The Incidence of Major Lepidopteran Pests on Cabbage and their Associated Parasitoids in Ghana
Date
2009-08
Authors
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Journal ISSN
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Publisher
University of Ghana
Abstract
The study was carried out at the Weija Irrigation Company site at Tubaman, Weija,
the Laboratories of the Department of Zoology (now Department of Animal Biology and
Conservation Science, DABCS), University of Ghana, Legon and some selected districts of
Ghana. It was designed to; identify and establish the major Lepidopteran pests on cabbage,
Brassica oleracea (L.) var. capitata and their parasitoids, determine the effect of the
commonly used insecticides on parasitism and the biology of the major parasitoid, describe
and illustrate the pests and their parasitoids to aid in identification. The rationale for the
study was to provide information for implementation of effective integrated pest management
in cabbage production in Ghana.
Field studies were carried out at the Weija site for three years. The design of the
experiments was a randomized complete block design. Indirect and direct methods were used
to investigate the effect of four insecticides namely, a synthetic pyrethroid (Karate), a
Bacillus thuringiensis formulation, neem seed water extract and a commercial neem
formulation, (Neemazal) on the major parasitoid. The illustrations were made using the
scanning electron microscope, photomicroscope, a digital camera and Camera Lucida
drawings.
The pests recorded were Plulella xylostella L., Trichoplusia rti (Hvibner). Hellula
undalis (F), Helicoverpa armigera (HUbner) and Spodoptera littoralis (Boisduval). Plulella
xylostella was the most abundant pest recorded during the study period. There was no
significant difference between its larval density per plant in the major rainy season (0.15 ±
0.04, p > 0.05) compared with the minor rainy season (0.20 ± 0.07, p > 0.05): and also
between the minor rainy season and the dry season (0.29 ± 0.5, p > 0.05). On the other hand,
T. ni was only abundant during the major rainy season. There was a significant difference between larval density per plant during this season (0.60 ± 0.11, p < 0.05) compared with the
other seasons. With regard to S. litloralis, there was a significant difference between its
larval density per plant in the minor rainy season (0.39 ± 0.10, p < 0.05) compared with the
other seasons. The other pests occurred in insignificant numbers.
Out of the 15 species of parasitoids recorded from the pests, 7 were identified to the
species level, 6 to the generic level and 2 to species groups. Seven of the species: Charops
sp.. Brachymeria sp.. Hockeria sp., Elasmus sp., Notanisomorphella sp., Tetrastichus
atriclavus S.L., Pediobius sp. and Trichomalopsis sp. were recorded for the first time in
Ghana and may also be new species. The major parasitoid was Cotesia plutellae
(Kurdjumov) which accounted for 92% and 60.9% of the parasitoids recorded from P.
xylosiella and T. ni, respectively. The second important parasitoid was Euplectrus
laphygmae (Ferriere) and it was specific to T. ni and S. litloralis. The facultative
hyperparasitoids of P. xylostella were Oomyzus sokolowskii (Kurdjumov), Elasmus sp.?
Aphanogmus reticulatus (Fouts) and Trichomalopsis sp. via C. plutellae. Blepharella vasta
(Karsh), Peribaea orbata (Wiedemann), Notanisomorphella sp and Chelonus cunirnaculaliis
(Cameron) were specific to the larvae of S. littoralis. No parasitoids were recorded from H.
armigera. No egg parasitoid was observed.
There was seasonal variation in parasitism of P. xylostella by C. plutellae. The rate of
parasitism (6 8 .6 ± 12.9%, p < 0.05) was significantly highest in the major rainy season and
significantly least (9.9 ± 7.1 %, p < 0.05) in the minor rainy season. The rate of parasitism of
P. xylostella by C. plutellae did not differ among the unsprayed and insecticide treated plots.
However, adult C. plutellae emerging from pests collected from 'Karate' treated plots
sometimes died in the process of eclosion or were short - lived. Neem seed water extract
applied at 50 g/l also adversely affected the ability of the parasitoid larva to spin its cocoon
for pupation. Of the three insecticides. ‘Karate’ was the most toxic causing 100% mortality to the adult Cotesia pluiellae within ten seconds of direct exposure. Neemazal and Bt.
applied at 1.0% and 1 .Og/liter of water respectively, were the least toxic to the adult.
It is concluded that P. xylostella is the major pest of cabbage in Ghana, while T. ni
and S. littoralis are abundant only during the major and minor rainy seasons respectively.
Cotesia pluiellae and E. laphygmae are the major parasitoids and they should be considered
in the development of any integrated pest management on cabbage. Karate has a harmful
effect on C. pluiellae adults. Even though, Neem seed applied at 50g/I adversely affected the
development of C. pluiellae, lower doses, or Bacillus thuringiensis formulations could be
applied in the development of an Integrated Pest Management programme on cabbage.
Description
Thesis(P.Hd)-University of Ghana, 2009