicum turgidum Forsk. 
and Zygophyllum simplex L. They found that a diet of 
Pennisetum, and Sorghum tended to enhance gregarious 
traits while Dipterygium accentuated solitarious 
characteristics. This was also apparent on the progenies 
of parents fed on these plants. Mishra and Singh (1992), 
Langewald and Schmutterer (1992), Schmutterer et 
al. (1993), and Doumbia (1994) found that feeding on, or 
topical application of, neem (Azadirachta indica L.)
32
extracts to nymphal and adult locusts tended to reverse 
their phase from gregarious to solitarious.
2.3 Methods of characterizing phase in locusts
The phase status of S. gregaria and L. migratoria 
has commonly been characterized on the basis of 
morphometric ratios (Uvarov, 1921; 1923), behaviour 
(Ellis, 1962), body colour (Stower, 1959), physiological 
characters such as eye colour (Stower, 1959), eye stripes 
(Uvarov, 1966), and fecundity as expressed by the number 
of ovarioles, egapods, and eggs per pod (Norris, 1950; 
Albrecht et al., 1953; Hunter-Jones, 1953; Papillon,
1960; Injeyan and Tobe, 1981a), haemolymph pigment 
composition (Mahamat et al., 1996), and pheromone 
emission (Torto et al., 1994). These phase 
charateristics are discussed below.
2.3.1 Morphometries
Morphometric measurements were one of the first 
methods used to separate phases (Uvarov, 1921) . It was 
established that Locusta danicus Linnaeus was the same as 
L. m. migratorioides, and this was the basis of Uvarov's 
phase theory. Uvarov (1923) used the morphometric E/F 
ratio {ratio of the length of the elytron (E) to that of
33
the hind femur (F) }, to separate the solitarious and the
gregarious phases of S. gregaria. Dirsh (1953) used uhe 
F/C ratio { ratio of the length of the hind femur (F), to 
the width of the head capsule (C) }, and found that it
was a more reliable phase parameter than the E/F ratio. 
The E/F and F/C ratios of S. gregaria and L. migratoria 
are summarized in Table 1.
34
Table 1. E/F and F/C ratios of S. gregaria and 
L. migratoria ( Meinzingen, 1993)
Locust
species
Phase E/F
males
F/C
males
E/F
Females
F/C
Females
S .gregaria solitaria <2 . 075 >3.75 <2.075 >3. 85
S.gregaria gregaria <2 .23 <3.15 <2 . 27 <3.15
L.migratoria solitaria <1 .83 3. 67 <1.83 3.46
L.migratoria gregaria <2 . 0 2 .96 <2 . 09 2.86
A method by which locust phase could be determined 
using both E/F and F/C ratios in a chart was proposed by 
Rungs (1954) and later modified by Duranton and Lecoq 
(1990) for more practical use in the field. It allows 
quick phase determination in a locust population. The 
proportion of any locust population of interest which 
undergoes phase shifts in the test population could be 
quickly determined. However, Chapman (1979) and Deng et 
al.(1996) suggested that morphometric parameters should
be considered with caution for phase description since 
they change slowly over several generations.
35
2.3.2 Body colour
Body colour patterns can be valuable for monitoring 
phase shift from the solitary to the gregarious phase, 
and vice-versa. Nymphal stages of the solitarious desert 
locust are often characterized by green straw colour and 
adult stage by greyish brown (Pener, 1991). In the 
gregarious phase, nymphs have a black pattern on a yellow 
background (Nickerson, 1956; Stower, 1959; Pener, 1991) 
while adults are pinkish when immature and bright yellow 
when mature (Norris, 1952, 1954). Stower (1959) and 
Duranton and Lecoq (1990) classified colour stages in 
nymphal desert locusts from 0-5, based on the extent of 
the black emaculation on the head and other body parts.
A grade of 0 corresponded to no black markings on either 
the head, hind femora, pronotum, or the abdominal 
tergites, while grade five corresponded to full black 
markings on these body parts. Norris (1954) used a 
colour classification scheme made of five grades for 
adult S. gregaria. Grade I corresponded to pink immature 
adults, with no flush of yellow at all on abdominal 
tergites and hindwings, grade III denoted adults with a 
flush of yellow on abdominal tergites, wings, and thorax,
and grade V corresponded to full bright yellow colour in 
fully mature adults. This classification was modified 
slightly by Loher (1960) into four stages to monitor the 
acceleration of maturation in the desert locust.
In L. migratoria, the solitarious adult locusts are 
green or brown while gregarious nymphs are black with an 
orange ventrum (Faure, 1942; Gunn and Hunter-Jones 1952; 
Hunter-Jones, 1958). Hunter-Jones (1958) described six 
colour types in hatchlings of L. migratoria : type 1 
corresponded to pale brown pattern on a yellow brown 
ground colour, type 3 a medium to dark brown covering on 
one third of the body surface, and type 6 to black 
hatchlings with little to no apparent ground colour. No 
such classification has been proposed for adults.
The number of eye stripes in S. gregaria and N. 
septemfasciata has been correlated with the number of 
hopper instars in crowded and isolated hoppers (Uvarov, 
1966). In S. gregaria, solitarious hoppers have seven 
stripes corresponding to six nymphal instars whereas the 
gregarious hoppers have six showing nymphal development
through five instars (Uvarov, 1966) . Stower (1959) and 
Duranton and Lecoq (1990) showed that eye colouration 
could also be a phase marker in the desert locust. They
2.3.3 Eye stripes and shading
found that while solitarious locusts had clear eyes with 
all the stripes visible, gregarious ones tended to have 
totally darkened eyes. Based on this, they graded the 
eye colouration from 0-5 as follows: 0, no shading of the 
eye; 1, one third of the eye shaded black; 2, one half 
shaded black; 3, one half shaded with additional black 
spots on the other half; 4, two thirds shaded with 
additional spots; and 5, eyes fully coloured black.
2.3.4 Haemolymph pigments
Mahamat et al.(1996) studied the presence of the 
blue biliverdin pigments in the haemolymph of S. 
gregaria. They showed that these pigments are associated 
with all stages of and ages of the solitary-reared 
(solitarious) insects, irrespective of their origin or 
diet, but absent or present in very small amounts in the 
crowd-reared (gregarious) ones. The ratio of absorbance 
at 460 nm (max for the carotenoids) and 680 run (max for 
the biliverdin pigments) in a UV spectrophotometer was 
found to be a convenient and reliable phase marker. Such 
ratio falls in the range of 3.99 - 4.78 in the gregarious 
and from 0.64 - 1.67 in the solitarious locusts (Mahamat 
et al., 1996).
37
38
2.3.5 Pheromones
The use of aggregation pheromones as a phase marker 
was recently demonstrated by Torto et al.(1994) and 
confirmed by Deng et al. (1996) by comparing pheromonal 
emissions of gregarious and solitarious adult desert 
locusts. Only adult mature male gregarious locusts were 
able to produce aggregation pheromone (Torto et al.,
1994; Obeng-Ofori et al., 1994a). Phenylacetonitrile is 
the major and key compound in the pheromone system of 
gregarious adult S. gregaria. Deng et al.(1996) showed 
that pheromone titres (as measured by the amounts of 
phenylacetonitrile) represent a more sensitive measure 
than morphometries of the onset of phase change in S. 
gregaria.
2.4 Developmental life cycle
The developmental cycle of both L. migratoria and S. 
gregaria comprises three stages: egg, nymph, and adult 
(Steedman, 1988; Meinzingen, 1993) (Fig. 2). Eggs are 
deposited by mature females in the soil where the whole 
embryonic development occurs. At the completion of the 
embryonic development, the eggs hatch and neonates crawl
39
Immatura
ADULT
Figure 2. Life cycle of gregarious desert locust and 
migratory locust (Steedman, 1988) (solitary 
S. gregaria and sometimes L. migratoria have 
a sixth-instar)
40
their way up to the ground surface. Nymphs develop 
through five to six instars depending on their phase and 
species before fledging into adults (Uvarov, 1966; 
Steedman, 1988; Meinzingen, 1993). The adult develops 
through two stages, a sexually immature stage 
corresponding to pink coloured adults and which do not 
copulate nor oviposit (Norris, 1954), and a mature stage 
corresponding to yellow coloured and sexually mature 
locusts which copulate and oviposit (Popov, 1954; Norris, 
1954). This developmental life cycle is reviewed in more
Eggs are laid in the soil where, under favourable 
conditions of soil humidity and temperature, they take, 
on the average, 15 days to incubate. Their development 
is affected by environmental conditions. For example, 
Ackowor and Vajime (1995) found that in the lake Chad 
breeding area of L. migratoria migratorioides, egg 
development is closely related to season, soil structure, 
and natural enemy complex in the area. The fastest egg 
development was recorded during the main rainy season in 
sandy soil at temperatures of around 29°C with an 
incubation period of 13.6 days while the slowest was 
recorded during the Harmattan (dry) season at
2.4.1 Egg development
detail in the following sections. I
temperatures ranging from 19°C to 26°C. Incubation time 
was 24 days in clayey soils.
No distinct diapause has been observed in the egg 
development in both S. gregaria and L. m. migratorioides, 
even though Meinzingen (1993), and Akowor and Vajime 
(1995) showed that in the field, some slowing or 
temporary arrest of the egg development is possible in 
conditions of low soil humidity and temperature 
conditions. In L. migratoria, the incubation period 
ranges between 20 - 40 days in cold conditions, but only 
10 - 20 days in warm conditions (Price and Brown, 1990; 
Meinzingen, 1993) .
The longer incubation time is a quiescent period 
(quiescens) and development and hatching proceed normally 
as soon as conditions become favourable.
2.4.2 Nymphal development
Nymphal developmental time varies from 24 - 57 
days depending on such conditions as temperature and 
humidity, and on locust phase. Under conditions of 
optimum temperature and humidity, gregarious locusts 
develop faster than solitarious ones (Duranton and Lecoq, 
1990). In S. gregaria, nymphal development goes through 
five instars over a period of 25 days in the gregarious 
phase, and through six nymphal instars in a period of 30
41
days in the solitarious phase. Under adverse conditions, 
development may take 50 days in the gregarious phase and 
up to 90 days in the solitarious phase (Duranton and 
Lecoq, 1990). In L. migratoria, nymphal development goes 
through five instars regardless of the phase, over a 
period of 24 to 35 days. However, under adverse 
conditions, solitarious individuals may go through six or 
seven instars and development could last as long as 60 
days (Meinzingen, 1993) .
2.4.3 Adult sexual maturation
At the end of the last instar, nymphs fledge into 
sexually immature adults. When water and food are 
adequate, the immature adults of S. gregaria become 
sexually mature, copulate and oviposit in about 18 to 30 
days (Steedman, 1988; Duranton and Lecoq, 1990) . A 
mature adult female can lay 2 to 3 eggpods containing 30- 
70 eggs each when gregarious, and more than 3 eggpods 
containing 55-140 eggs each, when solitary (Steedman, 
1988; Duranton et Lecoq, 1990). In L. migratoria, this 
number varies from 55-140 with an average of 67 eggs/pod 
(Steedman, 1988; Meinzingen, 1993) in the solitarious 
phase, and an average of 39.4 eggs/pod in the gregarious 
phase (Meinzingen, 1993). The time from egg to first 
oviposition in s. gregaria can vary from 50 to 332 days,
42
depending on soil moisture, relative humidity and 
temperature conditions, and locust phase status (Duranton 
and Lecoq, 1990). In both species adult longevity varies 
from 75 to 150 days.
On the average, S. gregaria may go through 3 
generations (1—5 generations) a year (Steedman, 1988; 
Meinzingen, 1993) depending on how fast the immature 
adults find suitable breeding conditions. Also, L. 
migratoria can produce up to five generations per year in 
the main outbreak area, but only two elsewhere in Africa 
(Meinzingen, 1993).
Adult sexual maturation is affected by such factors 
as rainfall, humidity, temperature (Norris, 1954; 1957; 
Steedman, 1988), chemical stimuli (Norris, 1954, 1968; 
Loher, 1960; Mahamat et al., 1993; Assad, 1995), host and 
nonhost plants (Jackson, 1978; Assad, 1995).
2.4.4 Rainfall, humidity, and temperature complex
When rainfall, humidity, and temperature are not 
adequate, immature adult locusts undergo arrested sexual 
maturation known as quiescence or imaginal diapause 
(Meinzingen, 1993). In S. gregaria, this state can last 
as long as seven months (Duranton and Lecoq, 1990) . When 
conditions are favourable, sexual maturation proceeds. 
Rainfall leads to growth and germination of nutritious
43
vegetation that supports rapid development (Carlisle and 
Ellis, 1965) and sexual maturation in immature adults.
This leads to mating and egg laying both in solitary and 
gregarious locusts (Steedman, 1988; Meinzingen, 1993) . 
Duranton and Lecoq (1990) and Meinzingen (1993) reported 
that at low temperatures (17CC and below), locusts tend to 
slow their maturation even if rainfall was adequate. 
Maturation is equally slowed down at high temperatures 
(above 30=C) in the absence of rainfall, but accelerated 
at temperatures between 27-30:C and adequate rainfall 20- 
lOCrr.m (Steedman, 1988; Duranton and Lecoq, 1990) .
2.4.5 Chemical stimuli
Pheromones play an important role in locust 
maturation and is particularly effective in inducing 
yellowing in males and females, testicular and ovarial 
development in male and female maturing locusts, 
respectively (Loher, 1960). Norris (1964) studied the 
accelerating and inhibiting effects of crowding on sexual 
maturation in two locust species, L. m. migratorioides 
and S. gregaria, and found that crowding accelerated 
maturation in the latter, but delayed it in the former. 
She further showed that young adults exerted inhibiting 
effects whereas older adults exerted accelerating effects 
on maturing adults. She concluded that the depression of
44
activity is a response to some non-mechanical, non­
visual, and presumably, a chemical stimulus which 
accelerated maturation in males and stimulated ovarial 
development and readiness to copulate in females of S. 
gregaria.
When immature adults of the desert locust, S. 
gregaria, were exposed to themselves, they tended to 
copulate after 28 days, however when they were exposed to 
mature males most of them copulated after 17 days (Loher, 
1990). In the migratory locust, L. migratoria 
migratorioides the acceleration was from 17-25 days to 
13-14 days (Loher, 1990). In the absence of mature adult 
locusts maturation is delayed in S, gregaria (up to 28 
days), but in L. migratoria, it is accelerated to only 7 
days after final molt (Loher, 1990). Norris (1964) 
observed that young adults of L. migratoria in crowded 
conditions tended to inhibit each other from maturing 
over a certain period. The nature of the inhibition was 
not identified. In S. gregaria, it was the opposite, 
isolation preventing pheromone production (Njagi et al., 
1996; Deng et al., 1996). In a recent study, Mahamat et 
al.(1993) applied the colour grading scale of Norris 
(1954) and pheromone release in gregarious adults 
described by Torto et al. ( 1994 ) to study maturation- 
acceleration in immature S. gregaria. They observed that 
the compounds identified by Torto et al.(1994),
45
especially phenylacetonitrile, were the ones responsible 
for hastening maturation of immature males and females. 
Assad et al. (1997) investigated the effects of the 
nymphal stage of 5. gregaria on the maturation of 
immature adults. Sexual maturation was significantly 
delayed in adults that were exposed to volatiles of 
fifth-instar nymphs. They further showed that live male 
and female nymphs placed in an upper compartment of two 
chamber cages were equally effective in inducing this 
delay. However, their faeces were ineffective, 
suggesting the mediation of a volatile signal from the 
nymphs themselves. Nymphal volatiles containing Z-, C;-C:; 
aldehydes and acids, phenol, and guaiacol which are 
nymphal aggregants (Torto et al., 1996), also act as 
maturation retardants for young adults of S. gregaria 
(Assad et al., 1997).
2.4.6 Interactions between species
Norris (1964) studied sexual maturation in S. 
gregaria and L. migratoria and reported that mature adult 
S. gregaria accelerated sexual maturation of young adults 
of L. migratoria. It was not clear whether mature adults 
of L. migratoria had the same effects on immature adults 
of S. gregaria.
46
2.4.7 Host and nonhost plants
Carlisle and Ellis (1967) studied the 
synchronisation of sexual maturation in desert locust 
swarms and found that locusts fledging at the beginning 
of the dry season and feeding on senescent vegetation 
failed to mature. This was attributed to the low 
concentrations of giberrellin and the monoterpenoid 
eugenol in their diet confirming their earlier findings 
(Ellis and Carlisle, 1965) . Maturation occurred in young 
adults which fed on green leaves containing these 
substances at the onset of the rainy season. Sexual 
maturation was also affected by odours of essential oils 
from certain desert shrubs (Carlisle and Ellis, 1967).
Jackson et al.(1978) showed that different food 
plants variably affected many aspects of locust growth 
and development. For example, Pennisetum, Diptarygium, 
Tribulus, and Chrozophora supported rapid growth and 
synchronized moulting. On the other hand, hoppers reared 
partly on a pure diet of sorghum completed their nymphal 
development, but many were retarded with poor moulting 
synchronization and low body weight. Sexual maturation 
was adversely affected.
Price and Brown (1992) found that migratory locusts 
had a high reproductive performance in maize monoculture 
during summer and in wheat monoculture during spring
47
seasons. They suggested that green food was essential 
for continuous reproduction, and that drying out of food 
plants could stop reproduction even at the oviposition 
stage, forcing locusts to enter into reproductive 
quiescence or delayed maturation.
Effects of a desert plant of the genus Commiphora on 
the sexual maturation of the immature males and females 
of the desert locust were investigated by Assad et 
al. {1997) . They found that extracts (containing 55 
detectable compounds mainly terpenoids) from this plant, 
when collected before winter rains accelerated sexual 
maturation of immature locusts. However, when collected 
after the winter rains no such acceleration was observed. 
Thus, volatiles from the host plant affect sexual 
maturation in locusts, and the effects vary with seasons 
(Carlisle and Ellis, 1967; Assad et al., 1997).
2.5 Methods of characterizing sexual maturation
2.5.1 Integumental colour
Maturing adults of S. gregaria become progressively 
yellow according to Norris (1954) and Loher (1960) colour 
classifications (Section 2.3.2 of this thesis). Such a 
scheme has not been applied in L. migratoria even though
48
in both locusts, yellowing is a sign of sexual maturation 
(Norris, 1950; 1964; Loher, 1990; Whitman, 1990).
49
2.5.2 Copulation
Norris (1950; 1954) found that mature males of S. 
gregaria, and L. m. migratorioides copulated as soon as 
they became sexually mature. This was applied later by 
Mahamat et al. (1993), and Assad (1995) and Assad et 
al. (1997) to study maturation acceleration and 
retardation, respectively, in the desert locust, S. 
gregaria.
2.5.3 Pheromone emission
Emission of the aggregation pheromone was used by 
Mahamat et al. (1993) and Assad (1995) to study sexual 
maturation of the desert locust, S. gregaria. They 
showed that, pheromone emission is also a sensitive 
indicator of sexual maturation since maximum pheromone 
production in males coincides with age at full maturity.
2.5.4 Length of basal oocytes and oviposition
The length of basal oocyte was used as an indicator 
of sexual maturity in studying the rate of maturation of
immature adult females of S. gregaria, since this indicates 
the rate of ovulation (Norris, 1954). Norris (1954) showed 
that oocyte length between 6 and 8mm corresponded to sexual 
maturity in females. Descamps and Wintrebert (1961) defined 
four maturity stages in female migratory locusts, L. m. 
migratorioides. Locusts in stage III maturity or 
reproductive stage had oocyte lengths of 3-7 mm. Highnam and 
Haskell (1964) studied the relationship between the endocrine 
system, flying activity and oocyte lengths in both S. 
gregaria and L. m. migratorioides. They showed that in S. 
gregaria, the oocyte lengths varied from 0.7 mm in fledglings 
to 8.0 mm in fully mature adults. In L . m. migratorioides 
the lengths varied from 0.7 mm in fledglings to 6.5 mm in 
fully mature adults. They further showed that the variations 
in oocyte lengths were also influenced by the locust phase as 
in isolated S. gregaria, and crowded L. m. migratorioides, 
oocyte growth was slow, but in crowded S. gregaria and 
isolated L. m. migratorioides, oocyte growth was faster.
There was a strong correlation between the variations in the 
lengths of oocytes and the size of the corpora cardiaca, on 
one hand, and the amount of neurosecretory material released 
by these glands in the pars intercerebralis of the brain, on 
the other (Highnam and Haskell, 1964).
50
The time to first oviposition is also a useful 
parameter in determining sexual maturation of the desert 
locust (Norris, 1950; 1954; Mahamat et al., 1993).
