Insecticidal potency of Hyptis spicigera preparations against Sitophilus zeamais (l.) and Tribolium castaneum (herbst) on stored maize grains

Food situation has remained insecure and unpredictable in sub-Saharan Africa, leading to high levels of cyclic famine and poverty. This has been exacerbated by perennial pest problems, which cause substantial post-harvest food grain losses. It is against this background that this study was designed to investigate the use ofHyptis spicigera preparations for pest control and hence generate effective biological control options compatible with the prevailing farmers’ circumstances. Laboratory and on-farm studies were conducted in order to assess the insecticidal potency of H. spicigera extracts on maize weevils, Sitophilus zeamaisMotschulsky (Coleoptera: Curculionidae) and red flour beetle (Tribolium castaneum Herbst (Coleoptera: Tenebrionidae). Fumigant toxicity was assessed at four rates (0, 2, 4, 6, 8, 10 μl/l air) in space fumigation whereas repellence was evaluated in choice bioassay system at five rates (0, 0.2, 0.4, 0.6, 0.8 and 1.0% w/w). Contact toxicity was evaluated by the perforated jar method using dry powder, fresh leaves and oils while antifeedant activity was studied by the flour disk method. The results established that both whole plant parts and steam-distilled essential oil extracts possess strong insect repellent activity at reasonably low doses. Following exposure of newly emerged adult insects to 12 h of fumigation using essential oils at a dose of 25 μl/vial, 70% mortality was recorded for H. spicigera and 0% in the control. Whole plant powder had high potency compared to polar extracts which were not significantly efficacious to the insects.Hexane extract also showed the highest feeding deterrence activity compared to either ethyl acetate or water extracts. Hyptis oil extract on maize was also the most effective treatment at reducing insect infestation and seed damage compared to other treatments in the field. Whole plant extracts and essential oils described could be useful for managing field populations of S. zeamais and T. castaneum. Therefore, this study introduces an innovative approach to the use of traditional plant-based pesticides for grain protection in Kenya.   Key words: Hyptis spicigera, bioactivity, antifeedant, toxicity, repellancy, stored-product insects.