Efficacy of oils from nine plant species as protectants against infestation by Callosobruchus maculatus F. (Coleoptera: Chrysomelidae)

Cowpea seeds bruchid (Callosobruchus maculatus F.) is an important pest of cowpea with a reputation of causing high grain loss both in quantity and quality. The insecticidal efficacy of nine plant oils [neem (Azadirachta indica A. Juss), groundnut (Arachis hypogaea L.), Castor (Ricinus communis L.), Palm fruit mesocarp and kernel (Elaeis guineensis L.), coconut (Cocos nucifera L.), Olive (Olea europaea L.), Soybean [Glycine max (L.)  Merr.] and melon (Cucumeropsis mannii Naudin.) against this arthropod was evaluated under laboratory temperature of 26 ± 3 °C and 74 ± 4% R.H at the Crop and Environmental Protection Laboratory of the Federal University of Agriculture, Makurdi, Nigeria. The experiment involved exposing adult C. maculatus to 3 mLkg-1 of the oils admixed with Ife brown cowpea seeds. The set up was a completely randomized design with four replicates. Results show that all the plant oils tested had a significant (p<0.0001) insecticidal effect on C. maculatus. Adult bruchid mortality was >70% at 72 hours after exposure to the oils. Oviposition, F1 progeny, the number of damaged seeds and weight loss were significantly reduced (p<0.0001) by the plant oils compared with the control (untreated cowpea seeds). For all the parameters evaluated in the study, oils from A. indica, R. communis, E. guineensis kernel and G. max appeared to provide better protection against C. maculatus than the other plant oils tested. We conclude that seed treatment with these oils could be an important pest control options for C. maculatus in smallholder's cowpea storage facilities.