51
CHAPTER THREE
3 General materials and methods
3.1 Insects
Crowded desert locusts, S. gregaria were obtained 
from a colony maintained at the ICIPE. The colony was 
propagated (for seventeen generations by the beginning of 
this work), from an original stock which was obtained 
from the Desert Locust Control Organisation for East 
Africa (DLCO-EA) in Addis Ababa, Ethiopia (Ochieng-Odero 
et al., 1991). Crowded migratory locusts were obtained 
from a stock maintained (for 16 to 17 generations at the 
beginning of the experiments) at the University of 
Nairobi, Kenya, and propagated for the experiments.
To keep the colony gregarious, locusts were reared 
crowded (100-200 nymphs or 60-100 adults) in aluminium 
cages of 50 x 50 x 50 cm (Fig. 3A; Plate H) whereas for a 
solitarious stock, locusts were reared individually in 10 
x 10 x 15 cm aluminium cages (Fig.3 B)(Ochieng-Odero et 
al., 1991). The rearing room (dimension: 2.5 x 2 m) was 
maintained at 29 ± 2°C., 58 ± 1 % RH, and 12:12 H 
light:dark period. The room was aerated by a duct system 
which maintained a negative pressure that facilitated a
52
53
Electric cab le
Wire m esn  
G la ss  w m ccw  N^iCer 
C a g e  ho icer
- ■- — Cage comcanmem 
aise Hoor seal
2c.*n
{Observation) 
^ewing/feeding !
window |
' L  5cm
OviDosid'on 
tube holder
Figure 3. Diagrams of standard cages for rearing crowded 
(A) and isolated locusts (B)
flow of fresh air at 10-15 air changes per hour (Ochieng-
Odero et al., 1991).
Fresh shoots of wheat (variety 'Nyangumi') and wheat 
bran obtained from Nakuru (Kenya) were provided daily to 
the colony.
3.2 Bioassays
3.2.1 Single chamber olfactometer
The olfactometer (Fig. 4) was a 60 x 30 x 30 cm 
single glass chamber with a two choice arena whose top 
was covered with a removable wire gauze (Obeng-Ofori et 
al., 1993). An aluminium metal plate drilled with 2 
mm-diameter holes (1cm apart) was fitted at the bottom of 
the chamber. Attached to each half of the arena and from 
underneath was a 28 cm (base length) pyramidal aluminium 
funnel. Each of the two funnels was connected by teflon 
tubing to a 2-litre round bottomed flask. Insects were 
placed in one of the flasks to serve as the source of 
volatiles; the other flask was left empty to act as a 
control. The olfactometer was placed under an extraction 
hood which maintained a continuous flow of air through 
the chamber, and prevented accumulation of volatiles.
Two 60 cm fluorescent light tubes, each 60 W, were fitted
54
55
Figure 4. Diagram of the olfactometric aggregation 
bioassay system
over the olfactometer arena to provide uniform 
illumination.
Temperature in the experimental room was maintained 
at 26-29°C.
3.2.2 Cage assays
Bioassays to investigate the gregarization of 
solitarious desert locusts in the presence of gregarious 
colony of migratory locusts were conducted in standard 50 
x 50 x 50 cm aluminium cages (Ochieng-Odero, 1991) which 
were easy to clean and had no risk of adsorbing volatiles 
(Fig. 3A) . The front side of the cage had a sliding 
plexiglass door to allow observations on the locusts.
One of the lateral sides had an opening with a sliding 
door to enable placement of food. The floor had a 
wiremesh (5 mm mesh) which allowed the faeces to drop out 
of the cages. The large cages had an outlet in the back 
of their ceiling for a 40 W bulb to provide light and 
heat to the insects inside.
For bioassays where only the effects of volatiles on 
phase shift or maturation were tested, double storey 
aluminium cages (15 x 15 x 30 cm), described by Mahamat 
et al. (1993) were used (Plate A). Wire gauze partition
56
57
A. Two chamber bioassay cages, a) source chamber, 
b) recipient (test) chamber.
between the two chambers allowed flow of volatiles from 
the top chamber to the lower one.
58
3.3 Collection of volatiles
Volatiles were adsorbed onto charcoal traps 
employing the method described by Torto et al.(1994)
(Plate B-l). The traps were 8mm-diameter and 8cm long 
glass tubes inside which, 60 mg of activated charcoal (80 
- 100 mesh Chrompack, Middlesburg, The Netherlands) was 
packed between two glass wool plugs (Plate B-2d) . Before 
use, the charcoal was cleaned thoroughly by Soxhlet 
extraction with dichloromethane (Merck, Germany) for 4 8 
hours followed by activation under nitrogen (250 ml/min) 
at 250:C for one hour. Air from a compressed air cylinder 
was cleaned by passing it through a charcoal filter 
before passing it over locusts contained in 0.5, 1.0, or 
2 1 round-bottomed flask (Plate B-la) depending on the 
number of locusts. When trapping was from smaller 
numbers of locusts ( 1 - 2  adults or 30 - 50 early instar 
nymphs), quick-fit glass tubes (male 2.8 cm ID, female
3.4 cm ID, and length 9 cm) (Plate B-lb; B-2c) were used. 
All joints were sealed with teflon tape to avoid air 
leakage during trapping sessions. Each trapping session 
lasted overnight (16 hours) after which, traps were
B
59
1
2
B. Volatile collection system (B-l) showing
connected to it a) three-neck round bottomed 
flask containing mature S. gregaria b) quick- 
fit trapping chambers, and the medical air 
source cylinders; quickfit chambers (c) with 
connected charcoal traps (d) (B-2)
eluted into a vial placed in ice with 5 ml HPLC grade 
(Aldrich Ltd, UK) dichloromethane. Volatile extracts 
were stored at -15°C, and concentrated under a stream of 
nitrogen at 0°C to approximately 300 ^1 prior to analysis.
3.4 Analyses of volatiles
Analyses of volatiles were carried out by capillary 
gas chromatography (GC) on a Hewlett-Packard (HP) 5390 
Series II gas chromatograph equipped with a flame 
ionization detector (FID) and a HP capillary column 
methyl silicone SPB-1 (30m x 0.2 mm ID x 0.2 /^ m film 
thickness), or Carbowax (50m x 0.2 mm ID x 0.2 ^m film 
thickness). When the Carbowax column was used, the 
analyses were run at initial temperature of 60-C 
isothermal for 10 min, then programmed to 180"C at a rate 
of 5'C/min and maintained there for 5 min, then to 220:C 
at a rate of 10DC/min and maintained isothermal for 15 
min. When the methyl silicone column was used, the 
analyses were run at an initial temperature of 45°C for 5 
min, then programmed to 150°C at the rate 5°C/min, then to 
250°C at a rate of 10°C/ min and maintained there for 13 
min. Chromatographic peaks were integrated using a HP 
3396 Series II integrator. Samples from L. migratoria 
were analyzed by GC-MS on a VG 12-250 mass spectrometer 
(El, 70 eV) coupled to a HP5790 gas chromatograph. The
60
GC conditions were the same as for analyses using 
Carbowax.
3.5 Integumental colour grading
Test locusts, S. gregaria were monitored for visible
colour changes of the cuticle as an indication of
gregarization (Plate C-E) or sexual maturity, in
accordance with the colour classification of Norris
(1954) for adult locusts as follows :
I : insect totally deep pink ;
II : insect clear pink;
III : insect with slight pink and yellow on the abdomen,
thorax, hindwings, and frons. This is obvious in
male Schistocerca and Locusta (frons);
IV : Yellow colour generalized over body and hindwings;
V : Insect totally yellow even females showing 
frons and hindwings yellow;
Pink immature adults (Plate C—1) or stage III yellow 
mature adults (Plate D-l) were considered to have reached 
the transient or gregarious phase.
Immature gregarious adult L. m. migratorioides 
(Plate C-2), also show some faint pink colour but usually 
pale to creamy white frons. The hind wings are hyaline 
at soft immature adult stage but become yellow after
61
C. Immature adults of S. gregaria{C-l) and L. 
migratoria(C-2) (note the pink and blackish 
body colours of the respective insects)... 62
D. Gregarious (D-l) and solitarious (D-2) mature 
adults of S. gregaria.
Gregarious (E-l) and solitarious (E-2) adults 
of L. migratoria migratorioides
65
1
F. Gregarious fifth-instar (F-l) and solitarious fourth- 
instar (F-2) nymphs of L. migratoria 
migratorioides.
cuticular hardening. The frons turns progressively 
yellow, especially in males, followed by the pronotum and 
legs as the locust matures. Solitarious adults (Plate E- 
2) have a green pronotum head and legs while the hind 
wings' hyaline colour turns yellow as the insect matures. 
The front wings are golden greyish. Mature solitarious 
males although green in general appearance can show 
yellow frons at maturity. Thus, in the adult migratory 
locust the yellow colouration on frons was a good 
indicator of sexual maturity. The colour grading was 
limited to the extent of the yellow colouration on frons 
of both gregarious and solitarious adult males, and the 
green colouration on the pronotum and legs for both male 
and female solitarious adults. The following scale was 
adopted:
Grade 1 : No yellow apparent, frons pale with the brown 
colour fading out. Hind femora with no yellow;
Grade 2 : Frons are pale with some faint yellow, less 
than 20% appearing at the base of the frons. 
Hind femora not yellow;
Grade 3 : About 50% of frons yellow with some yellow 
appearing on the pronotum and hind femora;
Grade 4 : More than 50 % of frons yellow and hind femora 
with more pronounced yellow;
Grade 5 : Almost whole frons yellow, hind legs and 
pronotum with more pronounced yellow 
colouration.
66
Grade 3 colouration corresponded to sexual 
maturation of the locusts as males stridulated,
copulated, and produced pheromone whereas adult females 
started ovipositing,
3.6 Statistical analyses
Statistical analyses were carried out by SAS General 
Linear Model Procedure (GLM) in which simple or multiple 
analysis of variance (ANOVA, MANOVA) were run, with the 
Least Significant Difference test (LSD test at P < 0.05) 
for mean comparisons (SAS, 1988). Counts in the 
olfactometer, morphometric ratios, and haemolymph 
absorbance ratios, were transformed into arcsine, square 
roots, log(x), or log(x+l), prior to analysis.
67
CHAPTER FOUR
68
4 Aggregation responses of S. gregaria and L.
migratoria migratorioides to their airborne 
volatiles.
4.1 Introduction
Fuzeau-Braesh et al. (1988) identified guaiacol, 
phenol, and veratrole as three of the four major volatile 
constituents in the air surrounding fifth-instar nymphs 
and older adults of S. gregaria, and L. m. 
migratorioides. However, they did not study the 
responses of the species to the crude volatile blend of 
the other. A more detailed study on the aggregation 
responses of different stages of S. gregaria has been 
recently carried out by Obeng-Ofori et al. (1993). They
found that, aggregation in the desert locust S. gregaria 
is mediated by two sets of releaser pheromones: a 
juvenile aggregation pheromone produced by nymphal stages 
to which only nymphs respond, and an adult aggregation 
pheromone which is specific to adult stages only. 
Aggregation responses in L. m. migratorioides are unknown 
and neither are cross-aggregation responses between the 
two locust species. In this chapter, comparative 
responses of the two locust species to their own and to 
each other's volatiles are described. In L. m.
migratorioides, it was further investigated whether there 
are stage and sex differences as reported for S. gregaria 
(Obeng-Ofori et al., 1994a)
4.2 Materials and Methods
4.2.1 Olfactometer assays
Aggregation responses of 5. gregaria and L. m. 
migratorioides to air-borne volatiles were conducted in 
olfactometric bioassays using the single chamber 
olfactometer (described in Chapter III, page 51 of this 
thesis), which eliminates visual and contact stimuli 
between source and test locusts.
Air from a compressed air cylinder was cleaned by 
passing it through a charcoal filter, then split into two 
streams, each passing into a round-bottomed flask, and 
then into either of the sides of the arena at a flow rate 
of 120 ml/rain. Locusts placed in one of the flasks 
provided volatiles for the assays. Sets of five, each of 
mixed or separate sexes of 3-4 day old fifth-instar 
nymphs, immature, and 25-40 days old mature adults of L. 
m. migratorioides and S. gregaria were tested against 
their own, and to each other's volatiles at five doses 3, 
7, 10, 20, and 40 Locust equivalents (LEQ). One LEQ 
represents the emission of 1 locust from 2L flask for 10
69
rain. The standard dose in this work was set at 10 LEQ, 
and the dose responses were studied in the range from 0 
to 10, and from 10 to 50. Thus in the lower range, doses 
7 and 3 {decrease by 3 units) were tested while in the 
upper range, doses 20 and 40 (increase by a factor of 2) 
were so. Test insects were introduced into the 
olfactometer via the entry box, and after 10 minutes, the 
number of insects on each side of the arena was recorded 
(Plate G). The aggregation index (Al) for each test was 
calculated using the equation:
Al = 100 ( T - C ) / N ; where 
T= number of locusts found on the treated side, C= number 
of locusts found on the control side, and N= total number 
of locusts tested.
Experiments involved different stages of S. gregaria 
and L. m. migratorioides (L. migratoria) as mixed (M) and 
separate (S) sexes and the controls (*) in the following 
combinations:
70
71
EXPT SOURCE RECIPIENT
(SEX)
Fifth-instar Fi fth-instar
1 L. migratoria L. migratoria (M, S)*
2 S. gregaria S. gregaria (M) *
3 L. migratoria S. gregaria (M)
4 S. gregaria L. migratoria (M)
Mature adults Mature adults
5 L. migratoria L. migratoria (M, S)*
6 S. gregaria S. gregaria (M) *
7 L. migratoria S. gregaria (M)
8 S. gregaria L. migratoria (M)
Fifth-instar Immature adult
9 L. migratoria L. migratoria (M) *
10 S. gregaria S. gregaria (M) *
11 L. migratoria S. gregaria (M)
12 S. gregaria L. migratoria (M)
Mature adults Immature adults
13 L. migratoria L. migratoria (M) -
14 S. gregaria 5. gregaria (M) '
15 L. migratoria S. gregaria (M)
16 S. gregaria L. migratoria (M)
Between experiments, the olfactometer was carefully 
cleaned with acetone. Air was also flushed through the 
tubes for 2-3 hours to remove any volatile residues. The 
source and the control flasks were systematically 
switched in between experiments to minimize bias.
Combinations 1 to 8 were also repeated and 
distribution of locusts to their volatiles after 10 and 
30 min of exposure at the standard dose of 10 LEQ was 
noted.
Additional tests were conducted with L. migratoria for 
stage and sex-specific responses to volatiles. These 
involved combinations 1, 5, 9, and 13 in the treatment 
combination list. Cross responses to eight to ten grams 
of 80% wet faeces of nymphs and immature adults of the 
respective locust species were also studied.
4.3 Data analysis
The data were analyzed using the SAS (1988) package. 
Tests for independence were run using the Chi-square 
test. For separation of means by treatment, the data 
were transformed into arcsine or square roots prior to 
analysis of variance using LSD test at P < 0.05.
72
73
G. olfactometric tests showing locusts (1,2,..) 
orienting themselves towards or sitting on 
the treated side of the arena
74
4.4 Results
4.4.1 Responses of S. gregaria to volatiles of
L. migratoria migratorioides
Aggregation reponses of fifth-instar S. gregaria 
nymphs to their own volatiles and to those of fifth- 
instar L. migratoria nymphs are shown in Fig. 5.
Responses were dose-dependent and fifth-instar S. 
gregaria nymphs responded maximally to volatiles produced 
by S. gregaria and L. migratoria at 10 and 20 LEQ, 
respectively. Fifth-instar S. gregaria nymphs had 
similar aggregation responses to their own volatiles and 
to those produced by fifth-instar L. migratoria nymphs. 
They also responded to faecal volatiles of fifth-instar 
nymphs of the latter species (Table 2).
Immature adults of the two species had significant 
aggregation responses to conspecific volatiles. S. 
gregaria immature adults were indifferent to volatiles 
from nymphs of L. migratoria while immature adults of L. 
migratoria were actually repelled by volatiles from 
conspecific nymphs and responded weakly and 
insignificantly to those of nymphal S. gregaria. On the 
other hand, immature adults of both species responded 
equally to volatiles produced by mature adults of the two 
species, but significantly more to those from their
Ag
gr
eg
at
io
n 
In
de
x 
(%
)
75
Doses (LEQ )
Figure 5. Dose-aggregation response curves for 
fifth-instar nymphs of S. gregaria 
(FSG) to volatiles of fifth-instar 
conspecifics[(FSG/FSG)] and to those of 
L. migratoria(FLM),[(FSG/FLM)] in the 
olfactometer (bars are SE).
76
Table 2. Aggregation responses of immature adults of 
S. gregaria (SG) and L. m. migratorioides 
(LM) to nymphal and adult conspecific and 
interspecific volatiles in the olfactometer.
Source Test Aggregation Index 
(% ± SE)
Fifth-instar Immature adult
LM LM -34 ± 10 c
SG LM 16 ± 8 a
LM SG -2 ± 8 b
SG SG 18 ± 8 a
Mature adult Immature adult
LM LM 60 ± 11 a
SG LM 22 ± 12 b
LM SG 2 4 + 1 1  b
SG SG 40 i 6 b
NvmDhal faeces Fifth-instar
LM SG 23 ± 8 a
SG LM 24 ± 7 a
NvmDhal faeces Immature adult
LM SG 32 ± 13 a
SG LM 32 ± 5 a
Mean indices in each group followed by the same 
letter are not significantly different(at 5% level, LSD- 
test) .
conspecifics (Table 2) . They also responded 
significantly higher than the untreated control 
(P < 0.05), to faecal volatiles of fifth-instar nymphs of 
L. migratoria similar to the responses to conspecific 
immature adult fecal volatiles (Table 2).
Responses of mature adults of S. gregaria to their 
own volatiles and those of L. migratoria were dose- 
dependent. Adults of S. gregaria aggregated strongly (P<
0.05) and significantly more to their own volatiles than 
to those produced by L. migratoria especially at doses of 
10 and 20 LEQ (Fig. 6).
4.4.2 Responses of L. migratoria to volatiles of S. 
gregaria.
The reponses of fifth-instar nymphs of L. migratoria
to their own volatiles and to those of fifth-instar
nymphs of S. gregaria are shown in Fig. 7. Responses to
volatiles of nymphal S. gregaria were not dose-dependent
and there were no significant differences in responses of
nymphal L. migratoria to their own volatiles and to those
produced by fifth-instar S. gregaria over the range of
doses tested (P <0.05) (Fig. 7) . However with regard to their own 
volatiles, nymphs of L. migratoria had significantly
higher (P <0.05) aggregation responses to volatiles from 
pecifics at 20 LEQ than at other doses. They also
77
Ag
gr
eg
at
io
n 
In
de
x 
(%
)
78
Doses (LEQ )
Figure 6. Dose-aggregation response curves for 
mature adults of S. gregaria(ASG) 
to volatiles of mature adult 
conspecifics[ (ASG/ASG)] and to 
those of L. migratoria (ALM),[(ASG/ALM)] 
in the olfactometer(bars are SE).
Ag
gr
eg
at
io
n 
In
de
x 
(%
)
79
Doses ( LEQ )
Figure 7. Dose-aggregation response curves for 
fifth-instar nymphs of L. migratoria 
(FLM) to volatiles of fifth-instar 
conspecifics[(FLM/FLM)] and to those 
of S. gregaria(FSG),[(FLM/FSG)] 
in the olfactometer(bars are SE).
responded significantly (P <-05 ) compared to untreated 
control, to faecal volatiles of fifth-instar nymphs of S.
gregaria (Table 2).
Immature adults of the two species aggregated weakly 
to volatiles of fifth-instar nymphs. They responded less 
to volatiles produced by mature adults of S. gregaria, 
compared to responses to their own volatiles. Responses 
were significantly greater within than between species. 
They also responded to faecal volatiles of fifth-instar 
nymphs of S. gregaria (Table 2).
Responses of mature adults of L . migratoria to their 
own volatiles and to those of S. gregaria were dose- 
dependent and there were no significant differences over 
the range of doses tested (Fig. 8).
4.4.3 Responses of S. gregaria or L. migratoria to 
choices of volatiles of both species
Nymphs of S. gregaria responded to volatiles of 
nymphs of L. migratoria similar to their own. Similarly, 
there were no significant differences (P > 0.05)in 
aggregation responses of nymphs of L. migratoria to 
volatiles of S. gregaria and to those of their 
conspecifics during 10 or 30 min exposure (Table 3) .
