Bioassay and efficacy of ethanol extracts of selected plant materials for the management of kola weevil Balanogastris kolae (Coleoptera; Curculionidae) on stored kola nuts

Kola weevil (Balanogastris kolae) is one of the major insect pests of stored kola nuts in Nigeria. A bioassay was conducted for contact and residual effects of four plant ethanol extracts on B. kolae adult and larvae. Cured kola nuts were treated with the extracts and stored for three months to evaluate their potency in protecting stored kola nuts against B.kolae infestation. The extracts were seeds of Azadirachta indica, Jatropha curcas, Piper guineese and Afframomum melegueta. They were applied at 50%, 75% and 100% concentration levels on adult and larvae while kola nuts were treated with 50% and 75% concentrations of the extracts and were stored for three months. Cypermethrin 2.5EC was used as a standard check. Larvae and adult mortality were recorded at 20 minutes intervals for 24hour on bioassay while exit holes, larvae and adult emergence were recorded on stored kola after three months. All the extracts at different concentrations had contact and residual effects on B. kolae. A. indica, P. guineense and A. meleguata gave 80-100%adult and larve mortality of B.kolae in all concentrations; their efficacies were comparable to cypermethrin. The B.kolae development and emergence from treated stored kola nuts at the various treatment levels decreased with increased concentration of the extracts applied. There were no significant differences (p>0.05) among the various treatments on exit holes, larval and adult emergence on stored kola nuts. The ethanol extracts of the four test plants can be used at concentrations of 50% and75% to reduce the menace of kola weevil.Keywords: Kola nuts; plant extracts; toxicity; managemen