Bioactivity of Novel Botanical Insecticide From Gnidia kaussiana (Thymeleaceae) Against Callosobruchus maculatus (Coleoptera: Chrysomelidae) in Stored Vigna subterranea (Fabaceae) Grains

Hexane, acetone, and methanol extracts from Gnidia kaussiana Meisn (Thymeleaceae), each at two dosages (0.2 and 1 ml/50 g grains corresponding, respectively to 1 and 5g/kg), and neem seed oil (NSO), used as standard insecticide were evaluated for repellence, toxicity to Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae) adults, F1 progeny inhibition, persistence and as grain protectant during storage. Experiments were laid out at complete randomized design with five replications for repellence test and four for others. All the extracts were effective in protecting stored Bambara groundnut (Vigna subterranea (L.) Verdcourt) from insect attack; however, their bioactivities were inversely correlated with solvent polarity. No adult survival was recorded in treated grains with hexane extract at 5 g/kg dosage within 2 d exposure. Also at 5 g/kg, all extracts hindered adults emergence, grain damage and weight loss after 4 months storage. Moreover, hexane extract was more repellent and exhibited averagely repellency. The insecticidal effectiveness of hexane extract did not decreased provided that the exposure time of insects to the product was high (7 d). The potency of acetone and methanol extracts decreased with storage time, although not linearly and remained significantly toxic to C. maculatus up to 60 d of storage. Therefore, hexane and acetone extracts are good candidates for incorporation in integrated pest management programs for control of cowpea weevils in stored grains by poor-resourced farmers and store keepers in Cameroon and other developing countries.