Mature adults of L. migratoria showed a 
significantly stronger response to their own volatiles
80
Agg
reg
atio
n 
Ind
ex 
(%)
81
Doses ( LEQ )
Figure 8. Dose-aggregation response curves for 
mature adults of L. migratoria (AIM) 
to volatiles of mature adult 
conspecif ics [ (ALM/ALM) ] and to 
those of S. gregaria(ASG),
[ (ALM/ASG) ] in the olfactometer 
(bars are SE) .
than to those of S. gregaria after 10 min of exposure (P 
< 0.05). However, after 30 min, mature adults of L. 
migratoria responded significantly more to volatiles of 
S. gregaria than to their own (P £ 0.05) . Mature adults 
of S. gregaria responded to volatiles of mature adults of 
L. migratoria similar to their own at the two time (10 
and 30 min) intervals. Aggregation responses to 
volatiles of S. gregaria of the two locust species after 
30 min of exposure to volatiles were not significantly 
different (P > 0.05) (Table 3) .
4.4.4 Sex responses to volatiles in L.m. 
migratorioides.
Male and female nymphs of L. migratoria 
migratorioides did not show any preference for their 
respective volatiles (Table 4) . Mature adult females 
were more strongly attracted (P< 0.05) to volatiles from 
mature males than males to those of females. However, 
mature females showed significantly (P < 0.05) lower 
aggregation responses to their own volatiles compared to 
the aggregation of males to their own, and this was not 
significantly different (P > 0.05) from the control 
(Table 4).
82
83
Table 3. Responses of S. gregaria (SG) and L. m.
migratorioides (LM) to volatiles emanating 
simultaneously from 10 locusts of each species 
placed in olfactometer choice tests.
Test (stages) Time (min)
Aggregation 
index (% ±SE) 
to LM 
volatiles
Aggregation 
index (% ±SE) 
to SG 
volatiles
Fifth-instar
LM 10 42 ± 6 a 52 ± 7 a
SG 10 42 ± 9 a 60 ± 8 a
LM 30 44 ± 9 a 50 ± 12 a
SG 30 46 ± 6 a 54 ± 6 a
Mature adult
LM 10 56 ± 8 a 32 ± 5 b
SG 10 44 ± 4 a 54 ± 4 a
LM 30 32 ± 5 b 66 ± 5 a
SG 30 44 ± 8 a 56 ± 9 a
All means in each column(3 and 4), followed by the 
same letter are not significantly different (at 5% level, 
LSD test) .
84
Table 4 Sex responses of nymphs and mature adults of 
L. m. migratorioides to volatiles in 
the olfactometer.
Source Test Aggregation Index 
(% ± SE)
Fifth-instar Fi fth-instar
Males Females 40 ± 9 a
Males Males 42 + 11 a
Females Males 42 ± 9 a
Females Females 46 ± 13 a
Mature adult Mature adult
Males Females 56 ± 6 a
Males Males 40 ± 8 b
Females Males 38 ± 9 b
Females Females 18 ± 8 c
Means in last column and by stage, followed by the 
same letter are not significantly different (at 5% level, 
LSD test) .
85
4.4.5 Intra and inter-stage aggregation responses in 
L. a.migratorioides.
Male and female fifth-instar nymphs responded highly 
to their volatiles which produced a repellent effect on 
immature adults (Table 5) . On the other hand mature 
adults were indifferent to nymphal volatiles, but 
responded highly to their own volatiles to which fifth- 
instar, young, and older adults were also responsive 
(Table 5).
86
Table 5 Cross-stage aggregation responses of L. m. 
migratorioides in the olfactometer.
Source Test Aggregation Index 
(% ± SE)
Fifth-instars
Fifth-instars
Fifth-instars
Fifth-instars 
Immature adults 
Mature adults
57 ± 8 a 
-34 ± 10 c 
6 ± 10 b
Mature adults 
Mature adults 
Mature adults
Fifth-instars 
Immature adults 
Mature adults
40 ± 10 b 
60 ± 11 ab 
7 6 ± 7 a
In each group, means followed by the same letter in 
the last column are not significantly different (at 5% 
level, LSD test).
4.5 Discussion
87
The present results show some similarity in the 
aggregation response patterns of different stages of S. 
gregaria and L.m. migratorioides. Particularly 
noteworthy is the extent of cross-reactivity between the 
nymphal and mature adults stages respectively of the two 
species. Thus, the aggregation responses of fifth-instar 
nymphs to their respective volatiles as well as to those 
of one another were remarkably similar (Figs. 5 and 7).
As in the case of S. gregaria (Obeng-Ofori et al.,
1994a), no sexual differences in the emissions of, or 
responses to, the nymphal pheromone was observed in L. 
migratoria. Nymphs, and immature adults cross-responded 
to their fecal volatiles. These results suggest the 
presence of either similar compounds or pheromone blends 
with similar releaser effects on each other's nymphs in 
the nymphal emissions of these species. This may account 
for the occurrence of mixed hopper bands of S. gregaria 
and L. m. migratorioides often observed in the field 
(Johnston and Buxton, 1949; El-Bashir and Abdel-Rahman, 
1991; Torto, pers. comm.). Similarly, mixed populations 
of immature adults and fifth-instar nymphs could be 
explained by the fact that immature adults of the two 
locust species are responsive to each other's nymphal 
faecal volatiles.
The responses of mature adults to their respective 
volatiles as well as to those of one another were also 
quite similar (Figs. 6 and 8). However, in this case, S. 
gregaria responses to volatiles of conspecifics was 
significantly higher at some doses (10 and 20 LEQ) 
compared to its responses to volatiles of mature adults 
of L. m. migratorioides (Fig. 6) suggesting some 
differences in the composition of the two pheromone 
systems or possibly in the amounts of the active 
compounds.
On the other hand, significant differences were 
observed between the two species when the aggregation 
responses of one stage of the insect to volatiles of 
another were compared. Thus, although immature adults of 
S. gregaria were indifferent or aggregated weakly to 
volatiles of conspecific fifth-instar nymphs as 
previously reported (Obeng-Ofori et al., 1993), those of 
L.m. migratorioides were significantly repelled by 
conspecific nymphs of similar stage (Table 2). 
Interestingly, immature adults of L. m. migratorioides 
were stimulated to aggregate, albeit weakly, in response 
to volatiles of fifth-instar nymphs of S. gregaria, 
whereas S. gregaria immature adults were indifferent to 
volatiles of nymphal L. m. migratorioides. These results 
clearly point toward compositional differences in the 
nymphal volatile blends of the two species. Comparison
88
of the aggregation responses of immature adults to 
volatiles of conspecific and interspecific mature adults 
also shows compositional differences in the adult 
pheromone blends (Table 2) . Thus, although immature 
adults of both species aggregated significantly to 
volatiles of conspecific mature adults, their cross 
responses were weak and statistically insignificant. 
Differences in pheromonal responses between the two 
locusts both at nymphal and adult stages may also be 
quantitative, inferring then that the two locusts have 
different treshold responses to the pheromones, and 
probably produce different amounts of the necessary 
compounds. This implies that in the ecological point of 
view these two locusts still maintain their specificity 
as two different species with distinct niches.
Cross responses between mature adults and nymphal 
stages also show differences between the two species 
(Tables 2 and 5) . Thus, whereas in S. gregaria mature 
adults and nymphs are indifferent to each other's 
volatiles, in L. m. migratorioides, although mature 
adults are also indifferent to nymphal volatiles, the 
nymphs respond positively to those of mature adults.
A major difference between the adults of the two 
locusts is that in S. gregaria males do not respond 
significantly to volatiles from females (Obeng-Ofori et 
al., 1994), while in L. m. migratorioides both males and
89
females cross responded to each other's volatiles. In 
the latter species (Table 4), females were strongly 
attracted to male volatiles, and, conversely males were 
significantly strongly attracted to female volatiles.
The attraction of females to female volatiles was very 
weak and statistically insignificant. These results 
suggest the mediation of two sets of volatile pheromones 
in mature adults of L.m. migratorioides: an aggregation 
pheromone produced by males that elicits aggregation from 
conspecifics of both sexes, and a sex pheromone that is 
produced by mature females that attracts/arrest male 
conspecifics.
In summary, the present study has demonstrated some 
similarities as well as differences in the pheromone- 
mediated aggregation behaviour of S. gregaria and L. 
migratoria migratorioides. The nymphs, as illustrated 
by the fifth stadium, were remarkably similar and indeed, 
the two species cross-responded to each other's 
pheromones; but S. gregaria showed some specificity for 
its own pheromone. Major differences between the two 
species occurred in cross- stage responses and in sex 
differentiation in the pheromone systems of mature 
adults. It is concluded that despite cross-activity 
between the aggregation pheromones of the two species 
both the nymphal and adult aggregation pheromones have
90
significantly different compositions possibly with some 
overlapping compounds. In addition, L. migratoria has a 
volatile sex-recognition signal produced by females.
91
CHAPTER FIVE
5. Primer effects of a gregarious population of one 
species on solitary individuals of the other.
5.1 Introduction
Locusts, as well as other living creatures, 
interact with their environment in general, and 
particularly with other insects sharing the same habitat. 
The behaviour and maturation of the desert locust and the 
migratory locusts have been reported to be affected by 
the presence of other locusts (Gillett, 1963; Loher,
1990) and host plants(Jackson et al., 1978; Assad, 1995). 
Nolte (1977), Ba-Angood (1976), and Gillett (1968) showed 
that when solitarious locusts were reared with certain 
grasshoppers their morphometries, colour, and eye stripes 
did not change, but when reared with other locust 
species, they acquired some gregarious characteristics.
Studies described in Chapter IV show that there are 
strong pheromone-mediated interactions between the two 
locust species. The long term (primer) effects resulting 
from interactions between the two locust species, S. 
gregaria and L. migratoria migratorioides, are the 
subject of this Chapter. Primer effects were by
more sensitive phase markers such as pheromone production
92
( Deng et al. 1996 ) and haemolymph pigment composition 
(Mahamat et al., 1996) in addition to previously known 
characters based on body colour changes and 
morphometries.
5.2 Materials and methods
This section addresses three different objectives. 
The first one is on the investigation of the effects of 
gregarious migratory locusts and two grasshopper species 
on the gregarization of solitarious desert locust in 
mixed population; the second is on the investigation of 
the effects of gregarious desert locusts on the 
gregarization of solitarious migratory locusts; and the 
third is on the effects of the volatiles of gregarious 
individuals of one species on the gregarization of 
solitarious ones of the other, with S. gregaria responses 
to volatiles of L. migratoria taken as a case study.
93
94
5.2.1 Effects of the presence of gregarious L.m.
migratorioides and two grasshopper species on the 
gregarization of solitarious S. gregaria
Insects and experiments
Preliminary rearing tests at random densities of 30, 
60, 70, 80, 110, 120, 130, 180, 200, 270, 400 nymphs and 
adults of L. migratoria migratorioides were conducted to 
determine the appropriate locust density to use as 
stimulus source on S. gregaria in subsequent assays. 
Integumental colour changes were used to monitor the 
degree of gregarization in test locusts. All locusts 
tested shifted to gregarious colour (grades 4 and 5, see 
page 59-52) at all densities tested. A standard density 
of 80 locusts was used in subsequent assays. The size of 
the cage (50 x 50 x 50 cm) allowed free movement of the 
locusts. One solitary desert locust, at nymphal or adult 
stages, was placed in the cage already described in 
Chapter III (Plate H) containing specific stages of 
either 80 two-day-old nymphs of gregarious L. migratoria, 
Phymateus viridipes Stal, Pyrgomorphidae, or 
Eyprepocnemis plorans (Charpentier), Eyprepocnemidinae. 
The latter two are grasshopper species collected as first 
instars behind the ICIPE fence.
95
H
H. One chamber bioassay cage used for mixed rearing 
experiments and described in Chapter III, and 
showing a)glass door, b)egg laying tube 
insertion holes.
96
T r e a t m e n t s  (TRT) w e r e  d e s i g n e d  a s  f o l l o w s :
RATIO 
80 : ]
TRT STIMULUS qnT.TTARIOUS TEST
1 . First-instar First-instar
L. migratoria S. gregaria
2. Two-day-old Two-day-old immature
immature adult 
L. migratoria
adult S. gregaria
3. Fifth-instar Two-day-old immature
L. migratoria adult S. gregaria
4 . First-instar First-instar
P. viridipes S. gregaria
5. First-instar First-instar
E. plorans S. gregaria
6. First-instar First-instar
5. gregaria S. gregaria
7 . First-instar First-instar
S. gregaria S. gregaria
80
80
19
10
19
19
Treatments 6 and 7 were referred to in the text as 
control gregarious or solitarious respectively. Test 
nymphal locusts were monitored for changes in 
integumental colour, and durations of instars (and stage) 
while adult locusts were monitored for changes in 
integumental colour, eye shading, morphometries (E/F and 
F/C ratios), eye colour, haemolymph pigment composition, 
and titres of the male-produced aggregation pheromone.
Integumental colour
Nymphal colour changes were recorded daily,
following Stower (1959) nymphal colour grading as follows 
(Fig. 9):
0 : no black maculation on head, abdomen and legs.
1 : a few black spots on the head; less than 1/4 of the
abdomen is black; a black line appearing in the mid 
horizontal axis of the femur.
2 : 1/3 of head including mouthparts black; less than 1/4
of the abdomen is black; black line on the femur 
conspicuous.
3 : 1/2 of head and mouthparts black; less than 1/4 of
the abdomen is black; femur over 1/3 black.
4 : 2/3 of head and mouthparts are black; 1/4 of the
abdomen is black; 1/2 femur is black.
5 : over 2/3 of head and 1/2 of abdomen and femur
black ( Plate 1-1; 1-2 ).
97
Adult colour changes from immature pink stage to 
mature yellow stage were also recorded daily until 
maturation colour stage III was attained as described in 
Chapter III.
Number of nymphal instars, nymphal and total 
developmental (to mature adult stage) times.
The nymphal developmental time in the desert locust 
was determined by monitoring the number of instars. This 
was done by recording each nymphal moult, and by counting
Head
0
Abdomen
J U S s ^ ^ *
Femur Eyes Grade
I
I
I
I
Figure 9. Black colour pattern used in grading nymphal 
colour changes in test s. gregaria
98
99
I
Gregarious fifth—instar and fourth-instar 
nymphs of s. gregaria from gregarious 
(1-1) and solitarious(1-2) control cages 
respectively.
the eye stripes of adult locusts. The duration of each 
instar was the time between consecutive moults whereas 
the nymphal developmental time was computed as the total 
of all instar durations. The total developmental time to 
end of immature stage was computed as the total nymphal 
developmental time added to the time the adult lived as 
immature.
Morphometric measurements
The length in millimeters (mm) of the elytron (E), 
the hind femur (F) , and the width of the head capsule 
(C), were measured on mature adult males and females of 
both species using an electronic Sylvac (T) caliper with 
a range of 0-150 mm and an accuracy of ± 0.03 mm. 
Morphometric changes were determined by calculating E/F 
and F/C ratios (Fig. 10).
Average morphometric ratios were calculated and 
compared to those of control locusts to determine the 
phase status of all test locusts. To determine the 
individual phase status and the proportion of transformed 
individuals of the desert locust, a standard morphometric 
chart (Duranton and Lecoq, 1990) was used (Fig. 10) .
100
cF/C
i GREGARIOUS
i
i
i i
i i 1
!o i i V
i i ! i
I 1
TRANSIENS
i 1
1 i i 1
1 J 1 1 I11 u I | 1 111 ! , .  i
' i ; iT i i i 1
i i i i
SOLITARIOUS 1
i
14 1-6 1-8 2-0 2-2 lk 2-6 2-8
E/F
Figure io. The standard morphometric chart for locust 
phase determination
Adult eye shading
102
The eye dark shading was recorded following Duranton 
and Lecoq (1990) grading scheme as follows: 0 : no 
shading of the eye, 1 : 1/3 of the eye shaded black, 2 : 
1/2 shaded black, 3 : 1/2 shaded with additional black 
spots on the other 1/2, 4 : 2/3 shaded with additional 
spots, and 5 : eyes fully black coloured. Grade 0 
corresponds to the solitarious phase, grade 3 to 
transiens congregans, and grade 5, fully gregarious 
desert locusts (Fig. 9).
Collection and analysis of haemolymph
Samples of locust haemolymph were collected from 33 
day-old male and female adults of S. gregaria. This was 
done by puncturing the membrane at the precoxal cavity 
with a sterilized needle, and sucking the fluid up to 15 
^1 using microcapillary pipets (Sigma Chemical Co. St 
Louis Missouri). The sample fluid was diluted to 300 n 1 
with phosphate buffer saline (PBS) pH 7.4 in Eppendorf 
tubes and kept at - 20°C until use (Mahamat et al. 1996).
The collected samples were analyzed for their uv- 
visible absorption spectra from 460 to 680 nm using a 
Beckman DU-50 UV Spectrophotometer with phosphate buffer 
saline as the reference (Mahamat et al., 1996) .
Absorbance ratios, R, were calculated as Al / A2 where Al
was the absorbance reading at 4 60nm, and A2 the 
absorbance reading at 680nm.
Collection and analysis of volatiles
Air-borne volatiles were collected following the 
procedure described in Chapter III. Air from a 
compressed air cylinder was cleaned through a charcoal 
filter and passed over locusts placed in a 2 liter three­
necked round bottomed flask. The volatiles released by 
the locusts in the air stream were adsorbed onto traps 
containing 60 mg of activated charcoal. After 16 hours 
(overnight), the traps were removed and eluted with 4 ml 
of cichloromethane (Aldrich Ltd., UK). The extract was 
then concentrated under a stream of nitrogen at 0:C to 
approximately 100 /ul and stored at - 15:C prior to 
analysis. Collections were done on adult locusts at age 
5-7, 10 - 12, 15 - 17, 20 - 22, 25 - 27, 30 - 32, and 35 
37 days after fledging.
Four microliters were analyzed by capillary gas- 
chromatography (GC) and by GC-Mass spectrometry (MS).
The columns used were methyl silicone SPB-1 (30m, 0.2 ^m,
and 0.2 mm ID), and Carbowax (50m, 0.2 /urn, and 0.2 mm ID)
(Chapter III, page 59) . Nitrogen was used as the carrier
gas at a flow rate of 0.35 ml/min.
103
5.2.2 Effects of gregarious S. gregaria on the
gregarization of solitarious L. migratoria
104
Insects and experiments
•it ' '* ,
Solitarious individuals of L.m. migratorioides •we.^e*^
reared under conditions as described in Chapter III of 
this thesis. For this experiment only immature adults 
from the 3rd generation of isolated locusts were used. 
Locust isolated rearing was required, at least, for three 
generations, to make sure that they were solitary enough 
to be used in such experiments. It was impossible 
(because of low hatchability of eggs) to obtain a 
sufficient number of nymphs from mature adults of this 
generation to run this experiment at the nymphal stage.
Two-day-old immature adult solitarious locusts were 
placed in the cages as described above (Plate H) and 
reared with same age gregarious immature adults of S. 
gregaria to the end of the adult stage. Three treatments 
(TRT) were set up as follows:
2E1 GREGARIOUS STIMULUS SOLITARIOUS TEST RATIO REPS
1. Two-day-old immature Two-day-old immature 80 : 1 9
adult S. gregaria adult L. migratoria
2. Two-day-old immature Two-day-old immature 80 : 1 9
adult L. migratoria adult L. migratoria
3. Two-day-old immature Two-day-old immature 0 : 1  9
L. migratoria L. migratoria
Parameters monitored were integumental colour 
changes (on pronotum and frons) in immature and mature 
adults, and the amounts of phenylacetonitrile produced by 
maturing adult male locusts (Chapter III and Section 
5.2.1) .
5.2.3 Effects of volatiles of gregarious L. migratoria 
on the gregarization of solitarious S. gregaria
Insects and experiments
One two-day-old first instar solitarious desert 
locust was exposed to the volatile emissions from crowded 
(15) first-instar migratory locusts of similar age in a 
15 x 15 x 30 cm bic'namber cage (Plate A) . Within each 
cage gregarious locusts placed in the top chamber, were 
visually separated from the solitary test locust by a 
black cloth placed at the bottom of the top chamber. 
Between cages, solitary locusts in the lower chamber were 
visually separated from each other by a black cloth taped 
over the window facing the neighbouring cage. Locusts 
were reared from nymphal to the adult stages. The 
following treatments (TRT) were set up:
105
TRT nRKGARinns STIMULUS S&Lt t a r tW $  TEST RAXIG REPS
1. First-instar 
L. migratoria
2. First-instar 
S. gregaria
3. First-instar 
S. gregaria
Test locusts were monitored for nymphal integumental 
colour, number and duration of instars (nymphal 
developmental time), and adult integumental colour, 
morphometries, duration of immature stage, and total 
developmental time (to the end of immature adult stage).
