Storage Temperature of Neem Kernel Extract: Differential Effects on Oviposition Deterrency and Larval Toxicity of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae)

Abstract

Neem has been used as an effective postharvest protectant for many crops. Neem is especially effective against the cowpea weevil, Callosobruchus maculatus (F.), a major pest of cowpeas. Although it is well known that azadirachtin A breaks down rapidly at high temperatures, the efficacy of neem kernel extract previously exposed to high temperatures for protecting stored pulses has not been conclusively investigated. Effectiveness of these materials would allow West African farmers to make a neem extract at their convenience and store it for later use. We found that neem kernel extract continued to reduce infestations of C. maculatus, after the neem kernel extract had been stored at high temperatures, including 2 wk at 50°C followed by up to 5 mo storage at 28°C. Neem kernel extract analyzed with high performance liquid chromatography revealed that azadirachtin A present in unheated or fresh neem kernel extract dissipated when stored at 50°C for 2 wk. Neem kernel extract heated to 28°C or above also lost effectiveness as an oviposition deterrent. However, the number of neem kernel extract-treated eggs that survive to become adults was significantly reduced even when the neem kernel extract was exposed to 50°C for 2 wk. We attribute the mortality we observed, which was maintained even when azadirachtin A was absent in neem kernel extract, to physical effects of the oil properties of neem kernel extract. We conclude that neem kernel extract can be stored at high temperatures for at least 5 mo without significant reduction in overall effectiveness.