Effects of Initial Infestation Levels of Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae) on Cowpea and Use of Nicotiana tabacum L. Aqueous Extract as Grain Protectant

Abstract

This study determined the effects of initial infestation of cowpea seeds (Ife brown variety) with different insect densities (0, 2, 4 and 6 pairs per 50 g seeds) of Callosobruchus maculatus (F.) and evaluated the effects of aqueous leaf extract of Nicotiana tabacum L. on C. maculatus in the laboratory. It was observed that adult beetle population increased significantly (p<0.05) with increase in insect density. The increase in population of beetles and corresponding weight loss of the seeds in different levels of infestation showed that the cowpea variety was susceptible to beetle infestation, emergence and survival of progeny. Significantly more adults emerged on higher infestation compared to lower and no infestation. In Nigeria, Nicotiana tabacum L. is a locally available plant, with known insecticidal properties. The plant leaf extract was easily extracted with water and confirmed its effectiveness as a protective agent for stored cowpea seeds. Experiment was conducted to assess the effects of aqueous extracts of N. tabacum at 0, 0.1, 0.2 and 0.3 mL · 50 g⁻¹ cowpea seeds on C. maculatus. Data was recorded and showed varying levels of effectiveness against C. maculatus. Result showed that seed appearance was dependent on levels of insect population, while N. tabacum aqueous extract exerted effects on survival of C. maculatus. Aqueous leaf extract of N. tabacum probably contained some insecticidal properties which might have significantly conferred beetle mortality and reduced beetle emergence leading to a decrease in seed weight loss.