The colours of nymphs and adults were graded daily 
as described for nymphs (page 97) and adults (Chapter 
III, see page 62). The number and duration of nymphal 
instars were obtained from recording moulting periods, 
and counting adult eye stripes. Adult morphometries were 
taken at age 12-15 days (Chapter III, ) after fledging 
when locust integument was expected to have fully 
hardened.
5.3 Data analysis
Data were analyzed using SAS (1988) . The data were
transformed into log(x) or square roots, and means were
separated, after analyses of variance, using LSD-test at 
P <; 0.05.
106
F i r s t - i n s t a r  15 : 1 6
S. gregaria
F i r s t - i n s t a r  15 : 1 6
S. gregaria
First-instar 0 : 1  6
S. gregaria
107
5.4.1 Effects of the presence of gregarious L.m
migratorioides, and two grasshopper species on the 
gregarization of solitarious S. gregaria.
Experiment 1. Effects of the presence of nymphal L.
migratoria on the gregarization of nymphal 
solitarious S. gregaria
Integumantal colour
Integumental colour patterns in all nymphal desert 
locusts varied from green (grade 0 to 1) to yellow and 
black (grades 4 and 5) when reared mixed (singly) with 
gregarious migratory locusts. Nymphs changed to 
gregarious colour by the third instar, about 15 days 
after rearing of the two locust species together (Plate 
J; Table 6) . About 13 (9 males and 4 females) out of the 
19 desert locusts reared with each other reached grade 4 
colour pattern and above by the fifth instar, 
representing a shift of 69% (75% in males, and 57% in 
females)( Table 6). These nymphs monitored to the pink 
fledglings attained stage III yellow colour at 29 ± 2 
days (Plates C-l; D-l). The appearance of the yellow 
colour (Plate K) was, however, somewhat delayed by 7 days 
compared to gregarious controls.
5.4 Results
Number of nymphal instars, nymphal and total 
developmental times (to mature adult stage) times.
Solitarious S. gregaria reared with gregarious L. 
migratoria went through five or six nymphal instars.
Only 50% (44% in males and 6% in females) of them went 
through five nymphal instars similar to the gregarious 
control while the remaining 50% (56% in males and 94% 
females went through six instars similar to the 
solitarious control. These results show that males were 
more responsive to the treatments and, therefore, 
transformed faster than females.
108
109
J. Solitarious S. gregaria{A) reared with
gregarious L. migratoria migratorioides (B).
110
K. Adult male S. gregaria[A) reared with crowded 
L. migratoria(B) and showing yellow colour 
typical of mature gregarious locust
Ill
Table 6. Time (days) taken by test S. gregaria (SG) and L. migratoria (LM) to attain
gregarious colour, and the corresponding percent shift when reared mixed with or 
exposed to volatiles
Source Test locust Rearing Days(± SE) in 
Test locusts
Days (± SE) 
in control 
locusts
% shift in 
test locusts
Nymphs LM Nymph SG mixed 15.0 + 2 a 4.0 ± 2 a 69 b
Nymphs PV Nymph SG mixed 17.0 ± 2 a 4.0 ± 2 b 63 b
Nymphs EP Nymph SG mixed - 4.0 ± 2 0 c
Nymphs LM Adult SG mixed 20.0 ± 0.6 b 22.4 ± 0.6 a 90 a
adult LM Adult SG mixed 15.3 ± 0.3 b 22.4 ± 0.6 a 89 a
adult SG Adult LM mixed 10.3 ± 0.3 b 11.8 ± 0.4 a 100 a
Nymphs LM Nymph SG exposure 25.0 ± 3.0 a 22 . 4 ± 0 . 6 b 100 a
Treatment and control means in each experiment followed by the same letter are not 
significantly different (at 5 % level; Student T-test). Percent (°o) shifts(to gregarious 
colour) in last column were compared between experiments.
PV refers to the grasshopper P. viridipes and EP to E . plorans.
Fifth and sixth instars were the longest while those 
of the other instars were quite variable compared to 
gregarious and solitarious controls. Solitarious S. 
gregaria reared with gregarious L. migratoria took 13 to 
19 days longer than the gregarious and solitarious 
controls, respectively, to complete their nymphal stage. 
There was, however, significant similarity (P<0.05) in 
development between nymphs from the test and those from 
the control colonies, especially at second, and from the 
fourth, fifth, and sixth-instars (Fig. 11). There 
significant synchrony in developmental time between the 
test desert locusts and the source migratory locusts 
(Fig. 12)
The total nymphal developmental time of the test 
locust was 46 days and immature adult took 23 days to 
reach maturation making it a total developmental time to 
mature adult of about 79 days.
112
CO
>N 50--
o
ov—y
CD 40--
E
__ 30--
O-4—'
c
CD
E 20-
Cl
_o
<D 10--
>
<u
Q
■■ SSG/GLM 
E S  GSG 
W  SSG
a “ ~ J a a ibe aaa jh b j bc Ii bh m  m  m  m  I
c
vV
VVV 
\V
VV
s
V
V \V
Nymphal
4
instars
Tota l
Dura
Figure 11. Developmental times of solitarious 
test S. gregaria(SSG) reared with 
gregarious L. migratoria(GLM) ,
[ (SSG/GLM)] compared to gregarious 
[(GSG)] and solitarious[(SSG)] 
controls.
stcqe
tion
De
vel
opm
ent
al 
time
 (
Da
ys)
114
50--
40--
30-
20
104-
■ ■  SSG  /  GLM 
E 3  GLM
10 ' ? ° <■ a ■o . j i  ■ i i  l i  I U l
Nymphal instars 6 To ta l s tage  Dura tion
Figure 12. Nymphal instar and stage durations of
solitarious test S. gregaria(SSG) reared 
with gregarious L. migratoria(GLM),
[(SSG/GLM)] and that of those which it 
was reared with[(GLM)].
Adult morphometries
115
Average E/F ratios showed that test S. gregaria males 
were more like solitarious while females were more like the 
gregarious controls. Average F/C ratios showed that test 
female locusts were all transients (Fig. 13). However, 
fewer males than females transformed to the gregarious 
phase.
Comparison with the standard morphometric chart showed 
that three out of the eleven males S. gregaria reared with 
L. migratoria changed to transiens while eight remained 
solitarious, representing a percent shift of 27% to 
transient phase. In females, only three out of eight 
changed into transients (37% shift) by the end of the first 
generation.
Eye shading
Nine (8 males and 1 female) out of sixteen solitarious 
adult desert locusts which were reared singly with 
gregarious migratory locusts attained grade 3 eye shading 
representing a total shift to the gregarious phase of 57%
M
or
ph
om
et
ri
c 
ra
ti
os
116
E/F F/C
Figure 13. Adult E/F and F/C ratios of solitarious 
S. gregaria(SSG) reared with gregarious 
L. migratoria(GLM) from nymphal stage 
[ (SSG/GLM)] compared to gregarious[(GSG)] 
and solitarious controls[(SSG)].
{67% in males, and 15% in females). Males were more 
responsive to treatment effects than females, and therefore, 
changed faster.
Haemolymph pigments
Haemolymph absorbance ratios (pigment composition) of 
all test desert locusts S. gregaria was intermediate between 
isolated individuals and crowded controls, especially in 
females (Fig. 14).
Volatile emissions
Volatile emissions from test S. gregaria mature adult 
males contained phenylacetonitrile similar to control 
gregarious mature adult locusts (Figs. 15;16). Its 
production followed the same pattern in both control and 
test insects, but reaching an optimum between 15 - 20 days 
after fledging in controls, and 25 - 30 days in test insects 
(Fig. 17) . Phenylacetonitrile was also detectable in the 
volatile emissions of mature adult males of gregarious L. 
migratoria (Fig. 18) which test locusts were reared with.
117
Ha
em
oly
mp
h 
pig
me
nt 
rat
ios
118
Males F em a le s
Figure 14. Haemolymph pigment ratios(at 460 over 
that at 680 ran wavelength) of test 
S. gregaria[(SSG/GLM)] compared to 
gregarious[(GSG)] and solitary[(SSG)] 
controls.
119
Figure 15. Gas chromatograms showing the production
of phenylacetonitrile (PAN) in adult males 
of S. gregaria reared with gregarious 
L. migratoria at 10-12, 15-20, 25-30, 
and 35-40 days after fledging (30m,
0.2mm ID methyl silicone column) .
120
Figure 16. Gas chromatograms showing peaks of
phenylacetonitrile (PAN) produced by 
gregarious locusts from control cages 
at 10-12, 15-20, 25-30, and 35-40 days 
after fledging (30m, 0.2mm ID methyl 
silicone column)
This compound was produced in relatively smaller 
proportions in adults of this species compared to the 
emissions from test and control mature adult males of S. 
gregaria.
Experiment 2. Effects of the presence of immature adults of 
L. migratoria on the gregarization of 
immature solitarious adults of S. gregaria
Integumental colour
Both male and female solitarious immature adults of S. 
gregaria reared with crowded immature adults of L. 
migratoria attained stage III yellow colour 15 days of mixed 
rearing representing a shift to gregarious phase of 89% (all 
males and 67% of females) of the test insects (Plate L). 
Immature adult males were more responsive to treatment 
effects than females (Table 6) .
Volatile emissions
Solitarious immature adult males of S. gregaria reared 
with gregarious immature adults of L. migratoria produced 
phenylacetonitrile after 10 days of rearing with maximum
121
Phe
nyl
ace
ton
itri
le 
(pg
/ma
le/h
our
)
122
Figure 17. Phenylacetonitrile titres (pg) produced
by test S. gregaria(SSG) when reared with 
gregarious L. migratoria(GLM)at nymphal 
stage[(SSG/GLM)] and immature adult 
[ (ISG/GLM)] as compared to titres of 
conspecifics from gregarious 
control[(GSG)] .
Time (min)
Figure 18. Gas chromatograms showing the production 
of phenylacetonitrile (PAN) in gregarious 
mature adults of L . migratoria at 10-12, 
15-17, 20-22, and 25-27 days after 
fledging (30m, 0.2mm ID SPB-1 methyl 
silicone column)
123
production occurring between 15 to 17 days, similar to 
gregarious control counterparts (Fig. 17) .
Eye shading
Seven (4 males and 3 females) out of the nine test 
desert locusts attained grade 3 eye shading stage and above 
by the end of the experiment, representing a shift of 78% 
(80% males and 75% females) .
Experiment 3. Effects of the presence of fifth-instar
nymphs of L. migratoria on immature adults 
of S. gregaria
Integumental colour
Solitarious male and female immature desert locusts 
reared with fifth-instar nymphs of the migratory locusts 
attained stage III yellow colour 20 ± 0.6 days after 
fledging representing a shift of 90% (80% in males and 100 
in females)(Table 6).
Volatile emissions
Solitarious immature desert locusts reared 
instar nymphs of the migratory locusts produced 
phenylacetonitrile about 15 days after fledging (Fig. 19) .
Eye shading
Solitarious immature desert locusts which were reared 
with fifth-instar nymphal migratory locusts attained grade 3 
eye shading stage typical of transient or gregarious phase. 
All males and females were in transient phase representing a 
100% shift.
Experiment 4. Effects of the presence of nymphs of P.
viridipes on the gregarization of nymphal S. 
gregaria
Integumental colour
About 63 % (75% in males and 75% in females) of nymphs 
reared with P. viridipes attained gregarious colour 
characteristics by the 4th-instar, 17 days after rearing
125
trace of
126
Figure 19. Gas chromatograms showing A) peaks of 
phenylacetonitrile (PAN) produced by 
S. gregaria reared from immature solitary 
adult stage with gregarious adults of L. 
migratoria ; B) produced by L. migratoria 
reared from immature solitary adult stage 
with gregarious adults of S. gregaria; and 
C)produced by adults of S. gregaria reared 
from immature solitary adult stage with 
fifth-instar nymphs of L. migratoria(50m, 
0.2mm ID Carbowax column) .
(Plate L;Table 6). Like in previous mixed rearing from 
nymphal stage (Experiment 1), gregarious colour in nymphs 
was observed to intensify after each moult, during the 
shifting process.
Number of nymphal instars, nymphal and total developmental 
time (to immature adult stage)
Solitarious S. gregaria reared with P. viridipes went 
through five or six nymphal instars. Fifty percent of them 
went through five nymphal instars (75% in males only) like 
in gregarious controls, while the other 50% went through 
six nymphal instars (females only) like solitarious controls 
(Table 7). Developmental times for first and sixth instars 
were the longest followed by the third, fourth and fifth 
instars. The second instar developed fastest (4 days). 
Compared to the control gregarious and solitarious locusts, 
the durations of the instars of the test S. gregaria were 
variable (Table 7).
Test nymphal desert locusts took a total of 45 ± 3 days 
to complete development. This represents a delay of about 
15 days compared to the nymphal development in gregarious 
and solitarious controls (Table 7) .
Fifty percent of the fledglings attained stage III 
yellow colour typical of mature gregarious locusts after 35
127
L
128
1
?* gregaria(A > reared with
(L*?i 1= 1P&s (B ) (L-l) , and E. plorans(B) 
> fr«  nymphal stage, P^°rans(B)
129
Table 7. Developmental time (days) of isolated S. gregaria(SSG) reared with P.
viridipes (PV),[(PV/SSG)] and E. plorans(EP), [(EP/SSG)] as compared
to those from gregarious[(GSG/GSG)] and solitarious (1 SSG/cage) 
controls.
Source/Test
1st-
instar
2nd-
instar
3rd-
instar
4 th­
ins tar
5th-
instar
6th-
instar
Total
stage
Young 
adul t 
stage
Total time 
to mature
PV/SSG 15±la 4±lbc 7±lb 9± 1 a 9+ lb ll±2a 45+3a 54±10a
99±10a
EP/SSG 5±lcd 6±la 9±0ab 6±2b 12 + la 9± la 47±3a 8±lc
GSG/SSG 5±lcd 5±lab 5±lc 5±lb 8± lb - 27±2c 22± lb 55±3c
1SSG/cage
-
4±ld 5±lab 4±lc 3±lc 7±lb 12±la 32±lb
74±2b
In columns, means followed by the same letter are not significantly 
different(at 5% level; LSD-test).
days. The remaining 50% stayed pink by the conclusion of 
the experiment (55-60 days after fledging).
130
Morphometries
E/F ratios of males of test S. gregaria were closer 
to solitarious controls while females were intermediate 
between solitarious and gregarious controls. F/C ratios 
of males were more were more like solitarious and females 
were transients (Fig. 20). Comparison with the standard 
morphometric chart (Duranton and Lecoq, 1990) showed that 
test locusts were more like transiens, a shift of about 
67% (75% in males and 50% in females).
Volatile emissions
Mature adult males of S. gregaria which fledged from 
nymphs reared with P. viridipes, produced trace amounts 
of phenylacetonitrile in their volatile emissions 15 and 
25 days after fledging.
Analysis of the volatiles from P. viridipes showed 
the presence of benzaldehyde, phenol, and guaiacol three 
of the aggregation pheromone components of mature desert 
locust.
M
or
ph
om
et
ri
c 
ra
ti
os
131
^  GSG 
VZZA SSG'
b a
Males Females Males Females 
E /F  F/C
Figure 20. Adult E/F and F/C ratios of solitarious
S. gregaria (SSG) reared with P. viridipes 
(PV) at nymphal stage[(SSG/FV)] compared to 
gregarious[(GSG)] and solitarious[(SSG)] 
controls.
These compounds are also present in the volatile 
emissions of the migratory locust (Appendix VII)
132
Eye shading
Five out of the six monitored desert locusts reared 
with P. viridipes showed significant dark shading of the 
eye of grade 3 and above, similar to those reared with L. 
migratoria and the control gregarious colony. This 
represents a shift of 84% (all males and 75% of females).
Experiment 5. Effects of the presence of nymphs of E.
plorans on the gregarization of nymphal 
solitarious S. gregaria
Nymphs of solitarious S. gregaria reared with E. 
plorans, maintained the colour characteristics of the 
solitarious controls (nymphs green and adults grey 
colour). Six nymphal instars were recorded and the total 
nymphal developmental time was up to 47 days (Table 7). 
Adults (which live for 8 days as immature) had eye 
shading and morphometries typical of solitarious locusts 
(Fig. 21), ancj did not produce phenylacetonitrile.
133
cn.2 '■4—'o
o
*L_
-J—'
<D
Eo
_c
C l
v_
O
ES3 GSG 
■ i  SSG/EP 
E 2  SSG
4-
b
I
a a
1es Females
E/F
Males
F/C
hemales
Figure 21. Adult E/F and F/C ratios of solitarious 
test S. gregaria (SSG) reared with E. 
plorans(EP) at nymphal stage[(SSG/EP)] 
compared to gregarious[(GSG)] and 
solitarious[(SSG)] controls.
134
5.4.2 Effects of gregarious S. gregaria on the
gregarization of solitarious L. migratoria
Immature adults of L. migratoria which were reared 
with gregarious immature adults of S. gregaria 
progressively developed brown to black colour on 
pronotum, thorax, and legs. At the same time, the green 
colour on these body parts faded. Mature adults showed 
yellow colour on frons, typical of gregarious locusts 
(Table 6). Changes in colour to gregarious form occurred 
in about 10 days after fledging, which was about 5 days 
faster than observed for solitarious S. gregaria reared 
with gregarious L. migratoria at the immature adult stage. 
All test migratory locusts had changed to gregarious phase 
at the end of the experiment.
Mature adult males of test L. migratoria produced 
significantly higher amounts of phenylacetonitrile than 
gregarious conspecific locusts in the control (Fig. 22). 
The maximum amounts of this compound in test L. 
migratoria occurred 15 days after fledging while in the 
control, it occurred after 20 days.
Ph
en
y 
lac
e 
ton
 i
tr
ile
(p
g
/m
a
le
/h
o
u
r)
135
Days a f te r  f ledg ing
Figure 22. Production(in pg) of phenylacetonitrile 
by solitary immature L. migratoria(ILM) 
when reared with gregarious immature 
adults of S. gregaria(ISG),[(ILM/ISG)] 
compared to those reared with gregarious 
conspecific controls[ (ILM/ILM)].
136
5.4.3 Effects of volatiles of gregarious L. migratoria 
on the gregarization of solitarious S. gregaria
Integumental colour
Nymphs exposed to volatiles of L. migratoria 
attained the gregarious colour grade 3 in 25 ± 3 days 
(Table 6). All locusts tested shifted to gregarious 
colour and nymphs fledged into pale pink immature adults 
and attained stage III yellow colouration at maturity.
Number of instars, nymphal and total developmental times 
(to mature adult stage).
Eighty percent of solitarious test locusts went 
through 6 nymphal instars typical of the solitary phase, 
and only 25% (only males) went through 5 nymphal instars, 
typical of the gregarious phase. This represents a lower 
(by 25 %) percentage shift in comparison to that observed 
from mixed rearing with either L. migratoria (Experiment 
1) or P. viridipes (Experiment 4). The instar duration 
pattern was irregular. The first and third instars were 
the longest (10 days) and the second the shortest (6 
days) (Fig. 23). The total nymphal developmental time 
was significantly longer than that of those of gregarious 
and solitarious controls. There was no clearcut synchrony 
in nymphal development between nymphs of L. migratoria in
137
W 60 
>, 
o 
O
CD
0)
>
CD
O
50-
40-
10 -
eza gsg 
ggg ssg
H  SSG : GLM
° 30J- 
c
<U
E  20 +
C l  
O . cb ~  ^ a a a
| a -i btJ  ab ■ M i Ml Ml _3m 111
2 3 4
Nympha l  i n s t a r s
6 lotal stage duration
Figure 23. Nymphal instar and stage duration of 
test solitarious S. gregaria[(SSG)] 
exposed to volatiles of gregarious 
source L . migratoria[(GLM)] from 
nymphal stage stage[(SSG:GLM)].
138
the source and the recipient S. gregaria (Fig. 24}.
Nymphal developmental time was 5 0 + 4  days, immature
adults attained stage III colour in 29 ± 4 days, leading
to a total developmental time of 79 ± 6 days, similar to
that of those reared with gregarious L. migratoria (P <
E/F ratios in males were intermediate between 
gregarious and solitarious controls whereas females were 
more like solitarious controls. The same pattern was 
observed with F/C ratios (Fig. 26). The standard 
morphometric chart suggests that about 50% (one half of 
males and one half of females) of the test S. gregaria 
were transiens, similar to those reared with L. m. 
migratorioides (Experiment 1), and P. viridipes 
(Experiment 4), but contrasted those reared with E. 
plorans (Experiment 5). Later in their development, five 
out of the six locusts studied had deformed (twisted) 
wings.
