The Incidence of Major Lepidopteran Pests on Cabbage and their Associated Parasitoids in 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.