Interactions Between Schistocerca Gregaria (Forskal) And Locusta Migratoria Migratorioides (Reich & Farmaire) In Relation To Phase Polymorphism

Abstract

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 dosedependent. 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 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 oocytelength 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. 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.