Morphometric ratios
0.05) (Fig. 25) .
O
De
vel
opm
ent
al 
tim
e 
(Da
ys)
139
■ I  SSG  : GLM
eza  g lm
40-
30--
20-
2 3 4 5
Nymphal instars
b
)
6 To ta l s ta ge  
D u ra tio n
Figure 24. Nymphal instar and stage durations of
solitarious test S. gregaria(SSG) exposed 
to volatiles of gregarious L. migratoria 
(GLM),[(SSG:GLM)] from nymphal stage 
compared to source gregarious L. 
migratoria [(GLM)].
D
ev
el
op
m
en
ta
l 
tim
e 
(D
ay
s)
140
60 -
50 --
SSS SSG/GLM  
■ ■  SSG : GLM
40 -  
30 -  
20-- 
10 -  b 
0-
a n a a a
a a a a a
I I I  a i  l l  I I  l i  l i
1 2  3 4 5 62
Nymphal instars
6 To ta l s tcge  
Dura tion
Figure 25. Nymphal instar and stage duration 
of test solitary S. gregaria (SSG) 
reared with gregarious L. migratoria 
(GLM) , [(SSG/GLM)] or exposed to its 
volatiles[(SSG:GLM)].
M
or
ph
om
et
ric
 
ra
tio
s
141
S 3  GSG 
H I  SSG : GLM 
eza SSG
2-
Males
b b
1
Fem a les
aTb a
b
J
Males
I
E/F
Fem a les
F/C
Figure 26. E/F and F/C ratios of solitary
S. gregaria(SSG) exposed to volatiles 
of gregarious L. migratoria(GLM) from 
nymphal stage[(SSG:GLM)].
Volatile emissions
1 4 2
Of the 10 starting replicates, three did not reach 
the fifth-instar stage due in part by ants preying on 
moulting nymphs, one died at fifth-instar nymphal stage 
(not preyed upon), and only one adult male survived up to 20 
days after fledging, therefore, volatile collections were 
discontinued.
In summary, S. gregaria reared with L. migratoria 
and P. viridipes shifted towards gregarious characters.
All locusts reared with other species than S. gregaria 
had an irregular nymphal development, leading to longer 
nymphal developmental time, with fledglings taking 
relatively longer period to attain stage III colour, 
compared to those from the control gregarious and 
solitarious cages (Fig. 27). Fledglings from nymphs 
reared with E. plorans did not reach stage III yellow 
colour since they died eight days after fledging.
Fledglings from locusts exposed to volatiles attained 
stage III colour at about the same time as those from the 
control gregarious cages.
Tot
al 
stag
e 
dur
atio
n 
(Da
ys)
143
120 
100
80 
60 
40 
20 
0
Nym pha l s tage  Im m a tu re  adu lt T o tc l d e ve lopm en ta l
time
■ H  SSG/GLM  
E 3  SSG /P V
- -  e za  s s g / e p
ES3 SSG : GLM 
[ZD  GSG
a
T
0
IJ l J
II
Figure 27. Stage durations of solitarious S.
gregaria(SSG) a) reared with[(SSG/GLM)] 
or b) exposed to volatiles of [(SSG:GLM)] 
gregarious L. migratoria(GLM) c)reared 
with P. viridipes(PV), [(SSG/PV)] and 
d) reared with E. plorans(EP),[(SSG/EP)] 
compared to gregarious conspecifics 
from the control[(GSG)].
144
Primer effects of gregarious L. migratoria on 
solitarious S. gregaria, and vice-versa, in mixed rearing 
experiments, were manifested by the changes from 
solitarious to gregarious integumental colour, the number 
of instars as reflected in eye stripes, morphometries, 
haemolymph pigment composition, titres of 
phenylacetonitrile, and the shading of the eyes. Since 
the experiments involved continuous mixed rearing of the 
two species, the shift observed in these phase 
characteristics may have been mediated by a combination of 
factors such as tactile, visual, acoustic, and olfactory 
cues (Chapman, 1979). From the mixed rearing experiments, 
it is not clear which of these are the most important in 
triggering and maintaining the process of phase shift. 
Chapman (1979) demonstrated that a stimulation by innate 
objects was enough to trigger the development of 
gregarious characters in the desert locust. Simpson 
(communication at the Locust Research Programme Workshop, 
1994) confirmed the effects of tactile factors by exposing 
solitarious insects to rolling plastic balls. Visual 
factors have also been shown to be important in inducing 
phase shifts (Chapman, 1979) . The lack of specificity of 
these cues led Chapman (1979) to comment that "anything 
could gregarize locusts". However, Gillett (1968) showed 
that visual and tactile isolation did not prevent the 
development of gregarious characters in locusts. It was
5.5 Discussion
further shown that while sound does not play an important 
role, olfactory information is quite important for food 
seeking, synchronisation of maturation time, and 
oviposition (Gillett, 1968).
The experiments which involved the exposure of 
solitarious individuals of S. gregaria to volatiles of 
gregarious L. migratoria during rearing, showed the 
development of gregarious characteristics similar to those 
reared together with gregarious individuals of the latter 
species. Even though gregarious nymphal and mature adult 
pheromone production (more sensitive phase parameter) had 
not been monitored, the results from the exposure 
experiments suggest that olfactory signals from the 
migratory locusts are as effective as the tactile, 
olfactory, acoustic, and visual stimuli altogether 
(present in mixed rearing ). They demonstrate the crucial 
role played by pheromones in gregarization (Gillett,
1968). This aspect needs more detailed and systematic 
investigations. These studies also show that, compared to 
gregarious or solitarious desert locusts in the controls, 
desert locusts reared with, or exposed to volatiles of 
migratory locusts, and to those of grasshoppers, showed 
delayed nymphal developmental time and adult maturation. 
Thus, only conspecific volatile emissions along with other 
factors are adequate for optimum locust development.
The results of rearing solitarious desert locusts 
with pheromone-producing and non-producing grasshoppers 
illustrate the key role of pheromones in locust phase
145
shift. When reared with the 'giant' pyrgomorphid 
grasshopper, P. viridipes, the test insects shifted to 
transient phase with respect to most parameters. Analysis 
of volatiles collected from this species has shown that P. 
viridipes produces, among others, such compounds as, 
guaiacol, benzaldehyde, phenol, and four acids, all of 
which are components of the nymphal and adult male- 
produced pheromonal systems of the desert locust (Torto et 
al., 1994; 1996). No such phase shift was observed in 
desert locusts reared with the grasshopper, E. plorans, 
which did not produce these compounds. Both groups of 
test insects, however, were abnormally affected. Those 
reared with P. viridipes changed colour and morphometries 
to those of transients, produced phenylacetonitrile, but 
stayed immature for up to 60 days. Those that had been 
reared with E. plorans did not show any evidence of phase 
change, and died within 10 days after fledging to immature 
adult stage.
Pheromone titres of S. gregaria under different 
treatments merit a special comment. Test locusts exposed 
to gregarious migratory locusts at the immature adult 
stage produced the highest titres of phenylacetonitrile 
followed by those associated at the nymphal stage. Both 
of these groups had significantly higher titres of 
phenylacetonitrile than the gregarious control desert 
locusts. Likewise, solitarious L. migratoria reared with 
gregarious S . gregaria at the immature stage also produced 
hicfher titres of nhenylacetinitrile than the gregarious
146
control migratory locusts. Similar observations were made 
by Deng et al. (1996) who showed that crowding of
solitarious locusts produces F0 and insects that had 
higher titres of phenylacetonitrile compared to those in 
subsequent generations. These results parallel those of 
Michel (1980) with reference to flight activity and 
migratory aptitude for a colony of crowded locusts from 
solitarious parents isolated for several generations. The 
colony used in this study has been crowded or isolated for 
several generations(17 generations at the time of this 
experiment). As Deng et al.(1996) have recommended, 
future studies on the relationship between phase history 
and specific phase characters may help throw further light 
on the relationship between the two.
Chemical analyses of volatiles from L. migratoria 
migratorioides confirmed the presence of the four 
compounds previously identified in the volatiles of 10 
day-old immature and mature adults of S. gregaria (Torto 
et al., 1994). Phenylacetonitrile, the key aggregation 
pheromone component in S. gregaria (Torto et al., 1994) is 
present in the volatiles of nymphal (Torto unpublished) 
and adult L. migratoria. This may account, in part, for 
the cross-aggregation responses observed between L. 
migratoria and S. gregaria.
In summary, the present study has established cross 
primer effects m  the gregarization of S. gregaria and L . 
W W » * * ™ O T O nes appeared to play a major role in
147
the gregarization of the two locust species, but the 
presence of conspecific pheromones is important for 
optimum development of S. gregaria.
148
CHAPTER SIX
6.0 Intra-and interspecific effects of volatiles of S. 
gregaria and L. m. migratorioides on sexual 
maturation times
6.1 Introduction
Chapters (IV) and (V) described the results of strong 
releaser and primer effects relating to gregarization 
resulting from the interactions between S. gregaria and L. 
migratoria, and also between S. gregaria and two 
grasshoppers. In the primer effects, volatile pheromones 
were shown to play an important role in phase change, 
developmental time of nymphs, and adult duration. The 
effects of these volatiles from nymphs and adults of the 
two species on the maturation of each other's young adults 
are not known. It is, however, known that in S. gregaria, 
nymphal aggregation pheromone retards sexual maturation of 
immature adults while adult aggregation pheromone 
accelerates it (Norris, 1954; Mahamat et al. 1993; Assad 
et al., 1996), but in L. migratoria, sexual maturation of 
immature has not been adequately studied.
Loher (1990) reported that males of L.m. migratorioides, 
in the presence of mature males, copulated with females of 
their own age earlier (13 - 14 days after fledging) than in 
the presence of conspecific young adults(17 - 25 days). Thus,
149
pheromones may also play an important role on the timing and 
synchrony of sexual maturation of the migratory locust(Loher, 
1990). In the desert locust, pheromones are also responsible 
for the change in body colour (Loher, 1990) . The maturation 
accelerating pheromones are believed to be produced in glands 
in the epidermis of sexually mature (yellow) adult males 
(Loher, 1960). In L. m. migratorioides a similar pheromone is 
suspected to be present in the epidermal glands of both sexes 
(Loher, 1960). Isolation of S. gregaria retards maturation 
while in L. m. migratorioides, isolation accelerates it 
(Loher, 1960). Maturation pheromones also act 
interspecifically since mixed rearing of mature S. gregaria 
accelerate sexual maturation of young L. migratoria 
migratorioides (Norris, 1964). Although it has been 
speculated that the reverse may be true, effects of mature 
adults of L. migratoria on the maturation of young adults of 
S. gregaria have not been reported.
The main objectives of this study were to compare intra 
and interspecific effects of volatiles of the nymphal and 
adult stages of the two locust species on sexual maturation 
of the adult stage.
150
6.2 Materials and methods
Maturation experiments were run in the two chamber 
bioassay cages described in Chapter III.
Insects and experiments
The insects used were from the same colonies as 
described in Chapter III of this thesis. Due to constraints 
in rearing L. migratoria (poor hatching, and space), 
different insect batches were used for the different 
experiments. Effects of volatiles on the sexual maturation 
of males and females were run separately. Fifth-instar 
nymphs (mixed sex) or adult males were used as sources of 
pheromones while male and female immature adults were the 
recipient (test) locusts in both species. Locusts used as 
signal sources were placed in the top chamber, and recipient 
ones in the bottom chamber as described in each of the 
experiments in 6.2.1 and 6.2.2.
6.2.1 Effects of gregarious L .m .migratorioides on
the sexual maturation of gregarious S. gregaria
These effects were studied in four different experiments 
as described below:
151
Experiment 1. Effects of fifth-instar nymphs of L.
migratoria on the sexual maturation of immature 
adult S. gregaria males
152
Treatments a) and c) were controls. The parameters 
monitored were:
Integumental colour and copulation
The integumental colour was monitored in males 
following the scheme (Norris, 1954)described in Chapter 
III, page 59-62. Each test insect in the lower chamber 
was considered as a replicate. Thus, for colour, 15 - 25 
insects were graded daily until at least 60% of them 
reached colour stage III.
Locusts which had attained colour stage III (even 
before the 60% reached that stage) were paired with fully
mature females for six hours and the number of pairs in
copulation was recorded on the particular day. The
average number of days taken by locusts to start
copulating was noted.
Pheromone emission
To monitor pheromone emission, locusts were randomly
Source Test(Immature) Ratio
a) Fifth-instar S. gregaria S. gregaria
b) Fifth-instar L. migratoria S. gregaria
c) Immature adult S. gregaria S. gregaria
5 : 5 
5 : 5 
5 : 5
picked from cages, and volatile collections were done at 10-
153
12, 15-17, 20-22, 25-27, and 30-32 days after fledging, 
following the procedure described in Chapter III.
Experiment 2. Effects of mature adult males of L.
migratoria on the sexual maturation of immature adult 
S. gregaria males
Source Test(Immature) Ratio
a) Mature adult S. gregaria S. gregaria 5
b) Mature adult L. migratoria S. gregaria 5
c) Immature adult S. gregaria S. gregaria 5
5
5
5
Treatments a) and c) were controls. The parameters 
monitored were the same as in experiment 1 described above:
Experiment 3. Effects of fifth-instar nymphs of L.
migratoria on the sexual maturation of immature 
adult S. gregaria females
Source Test(Immature) Ratio
a) Fifth-instar S. gregaria S. gregaria 5
b) Fifth-instar L. migratoria S. gregaria 5
c) Immature adult S. gregaria S. gregaria 5
5
5
5
Where a) and c) served as control treatments. 
Maturation was monitored by measuring the oocyte lengths 
in relation to the immature adult age. Basal oocyte 
length was considered to be a more reliable measure of 
sexual maturation time as colouration in females appeared 
to be less discernable at early immature adult stage. 
Thus, test females were dissected every five days. 
Isolated egg masses were dipped into 70% ethanol and 
spread open before measurements with a Wild Heerbrugg 
microscope (Switzerland) with a 10 grid micrometer
adapted to a X 10 lens. Readings were converted into 
millimeters using the following conversion formula.
Length ( mm ) = ( units in scope x 10 ) / magnification
Five females were dissected at 5-7, 10-12, 15-17, 
20-22, 25-27, 30-32, and 35-37 days after fledging. For 
each age, 50 oocytes were measured and the mean oocyte 
length calculated. Insects were considered mature at the 
time when the insects dissected had an average oocyte 
length between 6-8 mm (Norris, 1954) in S. gregaria.
Experiment 4. Effects of mature adult males of L. 
migratoria on the sexual maturation of immature adult 
S. gregaria females
Source Test(Immature) Ratio
a) Mature adult S. gregaria S. gregaria 5 : 5
b) Mature adult L. migratoria S. gregaria 5 : 5
c) Immature adult S. gregaria S. gregaria 5 : 5
Where a) and c) served as controls. Maturation was 
monitored using the same parameter as in experiment 3.
6.2.2 Effects of gregarious S. gregaria on sexual
maturation of gregarious L. m. migratorioides
These were similarly studied in four different 
experiments as described below :
154
Experiment 5. Effects of fifth-instar nymphs of S. 
gregaria on the sexual maturation of immature adult L. 
migratoria males
Source Test(Immature) Ratio
a) Fifth-instar L. migratoria L. migratoria 5 : 5
b) Fifth-instar S. gregaria L. migratoria 5 : 5
c) Immature adult L. migratoria L. migratoria 5 : 5
Where combinations a) and c) were controls. The 
parameters monitored were:
Integumental colour and copulation
The integumental colour was monitored in males 
following Norris (1954) scheme described in Chapter III. 
In addition, the appearance of the yellow colour on the 
frons- was monitored. Each test insect in the lower 
chamber was considered as a replicate. Thus, for colour, 
15 - 25 insects were graded daily.
Locusts that were sexually mature were also paired 
with fully mature females for six hours and the number of 
pairs in copulation was recorded on the particular day, 
and the average number of days taken by locusts to start 
copulating was noted.
Pheromone emission
155
As described in Section 6.2.1, locusts were randomly 
picked from cages to monitor pheromone emissions, and volatile
collections were done at 10-12, 15-17, 20-22, 25-27, and 30-32 
days after fledging, following the procedure described in 
Chapter III.
156
Experiment 6. Effects of mature adult males of S. gregaria
on the sexual maturation of immature adult L. migratoria 
males
Source Test(Immature)
a) Mature adult L. migratoria L. migratoria
b) Mature adult S. gregaria L. migratoria
c) Immature adult L. migratoria L. migratoria
Ratio
5
5
5
5
5
5
Where treatments a) and c) served as controls.
The parameters monitored were the same as described in 
exoeriment 1 of this section.
Experiment 7. Effects of fifth-instar nymphs of S. gregaria 
on sexual maturation of immature adult L. migratoria 
females
Source
a) Fifth-instar L. migratoria
b) Fifth-instar S. gregaria
c) Immature adult L. migratoria
Test(Immature)
L. migratoria 
L. migratoria 
L. migratoria
Ratio
5
5
5
where a) and c) were control treatment combinations.
The parameter monitored was the length of basal oocyte as 
described in Section 6.2.1. Test locusts were considered 
mature when their basal oocyte lengths were between 4 - 6.5 mm 
(Descamps and Wintrebert, 1961; Highnam and Haskell, 1964).
157
Experiment 8. Effects of mature adult males of S.
gregaria on the sexual maturation of immature adult 
female L. migratoria females
Source Test(Immature adult) Ratio
a) Mature adult L.migratoria L. migratoria 5 : 5
b) Mature adult S. gregaria L. migratoria 5 : 5
c) Immature adult L. migratoria L. migratoria 5 : 5
Combinations a) and c) were controls. Maturation 
was monitored using the same parameter as in experiment
6.3. Data analyses
Data were analyzed using SAS (1988). Square root or 
log (x+1) transformations were used when necessary and 
means were separated after ANOVA by LSD test.
158
6.4.1 Effects of gregarious L.m. migratorioides on 
sexual maturation of gregarious S. gregaria
Experiment 1. Effects of fifth-instar nymphs of X.
migratoria on the sexual maturation of immature 
adult S. gregaria males
Young adults of S. gregaria exposed to volatiles of 
fifth-instar nymphs of L. migratoria changed colour and 
mated at about the same time as control conspecific 
immature adults, but significantly earlier than those 
exposed to conspecific fifth-instar nymphal volatiles 
( fig. 28) .
Males of S. gregaria exposed to volatile emissions 
of fifth-instar nymphs of L. migratoria produced peak 
pheromone titres by 15-17 days after fledging, that is, 
about 10 days earlier than those exposed to volatiles of 
conspecific immature adults and fifth-instar nymphs (Fig. 
29) .
Experiment 2. Effects of mature adult males of X. 
migratoria on the sexual maturation of immature 
adult S. gregaria males
Young adults of S. gregaria exposed to volatiles of 
mature adults of L. migratoria changed to colour stage 
III significantly (p  ^ 0.05) earlier than control 
conspecific immature adults, but at about the same time 
as those exposed to volatiles of conspecific mature adults
6.4 Results
P
he
ny
la
ce
to
ni
tr
il
e(
pg
/m
al
e/
ho
ur
)
160
Days after  fledg.ing
Figure 29. Amounts(pg) of phenylacetonitrile 
produced by maturing males of 
S. gregaria(ISG) exposed to volatiles 
from live conspecific fifth-instar 
nymphs(FSG) , [(ISG/FSG)]; immature 
adults[(ISG/ISG)] and mature adults 
(MSG),[(ISG/MSG)]; and to those of 
fifth-instar(FLM),[(ISG/FLM)] and 
mature adults(MLM), [ (ISG/MLM)] of 
L. migratoria.
161
(Fig. 28).
However, in mating experiments there was no significant 
difference.
Sexually immature males of S. gregaria exposed to 
volatile emissions of mature adults of L. migratoria had 
peak pheromone production at 25 days after fledging 
similar to conspecific immature adults. This peak was, 
however, delayed (10 days) compared to those exposed to 
volatiles of conspecific mature adults (Fig. 29).
Experiment 3. Effects of fifth-instar nymphs of L.
migratoria on the sexual maturation of immature 
adult S. gregaria females
Young adult females of S. gregaria exposed to 
volatiles of fifth-instar nymphs of L. migratoria matured 
significantly earlier (10-12 days), with respect to 
average oocyte length, compared to those that were 
exposed to volatiles of conspecific immature adult and 
fifth-instar nymphal controls (5-7 days) (Figs. 30).
