Abstract:

 Effects of insecticides on the cocoon formation and adult emergence in the field population of Cotesia plutellae were studied. Sublethal dosage of avermectin or fipronil was fed to the 2nd instar larvae of host Plutella xylostella (DBM), in which the parasitoid C. plutellae were at the egg, early and mid larval stages. When compared to the control, the cocoon formation of C. plutellae reduced by 26.6%, 22.8% and 5.8% if parasitized DBM larvae were fed on leaves treated with avermectin when the immature parasitoids were at the egg, early larval and mid larval stages, respectively, and by 76.9%, 42.5% and 18.5%, respectively if parasitized DBM larvae were fed on leaves treated with fipronil. No significant depression on the adult emergence of C. plutellae was found in avermectin treatment. However, the adult emergence of the parasitoid reduced by 53.1%, 36.1% and 47.8% at the egg, early and mid larval stages of the parasitoid in fipronil treatment. The results indicated that avermectin and fipronil, even at very low dosages (=LC₁₀ to the host insects), might still cause serious damage to the parasitoid's eggs or larvae. The effects of the insecticides on cocoon formation and adult emergence of C. plutellae varied depending on different insecticides and different developmental stages of the endoparasitoid.

Key words: Cotesia plutellae, avermectin, fipronil, sublethal dosage, cocoon formation, adult emergence