Experiment 4. Effects of mature adult males of L.
migratoria on the sexual maturation of immature 
adult S. gregaria females
Young adults of s. gregaria exposed to volatiles of 
mature adults of L. migratoria matured at about the same 
time, with respect to average oocyte length, as those exposed
to volatiles of conspecific control immature adults, but 
suffered significantly delayed maturation when compared to 
locusts that had been exposed to volatiles of conspecific 
mature adults (Fig. 30).
6.4.2 Effects of gregarious S. gregaria on the sexual 
maturation of gregarious L.m. migratorioides
Experiment 5. Effects of fifth-instar nymphs of S. gregaria 
on the sexual maturation of immature L. migratoria males
Young adult males of L. migratoria exposed to volatiles 
from fifth-instar nymphs of S. gregaria showed delayed 
changes in integumental colour compared to those exposed to 
volatiles of conspecific control immature adults and 
conspecific fifth-instar nymphs. They mated at the same time 
as conspecific control immature adults, but later than those 
exposed to conspecific fifth-instar nymphs (Fig. 31).
Immature adult males of L. migratoria exposed to 
volatiles of fifth-instar nymphs of S. gregaria produced peak 
amounts of phenylacetonitrile at 15 days after fledging, that 
is 5 days earlier than conspecific immature adult controls, 
but a delay of 5-7 days compared to those exposed to 
conspecific fifth-instars. Pheromone production was 
significantly higher than in locusts that had been exposed to 
volatiles of conspecific fifth-instars which produced 
relatively smaller amounts of the compound (Fig. 32) .
162
M
at
ur
at
io
n 
tim
e 
(d
ay
s)
163
4 0 - -  
3 0 - -  
2 0 - -  
1 0 -
0 -
Figure
KS9 ISG /  SG 
■ ■  ISG /  LM 
E2ZI ISG /  ISG
II
Exposu re  to  E xp o su re  to
f i f th— in s ta r  Matu re  adu lt
vo lat i les  vo la t i les
Basal oocy te  length
30. Time taken by maturing adult females 
of S. gregaria[ISG) to attain mature 
oocyte length when exposed to volatiles 
of conspecific(SG) fifth-instar nymphs 
and mature adults[(ISG/SG)]; conspecific 
immature adults[(ISG/ISG)]; and to those 
of similar stages of L. migratoria(LM), 
[(ISG/LM)].
Ma
tur
atio
n 
time
 (
day
s)
164
Co lou r  Copu la t io n  C o lou r  Copu la t ion
Exposure to f i f th  —ins ta r  Exposure.to mature  
Nymphal volat i les Volati les
rigure 31. iime(days) taken by maturing adult 
males of L. migratoria(ILM) to 
attain stage colour I I I  when exposed 
to volatiles of live conspecific(LM) 
fifth-instar nymphs and mature adults, 
[ ( I L M / L M ) ] ;  conspecific immature 
adults, [ ( I L M / I L M ) ]; and to those of 
similar stages of S. gregaria(SG) ,
[ ( I L M / S G ) ] .
P
he
ny
la
ce
to
ni
tr
ile
(p
g/
m
al
e/
ho
ur
)
165
1000 
800 
600 
400 
200 
0
0 5 10 15 20 25 30 35
Days a f te r  f ledging
Figure 32. Amounts(pg) of phenylacetonitrile 
produced by maturing males of L. 
migratoria(ILM) exposed to volatiles 
Of live conspecific fifth-instar 
nymphs (FLM) ,[ ( I L M / F L M )  ] immature adults 
[ ( I L M / I L M ) ] ,  and mature adults(MLM)
[ (ILM/MLM)]; and to those of fifth-instar 
(FSG),[(ILM/FSG)] and mature adults(MSG), 
[(ILM/MSG)] of S. gregaria.
166
Experiment 6. Effects of mature adult males of S. gregaria
on the sexual maturation of immature L. migratoria males
Young males of L. migratoria exposed to volatiles of 
mature adults of S. gregaria changed colour and mated later 
compared to those exposed to volatiles of both conspecific 
control immature adults, and mature adults (Fig. 31).
Immature adults of L. migratoria that were exposed to 
volatiles of mature adults of S. gregaria produced peak 
amounts of phenylacetonitrile also at 15 days after fledging 
which was 5 days earlier than those that were exposed to 
conspecific immature adult controls, but 10 days later than 
those exposed to volatiles of conspecific mature adults 
(Fig. 32) .
Thus, both fifth-instar nymphs and mature adults of S. 
gregaria delayed sexual maturation of immature L. migratoria.
Experiment 7. Effects of fifth-instar of S. gregaria on
the sexual maturation of immature L . migratoria females
Immature adult females of L. migratoria exposed to 
volatiles of fifth-instar nymphs of 5. gregaria matured 
significantly earlier (5-10 days), with respect to lengths of 
basal oocytes, than those exposed to volatiles of conspecific 
immature adults, but later (5-10 days) than those exposed to 
volatiles of conspecific fifth-instars (Fig. 33).
M
at
ur
at
io
n 
tim
e 
(d
ay
s)
167
30
20
10
Figure 33. Time taken by maturing adult females
of L. migratoria(ILM) to attain mature 
oocyte length when exposed to volatiles 
of conspecific(LM) fifth-instar and 
mature adults,[ ( I L M / L M ) ]; conspecific 
immature adults [ ( I L M / I L M ) ] ;  and to those 
of similar stages of S. gregaria (SG) ,
[(ILM/ISG)].
S 3  ILM /  LM 
■ I  ILM /  SG 
£Z2 ILM /  ILM a a—
i-----------------------------i--------- [
b
iiIIIIII !
E xposu re  to  E xp osu re  to
F i f th— in s ta r  matu re  adu lt
N ympha l vo la t i les  vo la t i les
Basal oocyte length
168
Experiment 8. Effects of mature adult males of L.
migratoria on the sexual maturation of immature S.
gregaria females
Immature adults of L. migratoria that had been exposed 
to volatiles of mature adults of S. gregaria also matured 
earlier than those exposed to conspecific immature adults (7- 
10 days) with respect to average basal oocyte length, but 
slightly later (3-5 days) than those exposed to conspecific 
mature adults (Fig. 33).
In summary, the effects of exposing the respective 
locusts to each other's volatiles are presented in comparison 
to the effects of conspecific immature adults first, then to 
conspecific nymphal or mature adult stages, respectively.
The following are the main effects observed:
1) - fifth-instar nymphs of L. migratoria accelerated 
the sexual maturation of immature S. gregaria. These 
acceleration effects were more obvious in pheromone 
production and ovulation. Mature adults produced variable 
effects in general, ranging from neutral to acceleration, and 
sometimes a delay.
2) - fifth-instar nymphs and mature adults of 5. 
gregaria retarded the sexual maturation of immature L. 
migratoria with fifth-instar causing more clear-cut 
retardation effects.
169
The present results show that fifth-instar nymphal 
volatiles of L. migratoria like those of mature males of 
S. gregaria accelerate sexual maturation of immature S. 
gregaria. This is consistent with the analyses of 
nymphal L. migratoria's volatiles which were shown to 
contain phenylacetonitrile (Torto unpublished) a compound 
also present in the volatiles of mature adults of L. 
migratoria, but in much smaller amounts compared to those 
in mature adults of S. gregaria. Phenylacetonitrile is 
one of maturation accelerants in maturing desert locusts 
(Mahamat and Hassanali, unpublished).
Fifth-instar nymphs of S. gregaria, in contrast to 
those of L. migratoria, retard the maturation of immature 
adults of both species. The effect differentiates 
between fifth-instar nymphal volatiles of S. gregaria and 
those of L. migratoria, which suggest that these stages 
of the two species, have different pheromonal blends.
Mature adults of L. migratoria did not have 
consistent effects on the maturation of immature S. 
gregaria whereas nymphs and mature adults of S. gregaria 
appeared to retard the maturation of immature L. 
migratoria. The cause of these effects are unknown, but 
it is possible that the differences in their volatile 
systems could again be the main one,
Mahamat et al.(1993) showed that in S. gregaria, 
aggregation pheromones play a dual role with regard to
6.5 Discussion
aggregation and acceleration of sexual maturation. Hence
t h e  ' a g g r e g a t i o n - m a t u r a t i o n - a c c e l e r a t i n g  p h e r o m o n e
(Obeng-Ofori et al. / 1993a; Mahamat et al./ 1993; Torto et 
al., 1994) system of S. gregaria. The roles of the 
aggregation pheromone in the maturation and other 
biological aspects of L. migratoria must await further 
research. The interspecific activity of pheromones on 
locust maturation (Norris, 1968; Loher, 1990) are clearly 
shown in these experiments. In contrast to what Norris 
(1964) postulated, after running mixed rearing maturation 
experiments, that mature adults of S. gregaria could 
accelerate maturation of young adults of L. migratoria. 
Studies on the effects of exposure to volatiles on the 
maturation of the respective locust species have shown 
that mature adults of the former species in most cases 
delay, rather than accelerate, the sexual maturation of 
immature adults of the latter. This is probably due to 
compositional and quantitative differences in their 
volatiles blends such that, high amounts of the 
maturation accelerant (phenylacetonitrile) tend to retard 
maturation of the migratory locust(sort of long term dose 
dose effect) . It was shown ealier (Chapter V, page 128) 
that mature male migratory locusts produced relatively 
little amounts of phenylacetonitrile, infering that only 
such amounts may be enough for aggregation and proper 
maturation, and that higher quantities could lead to 
abnormal effects such as the delay observed in 
maturation. It is possible that aggregation phei
170
jromones
>" "-Ii -cies as a maturation accelerant im
accordance to what was observed in S. gregaria by Mahamat 
et al.(unpublished). Thus, pheromones may also 
contribute to synchrony of maturation within and between 
populations of the respective species. Whitman (1990) 
pointed out that if stimulatory and inhibitory pheromonal 
effects operate in the field, the demographic 
consequences would be great especially in the 
synchronisation locust maturation and in phase 
transformation. It was observed in these studies that L. 
migratoria matures relatively faster than S. gregaria. 
This is an important difference between the two species 
which in the field suggests their separation even if they 
coexisted as mixed bands, or up to the immature adult 
stage. The desert locust and the migratory locust appear 
to differ in maturation such that, within similar stages, 
they tend to inhibit each other's sexual maturation 
whereas between stages they tend to either inhibit or 
accelerate each other's maturation. Thus the cross­
effects on maturation may be affected by conditions in 
which they coexist. The inhibitive interactions between 
the two locusts were also apparent in the primer effect 
studies on phase shift and development. The antagonistic 
primer effects between S. gregaria and L. migratoria have 
ecological significance in that they lead to species 
divergence in the field, thus to separation in space and 
time. Therefore, these locusts may not be seen in mixed 
swarms, unless by accident and for a limited period.
CHAPTER SEVEN
7.1 General discussion
The results of the present study confirm that the 
interactions between L. migratoria migratorioides and S. 
gregaria observed in the field are not fortuitous. 
Interspecific and intraspecific assays have shown that 
the signals from the two locust species can elicit 
aggregation responses from each other, and affect each 
other's phase shifts, development, and maturation. An 
important finding of these studies is the fact that the 
two species share some similar components in their 
volatile pheromone systems. It is pertinent to compare 
the chemical communication system of the two species and 
to discuss the implications of interactions in relation 
to their possible biological role.
General comparisons between the two species
The present study revealed several similarities and 
differences in the pheromone systems of S. gregaria and 
L. migratoria. In both species, there was no sex 
differentiation in either the production or responses to 
nymphal aggregation pheromone. In the adult stages, the 
older maturing males produce the adult aggregation 
pheromones in both species. Also, in both species, the
172
young adults use their fecal volatiles and those of 
nymphal conspecifics to aggregate. However, there are 
major sexual differences in the adults of the two 
species. First, whereas young adults of S. gregaria are 
indifferent to the conspecific nymphal pheromone, those 
of L. migratoria are actually repelled by pheromones of 
conspecific nymphs. Among mature adults of S. gregaria, 
there was no evidence of a volatile sex pheromone in 
females. However, in L. migratoria, the female attracted 
male conspecifics but not the females, indicating the 
mediation of a volatile sex pheromone in the gregarious 
phase of this species.
Differences between the two species are also 
apparent in the maturation process. L. migratoria 
matures relatively faster (7-13 days) than S. gregaria 
(18-30 days). Interestingly, whereas S. gregaria nymphal 
pheromone retards the maturation of conspecific young 
adults, that of L. migratoria accelerates it. The adult 
S. gregaria pheromone accelerates the maturation of 
conspecific young adults. The sequential retardation by 
the nymphal pheromone and acceleration by that of adults 
ensure maturation synchrony in the adults of this 
species. In L. migratoria, the male-produced adult 
pheromone also accelerates the maturation of young adults 
albeit to a lesser extent than that emitted by 
conspecific nymphs. Synchrony possibly arises from the
173
augmentative accelerated effects of the nymphal and 
mature adult pheromone systems.
The present study did not include the oviposition 
behaviour of L. migratoria nor the aggregation behaviour 
of first to fourth-instars nymphs of this species. These 
are, by themselves, whole areas of research, and 
difficult ones. However, future behavioural studies in 
such aspects will help throw light on the extent of 
similarities in the pheromone systems mediating behaviour 
in the two species.
Interactions between the two locusts
The present study has shown remarkable interspecific 
aggregation responses to pheromone emissions of the two 
species at both nymphal and adult stages. In particular, 
the nymphs responded to one another's pheromone without 
any discernable specificity. Likewise the young adults, 
which do not produce any significant amount of the adult 
pheromones, also responded to each other’s fecal 
volatiles. Among mature adults, S. gregaria was less 
responsive to volatiles of L. migratoria. However, the 
latter was less discriminatory. This suggests the 
existence of a specific factor in the volatile blend of 
L. migratoria not present in that of S. gregaria.
The existence of similarities as well as differences 
in the pheromone blends of the two species is also
174
suggested by the cross-primer gregarization and 
maturation effects of the two species. Solitarious 
individuals from both locust species gregarized when 
reared with gregarious groups of the other species. 
However, mixed rearing led to longer developmental cycles 
compared to rearing with conspecific gregarious locusts 
in both species. Thus though intra and interspecific 
locust pheromone systems contributed to gregarization, 
only conspecific pheromone leads to normal development.
Cross-maturation effects also reflect some 
differences in the pheromone systems of the two species. 
Thus, whereas the maturation of adult S. gregaria is 
retarded by conspecific nymphal pheromone, on the other 
hand, it is accelerated by that of L. migratoria. The 
maturation of adult L. migratoria is retarded by S. 
gregaria nymphal pheromone. Likewise, mature S. gregaria 
pheromone retards the maturation of young L. migratoria; 
but that of mature L . migratoria had no consistent 
effects on young adult S. gregaria.
Pheromone composition
The similarities or differences between locust 
pheromonal systems is of special interest. One of the 
key findings in these studies is that the two locust 
species share some components in their aggregation 
Pheromone systems. However, in contrast to what was
175
suggested in the past by other authors (Fuzeau-Braesh] 
1988; Whitman, 1990} there are important qualitative and 
quantitative differences between the pheromone systems of 
the two species. Analyses of volatiles had shown that 
both species produce phenylacetonitrile. It is however 
not produced by nymphs of S. gregaria while mature adult 
males produce it in relatively high amounts (Torto et 
al., 1994). In contrast to nymphal S. gregaria, both 
male and female nymphal L. migratoria produce it in 
substantial titres in their volatiles (Torto unpublished) 
whereas mature adult males produce it in very small 
amounts, nearly 100 times less than by mature male S. 
gregaria. Other aggregation compounds i.e. guaiacol, and 
phenol, are present in the volatiles from the respective 
locust species. However a detailed chemical study of L. 
migratoria needs to be conducted. The presence of 
substantial amounts of phenylacetonitrile in nymphal L. 
migratoria explains why fifth-instar nymphs of this 
species accelerated sexual maturation of immature males 
and females of S. gregaria.
Pheromone-mediated interspecific interactions: biological 
role
Some of the key findings described in this thesis 
revolve around the volatile (pheromonal) factors in these 
interactions. Therefore, it is pertinent to discuss 
briefly the importance of pheromones in locust biology
176
and in particular interspecific interactions that may be 
mediated by pheromones. Pheromone communication is 
probably the most important mode of communication in 
insects (Whitman, 1990). Many insects use pheromones for 
either simple communication within species or for 
interspecific defense, as in pyrgomorphids, against 
natural enemies. In most social insects e.g. termites, 
ants, bees, and locusts, the survival of the colony is 
dependent on these chemical messengers which have various 
functions: aggregation, dispersion, alarm, and defense. 
The role played by pheromones is primarily to ensure 
communication within species as a way of keeping 
populations either together or separated when necessary, 
and for species and mate recognition during courtship. 
However, in locusts, pheromones are the most important 
mediators of key attributes of the gregarious phase of 
the insect. In grasshoppers, chemical communication is 
less well known probably because of lack of research in 
this area. These insects also make extensive use of 
acoustics and vision besides olfactory signals within 
their habitats (Whitman, 1990).
Interactions between species occur when species 
become receptive to each other's semiochemicals, due to 
some ecological relationship or coevolution, such as in 
predator/prey, symbiotic, and mutualistic relationship 
(Jaffe et al., 1995). A list of compounds shared by 
grasshoppers and locusts was summarized by Whitman 
(1990). For example, phenol, guaiacol and benzaldehyde,
177
which are components of the male produced aggregation 
pheromone in S. gregaria (Torto et al., 1994) are also 
present in the defense secretions of Romalea sp., a very 
aposematic pyrgomorphid grasshopper. They are also found 
in the volatiles of P. viridipes. Interestingly, 
solitary S. gregaria reared with P. viridipes turned 
gregarious similar to those reared with L. migratoria.
The results on the cross reactivity of the aggregation 
pheromone systems of L. migratoria and S. gregaria 
account for the occurrence of mixed bands and mixed 
populations of immature adults and 5th-instar nymphs of 
the two locust species often observed in the field (El— 
Bashir and Abdel-Rahman, 1991; Johnston and Buxton, 1949; 
Torto, pers. comm.). I can only speculate on an 
adaptative value of these pheromone mediated interactions 
to both species. They share common ecological niches and 
outbreak areas. Solitarious S. gregaria and L. 
migratoria occupy arid to semi-arid zones where food 
source is scarce. They are in the constant need of 
finding new areas where they can develop and reproduce.
To achieve this, both species have developed phase 
polymorphism which allows them to transform their 
behaviour into migratory phase with long flight 
characteristics. These are important for recolonizing 
lost (formerly colonized) habitats and the discovery of 
new ones. Gregarization in both species is density- 
dependent. In .«? gregaria and L. migratoria, but
178
particularly in S. gregaria, populations occurs in very 
patchy vegetation with limited resources. Rapid 
gregarization and the resulting migratory capability of 
the gregarious phase are common to both species ano aj-low 
the insects access to a wide profile of food plants, and 
thus a rapid increase in numbers. Cross-facilitation of 
gregarization in the early stages when the densities of 
each species is low can thus be adaptatively advantageous 
to both species.
However, the question which arises is "how long would 
this situation persist in the field"? In breeding areas 
where green vegetation and moisture are found in restricted 
areas such as water courses, locusts are also forced to 
encounter each other and interact. This also occurs during 
hot or cold temperatures in the habitats when insects have 
to seek for refuge under such plants as Zygophyllum simplex 
L. or other shrubs or straw (Descamps, 1961a; Steedman,
1988; El-Bashir and Abdel-Rahman, 1991). Nymphs are more 
subject to these sorts of interactions because they cannot 
move over long distances. Long contact between the two 
species, however, may lead to mutual inhibition between 
different stages such that development and maturation 
between the two species are not synchronized, leading to 
divergence in developmental rates. Our studies on mixed 
rearing have shown that L. migratoria slows down nymphal 
development of S. gregaria; therefore, in mixed nymphal 
populations the former species tends to develop faster,
179
fledge into immature adult stage and swarm earlier than the 
latter. Thus, the occurrence of mixed bands is a temporary 
phenomenon which may last until locusts reach the soft 
immature stage, not beyond. Under optimal conditions, 
immature L. migratoria matures relatively faster than 
immature S. gregaria. If immature adults of the latter are 
in the presence of fifth-instar nymphs of the former, their 
sexual maturation is accelerated; if immature adults of the 
former are in the presence of fifth-instar nymphs of the 
latter, their maturation is delayed. Thus divergence also 
occurs in mixed populations that may persist to the 
fledgling stages.
Another limiting factor to prolonged coexistence 
(interactions) of the two species is their food habits. S. 
gregaria is a general feeder while L. migratoria is 
restricted to graminaceous plants, thus the two are likely 
to diverge into different habitats even in the nymphal 
stage.
Whether speculations outlined here represent a valid 
description of real events in the field must await more 
detailed ecological studies of mixed locust populations.
180
7.2 Recommendations
181
The preceeding discussions suggest the following lines 
for future research:
1) - Studies of the same scale as those by Obeng-Ofori et
al. (1993, 1994a;b), and Torto et al. (1994) need to 
be done for L. migratoria, L. pardalina and other 
locusts and grasshoppers, in order to have a better 
understanding of the possible cross influence 
between chemical communication systems in these 
species, and where they co-occur, locust phase 
dynamics and swarm formations;
2) - Detailed chemical studies involving the isolation
and characterization of aggregation pheromones of the 
migratory locusts, and their similarity or differences 
with those of the other grasshoppers and locusts, in 
order to have a more comprehensive understanding of 
the general effects of pheromones on the ecology of 
acridids in general.
3) - Similar to work done in S. gregaria (Hansson et al.,
1996; Anton and Hansson, 1996), physiological studies 
may be conducted to elucidate the detection of the 
aggregation pheromones in the central nervous system 
 i_2— — — ——  ' ratorioides.
REFERENCES cited
182
Ackowor, J.B., andVajime, C.K., 1995. Factors affecting 
Locusta migratoria migratorioides egg 
development and survival in Lake Tchad basin 
outbreak area. Int. J. Pest manag. 41 (2) 87-9 
Albrecht, F.O., 1964. Etat hygrometrique, coloration et 
resistance chez 1'imago de Locusta migratoria 
migratorioides (R & F). Experentia 20, 97-98. 
Albrecht, F.O., and Lauga, J. , 1978. Influence du
photoperiodism et de la temperature d'elevage sur la
morphologie et la phase de Locusta migratoria
migratorioides (R et F.) (Orthopteres, Acridiens) .
C.r.hebd. Seanc. Acad. Sci., Paris 289D, 1799-1801. 
Albrecht, F.O., and Lauga, J. , 1979. Effets de la 
photoperiode l'humidite sur le polymorphism de 
Locusta migratoria migratorioides (R et 
F.) (Orthopteres, Acridiens) eleve en isolement: 
etude morphologique des solitaires verts et bruns. 
C.r.hebd. Seanc. Acad. Sci., Paris 289D,753-755.
Albrecht, F.O.; Verdier, M. , and Blackith, RE, 1958.
Determination de la fertilite par l'effet de groupe
chez le criquet migrateur (Locusta migratoria
migratorioides R.et F.). Bull. biol. Fr. Belg.
92,349-427
Anton, S., and Hansson, B.S., 1996. Antennal lobe
interneurons in the desert locust Schistocerca 
gregaria (Forskal): Pocessing of aggregation 
pheromones in adult males and females. J. Comp 
Neurol. 370:85-96.
Assad, Y. O. H, 1995. Effects of some inter-and intra­
specific signals (Kairomones and pheromones on the 
maturation of the desert locust Schistocerca gregaria 
(Forsk.). (Orhtoptera: Acrididae). Ph.D thesis, 
University of Khartoum, Sudan; 222. pages.
Assad, Y. O.H.; Hassanali, A.; Torto, B.; Mahamat, H.;
Nabel, Hamed, Hassan, and Sayed El Bashir, 1997. 
Effects of fifth-instar nymphal volatiles on the 
sexual maturation of adult desert locust, Schi stocerca 
gregaria (Forskal) (Orthoptera Acrididae. J. Cham.
Ecol. (in press)
Ba-Angood, S.A.S., 1976. Preliminary observations on the 
effects of crowdness by grasshoppers on phase 
development of the desert locust, Schistocerca 
gregaria (Forsk). Z. Ang. Entomol., 80 : 228-231
Barnes, O.L., 1955. Effects of food plants on the Lesser 
migratory grasshopper. J. Econ. Entomol. 48 (2) : 
119-124
183
Bouaichi, A.; Roessingh, P.; and Simpson, S. J. , 1995.
An analysis of the behavioural effects of crowding 
and re-isolation of solitary reared adult desert 
locusts(Schistocerca gregaria) and their offspring. 
Physiol. Entomol., 20 (3) : 199-208.
Borror, D.J.; Delong,D.M.; and Tripplehorn, C.A., 1976.
An Introduction to the study of insects. 4th ed. 
Library of Congress cat. Publ.
Brown, C.R.; and Lockwood, J.A., 1994. Olfactory basis 
of cannibalism in grasshoppers (Orthoptera :
Acrididae) in laboratory assessment of attractants.
J. Chem. Ecol. 20 (9): 2249-2260 
Brown, M.W., Hoguire, H.W., and Schmitt, J.J., 1995.
Competitive displacement of apple aphid by Spirea 
aphid(Homoptera: Aphididae) on apple as mediated by 
human activities. Environ. Entomol. 24 (6) : 1581- 
1591.
Brust, E.G.; Stinner, B.R., and Mccartney, D., A., 1986. 
Predator activity in corn agrosystems. Environ. 
Entomol. 15 (4):1017-1021.
Carlisle, D.B., and Ellis, P.E., 1962. Endocrine glands 
and phase in locusts. 4e congres. Un. Int. Etud. 
Insect. Sociaux. Pavia 1961. 10 : 219-224.
Carlisle, D.B., and Ellis, P.E., 1965. The influence of 
aromatic shrubs on sexual maturation in the desert 
locust Schistocerca gregaria. J. insect. Physiol. 11
Carlisle, D.B., and Ellis, P.E., 1967. Synchronization 
of sexual maturation in desert locust swarms. 5e 
Un.int1. Etudes Insect. Soc., Toulouse (1965) : 63- 
67.
Chapman, R.F., 1979. A biology of locusts. Studies in 
Biology No 71. Edwards Arnold Publisher Ltd.67 pp.
Chauvin, R., 1941. Contribution a 1'etude physiologique 
du criquet Pelerin et du determinisme des phenomenes 
gregaires. Ann. Soc. Entomol. France. 1 : 1-173.
Davey, J.T., and Johnston, H.B., 1956. The African
Migratory Locust. Anti-locust Bulletin No 22, 93 
pp. ALRC 1, Prince Gate London, S.W.7
Deng, A.L., 1995. Studies on some factors that influence 
phase dynamics of the desert locust, Schistocerca 
gregaria (Forskal) (Orthoptera : Acrididae). Ph.D 
Thesis, University of Khartoum, Sudan. 177 pages.
Deng, A.L.; Torto, B. Hassanali A., and Ali, E.E., 1996.
Effects of shifting to crowded or solitary conditions 
on pheromone release and morphometries of the desert 
locust, Schistocerca gregaria (Forskal) (Orthoptera : 
Acrididae). J. Insect. Physiol. 42 (8) 771-776.
Descamps, M. , and Wintrebert, D. , 1961. Quelques
remarques de recherches preliminaires a propos des 
criteres de maturite sexuelle et de ponte chez les 
femelles d'Acridiens migrateurs. Revue de pathologie 
vegetale et d'Entomolgie Agricole. No 3 : 1-143.
185
Descamps, M., 1961a. Notes sur le comportement, dans les  ^
conditions naturelles, des larves solitaires 
Locusta migratoria migratorioides R & F.(Orth 
Acrididae). Revue de pathologie vegetale et 
d'Entomolgie Agricole No 3 : 144-186.
Descamps, M . , 1961b. Cycle biologique de Gastrimargus
nigericus Uv. (Orth. : Acrididae) dans la vallee du 
Bani (Mali). Revue de pathologie vegetale et 
d'Entomolgie Agricole No 3 : 187-199.
Dietrich, C.A. , Ted, H.E.,and Deitz, 1.1., 1991.
Morphometric discrimination among females of sibling 
species of Aconophorini (Homoptera : Membracidae). 
System. Entomol. 16 : 311-318 
Dirsh, V.M., 1953. Morphological studies on phases of 
the desert locust (Schistocerca gregaria Forskal). 
Anti-Locust Bull. No 16, 34 pp.
Doumbia, L., 1994. Effects of Melia azedarach on nymphs 
of the desert locust S. gregaria (F). Sahel P.V 
Info No 60: 2-10.
Dudley, B., 1964. The effects of temperature and 
humidity upon certain morphometric and colour 
characters of the desert locust (Schistocerca 
gregaria Forsk.) reared under controlled conditions. 
Trans. R. Entomol. Soc. Lon. 116 (6) : 115-129 
Duranton, J.F., et Lecoq, M. , 1990. Le Cri miPt P s W i n  
a.U_.S.&hel. Ministere des Affaires Etrangeres Pays 
Bas et Cirad/Prifas France.
El-Bashir, S., a n d  Abdel-Rahman, H.E., 1991. Factors 
inducing phase change in the desert locust, 
Schistocerca gregaria (F) at recession and breeding 
habitats. 19th ICIPE Annual Report. ICIPE Science 
Press.
Ellis, P.E., 1959. Learning and social aggregation in 
locust hoppers. Anim. Behav. 7 : 91-106.
Ellis, P.E., and Carlisle, D.B., 1965. Desert locust: 
sexual maturation delayed by feeding on senescent 
vegetation. Science 149 : 546-547
Ellis, P.E., and Pearce, A., 1962. Innate and learned 
behaviour patterns that lead to group formation in 
locust hoppers. Anim. Behav. 10 : 305-318.
Everts, J.W., 1990. Effets sur 1'Environnement de la
lutte antiacridienne. Rapport de Projet. FAO, Projet 
ECLO/SEN/003 NET. A Pilot study PP 312
Ferenz, H.J.; Luber, K.; Wieting, J., 1994. Pheromones
as a means of controlling migratory locusts. In New 
trends in locust control (Ecotoxicology. Botanical s. 
Pathogens. Attractants. Hormones. Pheromones. Remote 
sensing). GTZ. Postfach5180,65726 Eschborn, Germany. 
Faure, J.C., 1942. The phase of locusts in South Africa.
Bull. Entomol. Res. 23, 293-424 
Fuzeau-Braesch, s,, 1972. Pigments and colour changes. 
Ann. Rev. Entomol. 17, 403-424.
187
Fuzeau-Braesch, S., Genin, E., Jullien, R., Knowles, E., 
and Papin, C., 1988. Composition and role of 
volatile substances surrounding two gregarious 
locusts, Locusta migratoria and Schistocerca 
gregaria. J. Chem. Ecol. 14 (3) : 1023-1033.
Gillett, S.D., 1968. An airborne factor affecting the
grouping behaviour of locusts. Nature 218 : 782-783.
Gillett, S.D., 1975. Changes in social behaviour of the 
desert locust, Schistocerca gregaria, in response to 
gregarizing pheromone. Anim. Behav. 23 : 494-503.
Gillett, S.D.; Packham, J.M.; and Papworth, S.J., 1976. 
Possible pheromonal effects on aggregation and 
dispersion in the desert locust, Schistocerca gregaria 
(Forsk.). Acrida 4 : 287-298.
Gillett, S.D., and Phillips, M.L., 1977. Faeces as a
source of a locust gregarizing stimulus. Effects on 
social aggregation and on cuticular colour of nymphs 
of the desert locust, Schistocerca gregaria 
(Forskal). Acrida 6 : 279-286.
Gillett, S.D., 1983. Primer pheromones and polymorphism 
in the desert locust. Behav. 31 : 221-230.
Gunn, D.L., and Hunter-Jones, 1952. Laboratory
experiments on phase differences in locusts. Anti- 
Locust Bull., 12 : 1-29.
188
Hanson, B.S.; Ochieng, S.A.; Grosmaitre, X. ; Anton, S., 
and Njagi, P.G.N., 1996. Physiological responses 
and central nervous projections of antennal 
olfactory receptor neurons in the adult deserl 
locust, Schistocerca gregaria (Orthoptera:
Acrididae). J. Comp. Physiol. A 179:157-167
Heinz, K.M.; and Nelson, J.M., 1996. Interspecific
interactions among natural enemies of Bemisia in an 
inundative biological control program. Biocontrol, 
6(3) : 394-406.
Hemming, C.F., and Symmons, P.M., 1969. The germination 
and growth of Schouwwia purpurea (Forsk.) Sweinf and 
its role as a habitat of the desert locust. Anti- 
Locust Bull. No 46. 43 PP.
Herok, C.A.; and Krall, S., 1995. Economics of desert 
locust control. GTZ Postfach 5180 D 65726 Eschborn 
Germany.
Highnam, K.C.; and Haskell, P.T., 1964. The endocrine 
system of isolated and crowded Locusta and 
Schistocerca in relation to oocyte growth and the 
effects of flying upon maturation. J. Insect. 
Physiol. 10 : 849-864.
Hunter-Jones, P., 1958. Laboratory studies on the
inheritence of phase characters in locusts. Anti-
Locust Bull. 29 : 35 pp. ALRC, 1 Prince Gate, S.W.7 
London.
189
Injeyan, H.S., and Tobe, S.S., 1981. Phase polymorphism
in Schistocerca gregaria-. Reproductive parameters. J. 
Insect. Physiol. 27 : 97-102.
Islam, M.S; Roessingh, P.; Simpson, J.J.; and McCaffery,
A.R., 1995. Effects of population density
experienced by parents during during mating and 
oviposition on the phase of hatchling desert 
locusts, Schistocerca gregaria. Proceedings of the 
Royal Society of London. Series B, Biological 
Sciences, 257 (1348) : 93-98.
Jackson, G.T., Popov, G.B.,Ibrahim, A.O., Alghamedi,
S.A., and Khan, A.M., 1978. Effects of food plants 
on the development, maturation, fecundity and phase 
of the desert locust, Schi stocerca gregaria 
(Forskal). COPR. London and LRS/ARS Jeddah, pp 1-17.
Jaffe, K.; Ramos, C.; and Issa, S., 1995. Trophic
interactions between ants and termites that share 
common nests. Ann. Entomol. Soc. Amer. 88 (3) : 328-
333.
Joffe, S.R., 1995. Desert locust management. A time for 
change. Washington, USA. World Bank. World Bank 
Discussion papers, No 284, xi + 58 PP.
Johnston, H.B., and Buxton, D.R., 1949. Field
observations on locusts in East Africa Anti-Locust
Bull., 5 pp. ALRC. British Museum (Natural History). 
London S.W 7.
190
Kennedy, J.S., 1956. Phase transformation in locust 
biology. Biol. Rev. 31 : 349-370.
L a n g e w a l d ,  J., a n d  S c h m u t t e r e r , H., 1992. Effects of 
neem oil treatments on the phase status of the 
desert locust, S. gregaria. In Proceedings 
workshop, IITA, Cotonou, by Lomer, C.J., and Prior, 
C., 1992.
Launois, M. , and Launois-Luong, M.H., 1989. Oedaleus 
senegalensis (Krauss, 1877) sauteriau ravaaeur du 
Sahel. Ministeres des affaires etrangeres des Pays- 
Bas. ClRAD/PRI FAS, FRANCE.
Leal, S. W. , Kawamura, F., and Ono, M., 1994. The Scarab 
beetles Anomala albipilosa sakishimana utilizes 
the same sex pheromone blend as a closely related 
and geographically isolated species, Anomala cuprea. 
J. Cham. Ecol. 20 (7) : 1667-1676.
Lean, O.B., 1931. On the recent swarming of Locusta
migratorioides (R & F) . Bull. Entomol. Res. 22 : 
365-378.
Loher, W . , 1960. The chemical acceleration of the
maturation process and its hormonal control in the 
male of the desert locust. Proc. Roy. Soc. B. 153 : 
380-397.
Loher, W., 1990. Pheromones and phase transformation in 
locusts. In. Biology 0f Grasshoppers. Chapman, R.F. 
and Joern, A., 1990. pp. 337-355.
191
Lomer, C.J., and Prior, C., 1992. Biological CQntrgl.p.f
1 nrnsts and arasshoPB££5.- Proceedings workshop IITA, 
Cotonou. CAB Int., IITA. PP 394 .
Mahamat, H. ; Hassanali, A., andMunyinyi, D., 1996.
Haemolymph pigments composition as a chemometric
indication of phase in the desert locust
Schistocerca gregaria (F) . Phytochem. (in press)
Mahamat, H., Hassanali, A., Odongo, H., Torto, B. , and
El-Bashir, E.S., 1993. Studies on maturation -
accelerating pheromone of the desert locust
Schistocerca gregaria (Orthoptera: Acrididae).
Chemoecol. 4 : 159-164.
Meinzingen, W.F., 1993. A guide to migrant pest
management in Africa. FAO Rome AGP.
Michel, R., 1980. Development of flight behaviour of
successive generations of the desert locust,
Schistocerca gregaria (Forskal) raised in isolation
then in group. Anim. Behav., 28 (4) : 1288-1289.
Mishra, P.K.; and Singh, R.P., 1992. Antifeedant
efficacy of neem (Azadirachta indica A.Juss) seed
kernels, seed coat, and fallen leave extracts
against the desert locust, Schistocerca gregaria.
Indian Journ. Entomol., 54 (1) : 89-96.
Nickerson, B., 1958. Pigmentation of hoppers of the
desert locust (Shistocerca gregaria Forsk.) in
relation to phase colouration. Anti-Locust bull. No 
24. ALRC. London.
192
Ngi-song, A.J., Overholt, W.A. , and Ayertey, J.N., 1995.
Suitability of African gramineacious steroborers for 
development of Cotesia flavipes and C. sesamiae 
(Hymenoptera: Braconidae). Environ. Entomol. 24 (4) 
978-984.
Njagi, P.G.N; Torto, B.; Obeng-Ofori, D., and Hassanali, 
A., 1996. Phase-independent responses to phase-
specific aggregation pheromone in adult desert 
locusts, Schistocerca gregaria (Orthoptera: 
Acrididae). Physiol. Entomol. 21, 131-137.
Nolte, D.J., 1963. A pheromone for melanization of 
locusts. Nature, 200 : 660-661.
4
Nolte, D.J., 1970. The gregarization pheromone.
Proceedings Int. conf. Current and Future Problems 
of Acridol. London pp 85-92.
Nolte, D.J., 1973. The relationship of locust species 
to locus to1. Acrida 2 : 53-60 
Nolte, D.J., 1974. The gregarization of locusts. Biol.
Rev. 49 : 1-14.
Nolte, D.J., 1977. The action of Locustol. J. Insect.
Physiol., 23 : 899-903.
Norris, M.J., 1950. Reproduction in the African
migrattory locust (Locusta migratoria migratorioides 
R & F ) in relation to density and phase. Anti- 
Locust Bull. No 6 pp 47.
Norris, M.J., 1952. R e p r o d u c t i o n  i n  t h e  d e s e r t  l o c u s t  
(Schistocerca gregaria F o r s k . )  i n  r e l a t i o n  t o  
d e n s i t y  a n d  p h a s e .  Anti—Locust Bull. London, No 13 
: 2 + 4 9 p p
Norris, M.J., 1954. Sexual maturation in the desert
locust[Schistocerca gregaria Forskal) with special 
reference to the effects of grouping. Anti-Locust 
Bull. No 18. ALRC.
Norris, M.J., 1957. Factors affecting the rate of sexual 
maturation of the desert locust (Schistocerca 
gregaria Forsk.) in the laboratory. Anti-Locust 
Bull. 28. ALRC, London. 2 6 pp
Norris, M.JJ, 1964. Accelerating and inhibiting effects 
of crowding on sexual maturation in two species of 
locusts. Nature, 203 (4946) : 784-785.
Norris, M.J., 1968. Laboratory experiments on oviposition 
responses of the Desert Locust, Schistocerca 
gregaria(Forsk.). Anti Locust Bull., 43.
Obeng-Ofori, D,, 1993. The behaviour of 9 stored product 
beetles at pitfall trap arenas and their capture in 
millet. Entomol. Exp. Appl. 66 : 161-169.
Obeng-Ofori, D., Torto, B. , and Hassanali, A., 1993. 
Evidence of mediation of two releaser pheromones 
in aggregation behaviour of the gregarious desert 
locust Schistocerca gregaria (Orthoptera:
Acrididae). J. chem. Ecol., 19 (8) : 1665-1676.
194
Obeng-Ofori, D., Njagi, P.G.N., Torto, B., Hassanali, A., 
andAmiami, H., 1994a. Sex differentiation studies 
relating to releaser aggregation pheromones of the 
gregarious desert locust Schistocerca gregaria 
(Orthoptera: Acrididae). Entomol. Exp. Appl. 73 : 85- 
91.
Obeng-Ofori, D. ; Torto, B.; Njagi, P.G.N.; Hassanali, A. ;
and Amiani, H., 1994b. Fecal volatiles as part of the 
aggregation pheromone complex of the desert locust, S. 
gregaria (Forskal) (Orthoptera: Acrididae). J.Chem. 
Ecol. 20 (8) : 2077-2087-
Ochieng-Odero, J.P.R.; Ndugo, S.M.; El Bashir, S.; and
Capstick, P.B., 1991. A method for rearing crowded 
(gregarious) and isolated (solitary) locusts 
(Orthoptera: Acrididae) in the laboratory.
Proceedings workshop. ICIPE. Nairobi.
Odum, E.P., 1971. Fundamentals of Ecology. 3rd Ed.
W.B.Saunders Company W. Washington Square Phil. PA 
19105., USA.
Papillon, M. , 1960. Etude preliminaire de la
repercussion du groupement des parents sur les 
larves nouveau-nees de Schistocerca gregaria Forsk. 
Bull. Biol. Fr. Bel. 93, 203-263.
Pasquier, 1950. Sur les causes de la gregarization chez 
les acridiens: la densation. Ann. Inst, agric.
Alger. 5 (9) :l-9.
Pedgley, D., 1981. Ee M a _ l a a iaL J flm : »5ttna 
, UK.
195
Pener, M.P., 1991. Locust phase polymorphism and its
endocrine relations. Advances in Insect Physiol.,
23 : 1-79.
Perfecto, I.Horwith, B. ; Vandermeer, J. , Schultz, B. ;
Mcguinness, H.; and Dos Santos, A., 1996. Effects of 
plant diversity and density on the emigration rate of 
two ground beetles, Harpalus pensylvanicus, and 
Evarthrus sodalis (Coleoptera : Carabidae) in systems 
of tomatoes and beans. J. Environ. Entomol. 15 (5) :
1028-1031.
Phillips, T. W. , Jiang, L. X., Burkholder, W. E.,
Phillips, J. K. , and Tran, H. Q. 1993. Behavioural 
responses to food volatiles by two species of stored 
product Coleoptera, Sitophilus oryzae(Curculionidae) 
and Tribolium castaneum (Tenebrionidae). J. Chem. 
Ecol.19 (4) : 723-734.
Popov, G.B., 1954. Notes on the behaviour of swarms of 
the desert locust (Schistocerca gregaria Forsk}, 
during oviposition in Iran. Trans. R.Entomol. Soc. 
Lond. 105 : 65-77.
Popov, G.B., 1969. Rapport de la mission au Niger et au 
Mali. 1968. Rapport de 1 1avancement des travaux. 
Station de recherches sur le terrain. FAO, Rome, 
Series techniques (Doc. multigr.).
196
Popov, G.B., Duranton, J.F., and Gigault, J. , 1991.
E&idss proloai crues des biotope_s du , Criguet Peleri_n_  
Sr.hi stonerca grecraria (Forskal, 1775J— en Afr.igue—
Nnrri Orr.i.dfintale: Mise en evidence et descrip_tion_ 
das unites territoriales ecoloaiguement homoaenes. 
CIRAD/PRIFAS. Ministere de la cooperation et 
developpement/Commission des Communautes 
Europeennes:Bruxelles/FAO : ROME/CIRAD 
PRIFAS:MONTPELLIER.
Price, P.W., 1984. Insect Ecology. 2nd Ed. John Wiley & 
Sons Inc. PP 1-181.
Price, R.E.; and Brown, H.D., 1990. Reproductive
performance of the African migratory locust, Locusta 
migratoria migratorioides (Orthoptera : Acrididae), 
in a cereal crop environment in South Africa. Bull. 
Entomol. Res. 80 : 465-472.
Price, R.E.; and Brown, H.D., 1992. Incubation and 
overwintering of the egg stage of the African 
migratory locust, L. migratoria migratorioides 
(Orthoptera :Acrididae) , on the Highveld of South 
Africa. Bull. Entomol. Res. 82 : 235-241.
Roffey, J., 1969. Radar studies on the desert locust, 
Schistocerca gregaria, Forsk., In the Niger 
Republic. September-October, 1969. Anti-Locust Res. 
Cent. Occ. Rep., 17 : 14 pp.
Roffey, J., and Popov, G.B., 1968. Environmental and
behavioural processes in a desert locust outbreak.
219 ; 44 6-44 9.
197
Rowell, C.H.F., 1970. E n v i r o n m e n t a l  c o n t r o l  of
colouration in an Acridide, Gastrimargus africanus 
(Saussure). Anti-Locust Bull.47.ALRC(London W.8).
Rungs, C., 1954. Une nouvelle representation graphique
de la gregarisation du Criquet Pelerin, Schistocerca 
gregaria Forsk.C.R. Societe des Sciences 
Naturelles et Physiques du Maroc 6 : 130-132.
Rupasas, H.R.; Bottrell, D.G., and Coll, M . , 1996.
Intraspecific variations in chemical attraction of 
the rice to insect predators. Biocont. 6 (3) 394- 
406.
SAS Institute, 1988. Users guide: Statistics. SAS
Institute, Release 6.03 edition. Cary, N.C. USA.
102 8 pp.
Saini, R.K., and Hassanali, A., 1991. Effective and 
sustainable ecotechnology development with a 
specific example of tsetse control. Proceedings 
Community-based and environmentally safe Pest 
Management. Saini, R.K., and Haskell, P.T. ICIPE 
Sciences Press, pp. 49-58
Saini, R.K.; Rai, M.M. ; Hassanali, A.; Wawiye,j. * Odongo, 
H., 1995. Semiochemicals from froth of egg pods 
attract ovipositing female S. gregaria. J. Insect. 
Physiol. 41 (8) : 711-716
198
Schmutterer, H.; Baumgart M . ; Freisewinkel, D.,
Langenwald, J.; and Nicol, C.M.Y., 1993. The 
effects of neem oil and other neem products on 
nymphs and resulting adults of Schistocerca 
gregaria, Nomadacris septemfasciata, Locusta 
migratoria migratorioides, and Zonocerus variegatus. 
J. Appl. Entomol. 116 : 178-186.
Shaeffer, G.W., 1972. Radar detection of individua 
locusts and swarms. In : Hemming C.F. and Tayl 
T.H.C., (Ed.Sc.). Proceedings Int. Study Conf. 
Current anf future problems of Acridology. Centre 
for overseas Pest Research (COPR), London : 379-380
Showier, A.T., 1995. Locust (Orthoptera: Acrididae) 
outbreak in Africa and Asia, 1992-1994 : an 
overview. Amer. Entomol. 1 (3): 179-185.
Steedman, A., 1988. Locust handbook (2nd edition)
Overseas Development Natural Ressource Institute, 
London. 180pp.
Stein, S.J.; and Price, P.W. , 1995. Relative effects of 
plant resistance on natural enemies by plant 
development age on sawfly (Hymenoptera: 
Tenthredinidae) preference and performance. Environ. 
Entomol. 24 (4) : 906-916.
Stower, W.J., 1959. The colour patterns of hoppers of
the desert locust (S. gregaria Forsk.). Anti-Locust 
Bull. No 32. ALRC London.
199
Thomas, M.D.; Wood, S.N.; Lomer, C.J., 1995. Biological 
Control of locusts and grasshoppers using a fungal 
pathogen : the importance of secondary cycling. 
Proceedings of the Royal Society of London. Series B. 
Biological Sciences, 259 (1356) : 265-270.
Torto, B.; Njagi, P.G.N.; Hassanali, A.; andAmiami, H., 
1996. Aggregation pheromone system of nymphal 
gregarious desert locust, S. gregaria (Forskal). J. 
Chem. Ecol. 22 (12) : 2273-2281
Torto B.; Obeng-Ofori, D.; Njagi, P.G.N.; Hassanali, A.,
and Arniami, H., 1994. Aggregation pheromone system of 
adult gregarious desert locust Schistocerca gregaria 
(Forskal). J. Chem. Ecol. 20 (7) : 1749-1762.
Toye, A.S., 1973. Effects of food on the development of 
the desert locust Schistocerca gregaria (Forskal).
J. Entomol. (A) 48 (1) : 95-102.
Uvarov, B.P., 1921. A Revision of the genus Locusta, L. 
{-Pachytylus Fieb.), with a new theory as to the 
periodicity and migrations of locusts. Bull. Entomol. 
Res. 12, 135-63 
Uvarov, B.P., 1923. A Revision of the old world 
Cyrtacanthacridinae (Orthoptera, Acrididae)
I,II,III. Ann. Mag. Nat. Hist. 11 (9) : 130-144; 
473-490.
Uvarov, B.P., 1966. grasshoppers and locusts. Cambridge 
University Press, Cambridge. 481 pp.
200
Wiedenman, R.N.; Smith, J.W. Jr.; and Darnell, P.O.,
1992. Laboratory rearing and biology of the parasite 
Cotesia flavipes (Hymenoptera:Braconidae) using 
Diatrea saccharalis (Lepidoptera : Pyralidae) as a 
host. Env. Entomol. 21 (5) 1160-1167.
Whitman, D.W., 1990. Grasshopper chemical communication.
In Biology of Grasshoppers. Chapman, R.F., and Joern, 
A., 1990. pp.357-361.
Whitman, D.W.; Blum, S.M.; and Slansky, F.Jr., 1994.
Carnivory in phytophagous insects. In Functional 
dynamics of Phytophagous insects by Ananthakrisnan, 
T.N., 1994 pp 161-205.
Zolotarevsky, B., 1938. Recherches sur les foyers
gregarigenes du criquet migrateur Africain (Locusta 
migratoria migratorioides Reich and Farmaire,
Orth.). Rapport de mission. Bull. Soc. D'histoire 
Naturelle Afrique du Nord. 20 : 122-240.
201
202
appendices
203
Appendix I. Number of S. gregaria (SG) or L. migratoria (LM) (out of 100 tested) 
that aggregated in treated and non treated (Blank) sides in the two 
choice olfactometer.
COMBINATIONS
Doses of source (LEQ)
3 7 10 20 40
Source / Recipient Fifth -instar nymphs
LM / SG 58 + 8 a 62 ± 6 a 66 + 7 a 74 + 6 a 60 + 8 a
SG / SG 66 + 9 a 72 ± 5 a 74 + 7 a 68 + 7 a 68 + 7 a
Blank / SG 36 + 7 b 36 ± 7 b 34 + 7 b 24 + 3 b 36 + 7 b
SG / LM 72 + 6 a 78 ± 5 a 74 + 9 a 74 + 5 a 78 + 7 a
LM / LM 56 + 7 b 68 ± 5 a 77 + 5 a 80 + 4 a 66 + 7 a
blank / LM 22 + 4 c 20 ± 4 b 20 + 7 b 16 + 4 b 20 + 7 b
Mature adults
LM / SG 52 + 8 a 70 ± 7 a 72 + 4 a 66 + 7 b 62 + 11 a
SG / SG 64 + 7 a 76 ± 7 a 82 + 5 a 88 + 4 a 62 + 7 a
Blank / SG 36 + 9 b 22 ± 6 b 24 + 5 b 22 + 6 c 36 + 10 b
SG / LM 66 + 7 a 74 ± 6 a 78 + 5 a 86 + 4 a 66 + 7 a
LM / LM 56 + 5 a 72 ± 7 a 82 + 6 a 76 + 6 a 72 + 4 a
Blank / LM 22 + 6 b 20 ± 4 b 18 + 5 b 8 + 4 b 28 + 6 b
204
Appendix II. Average responses (% ± SE) of nymphs and mature adults of S.
gregaria (SGR) and L. m . migratorioides (LMI) to each other's 
volatiles in the olfactometer.
Stage Treatment (Source / Test) combinations
Fifth-instar 
Mature adults
LMI / LMI SGR / LMI SGR / SGR LMI / SGR
46 ± 7 ab 
53 ± 8 a
56 ± 8 a 
52 ± 8 a
40 ± 10 ab 
37 ± 10 b
31 ± 10 b
55 ± 8 a
In lines and columns, means followed by the same letter are not 
significantly different (P < 0.05 LSD test). Treatments are compared 
horizontally, and stages vertically (80 replicates of 10 min each).
Appendix III. Dose responses of nymphal and mature adult locusts
of species S. gregaria and L . migratoria in a two choice 
olfactometer.
Locust
Stage
Doses (LEQ)
3 7 10 20 40
Nymphs
Mature adults
33 ± 10 be 
31 ± 10 c
43 ± 7 ab 
51 ± 8 b
48 ± 9 ab 
64 ± 8 ab
54 ± 7 a 
66 ± 7 a
39 ± 10 ab 
36 ± 10 C
Within each line and column, means followed by the same letter are not 
significantly different ( P < 0.05 LSD test).
205
Appendix IV. Densities of crowded L. m. migratorioides (LM) in relation to the 
gregarization of each isolated S. gregaria (SG) under study.
NYMPHAL STAGE ADULT STAGE
Density Density of Density Eyes of test SGR Phase of test
No Sexe of LM test SG of adult SG
LM
stripes shading
1 Female 273 5 154 7 Transiens
2* Male 117 5 117 6 3 Transiens
3 Female 71 5 66 7 - ii
4 Female 120 5 33 7 - ii
5 Male 122 5 57 7 3 ii
6 Male 200 5 185 6 5 gregarious
7 Male 400 5 244 6 3 ii
8 Female 200 - 200 7 -
9 Male 131 5 108 6 1 transiens
10 Male 200 5 154 7 3 ii
11 Male 400 5 200 6 3 gregarious
12 Male 80 2 120 7 1 Solitary
13 Male 80 3 . 5 120 7 1 Solitary
14 Male 80 3 . 5 80 7 1 Solitary
15 Male 80 2 80 7 1 Solitary
16 Male 80 5 80 6 3 gregarious
17 Female 80 5 80 6 3 II
18 Female 80 5 80 6 3 II 1
19 Female 80 2 80 7 1 Solitary
Mean h SE 152 ± 24 4 ± 0 . 3 118 + 13 6.6 ± .1 2.2 ± 0.4 Transiens
206
Appendix V. Developmental time of s. gregaria (SG) reared with L. migratoria 
migratorioides (LM), P. viridipes (PV) and E . plorans (EP), 
exposed (:) to volatiles of L. migratoria, and in controls.
Trts Nymphal instars Im. ad Total
S/C 1 2 3 4 5 6 Total
SG/LM 6 + lc 5 + lab 8 ± lab 8 ± lab 9 + 2b 10 + 2a 46 + 3a 29 + 2b 75 + 6.4 b
SG/PV 15 + 0a 4 + lbc 7 ± lb 9 ± la 9 + lb 11 + 2a 45 + 3a 54 + 10a 99 + 10 a*
SG/EP 5 + 0c 6 + la 9 ± Oab 6 ± 2b 12 + la 9 + la 47 + 3a 8 + Id 55 + 3 c**
SG: LM 10 + lb 6 + la 10 ± la 7 ± 2ab 8 + 2b 9 + 2a 50 + 4a 29 + 6c 79 + 8 b
GLM 6 + lc 4 + lbc 7 ± lb 6 ± lb 10 + lb . 33 + 2b 12 + Id 45 + 3 c
GSG 5 + led 5 + lab 5 ± lc 5 ± lb 8 + lb . 27 + 2c 22 + lc 49 + 2 c
SSG 4 + Od 5 + lab 4 + lc 3 ± lc 7 + lb 12 + la 32 ± lb . .
SLM 4 + Od 3 ± 0c 4 ± 0c 8 ± 2ab 12 + la ■ 31 + lb • •
SSG: solitary control; Trts = treatment combinations; S: solitarious test 
locust; / : over. C : gregarious source. In columns, means followed by the same 
letter are not significantly different (P < 0.05, LSD-test). Im. ad = immature 
adult; * : 50% reached maturation colour in 34 ± 3 days. The other 50% never did by 
the end of the experiment. ** : died 8 days after fledging
207
Time (min)
Appendix VI. Chromatograms showing phenylacetonitrile (PAN) 
produced by gregarious fifth- instar nymphal 
male (A) and female (B) L. migratoria 3 - 4  
days after molt (50m, 0.2mm ID Carbowax 
column).
208
Appendix VII : Compounds identified from the volatiles of adult P. viridipes Staal,
1873 used in the studies of primer effects of pheromones ( + means 
that the compound is found in that species ).
Compound P. viridipes L. migratoria S . gregaria
Benzaldehyde + + +
Pentanoic acid +
Hexanoic acid +
Guaiacol + + +
Heptanoic acid +
Phenol + + +
Octanoic acid +
Nonanoic acid +
Decanoic acid +
209
Appendix VIII. Percent (%) of S. gregaria showing phase change after rearing with 
different stages of L. migratoria
Parameters Nymph/Nymph Immat. /Immat. Immat./Nymph Nymph : Nymph
M F Tot. M F Tot. M F Tot. M F Tot.
Body colour 75 57 69 100 32 89 80 90 75 100 100 100
Morphometries 27 38 33 • . . . . . 50 50 50
Haemolymph 80 100 81 • . . . . . . •
PAN * * * * * * * * * *
Eye stripes 44 6 50 . . . . .  . 50 0 25
Eye shading 100 13 56 80 75 78 100 100 100 • • .
/ = mixed rearing; : = exposed to volatiles of L.m. migratorioides. M = males 
F = Females. PAN = phenylacetonitrile. * : males were grouped for pheromone 
collection; they produced the pheromone; . : parameter not monitored for this 
experiment. Tot. : total immat. = immature adult
210
Appendix IX. Mean basal oocyte lengths in maturing females of S. gregaria (ISG)
and L. m . migratorioides exposed to volatiles of fifth-instar nymphs 
(FSG or FLM), conspecific immature, and mature adult (MSG or MLM) 
volatiles.
Combinations Days after fledging (range of ± 1 day)
Source / Test 6 11 16 21 26 31
ISG / ISG 2.4 ± 0.2 4.1 ± 0.3 4.9 ± 0.2 5.3 ± 0.3 5.6 ± 0.1
MSG / ILM 1.7 ± 0.6 2.4 ± 0.3 5.0 ± 0.5 4.3 ± 0.3 4.8 ± 0.5 3.0 ± 0.2
MLM / ISG 1.8 ± 0.2 2.8 ± 0.5 3.8 ± 0.9 5.0 ± 0.9 3.2 ± 0.5 .
FSG / ISG . 2.7 ± 0.9 4.9 ± 0.8 5.9 ± 0.4 5.6 ± 1.0 4.1 ± 0.8
FSG / ILM 1.0 ± .01 1.8 ± 0.3 5.5 ± 0.0 4.2 ± 0.2 4.9 ± 0.5 3.9 ± 0.5
FLM / ISG . 2.2 ± 1.0 6.5 ± 0.3 5.0 ± 0.6 6.0 + 0.8 5.6 ± 0.1
MSG / ISG . 2.0 ± 0.5 5.2 ± 0.5 6.4 ± 0.7 3.3 ± 0.7 5.4 ± 0.6
MLM / ILM 1.9 ± 0.1 4.9 ± 0.3 5.5 ± 0.0 4.4 ± 0.4 4.7 ± 0.2 4.3 ± 0.1
ILM / ILM 1.0 ± 0.4 3.2 ± 0.0 4.1 ± 0.1 4.9 ± 0.8 4.9 ± 0.2 3.6 ± 0.1
ILM / ILM 2.7 ± 0.8 5.8 ± 0.7 3.0 ± 0.6 4.9 ± 0.5 5.1 ± .04 5.5 ± 0.1
A total of 25 female locusts in clusters (blocks) of 5 locusts were tested for 
each experiment or combinations.
211
Appendix X. Summarized effects of L. migratoria and S. gregaria on each other's 
sexual maturation with respect to integumental colour, copulation, 
oocyte length, using two conspecific effects as references.
Combinations Parameters Effects
Source Test Colour On ISG On ILM
ISG FSG MSG ILM FLM MLM
FLM ISG + + . + + + Acceleration
FSG ILM . . . - - . Delay
MLM ISG - . + + + - intermediate
MSG ILM - • • - • + Delay
Copulation
FLM ISG + + . + + + Accelerate
FSG ILM . . . + - . delay
MLM ISG - . - + + - Delay
MSG ILM - • • - • — Delay
Pheromone
FLM ISG + + . + + + Accelerate
FSG ILM + - . Intermediate
MLM ISG + . + + + - Accelerate
MSG ILM - • ■ + • + Accelerate
Oocyte lengths
FLM ISG + + + + + Accelerate
FSG ILM . . . + - . Intermediate
MLM ISG + . - + + - Delay
MSG ILM • • • + • + Accelerate
FLM : fifth-instar L. migratoria; FSG : fifth-instar S. gregaria MLM : mature
adults L. migratoria; MSG : mature adult S. gregaria.