Abstract

A major problem in cowpea seed storage in the tropics is infestation by bruchids, especially the cowpea bruchids Callosobruchus maculatus (F.). Plant mixtures containing lemon grass (Cymbopogon citratus) (L) and wild basil (Ocimum suave) (W) in the ratio of Lemon grass : Wild basil - 100:0, 80:20, 60:40, 50:50, 40:60, 20:80, 0:100 and 0:0 were used under ambient laboratory conditions with the aim of evaluating the efficacy of these plant powders against the cowpea bruchids. The mixed powders were each applied at 1, 2 and 3 g concentration to 20 g of cowpea seeds and acute toxicity was assessed after up to 96 h exposure. Insect mortality was recorded to estimate the LC50s and LT50s. All mixtures with the exception of 100L: 0W and 80L: 20W) caused significant insect mortality (p<0.05) as the concentration increased from 1 - 3 g/20 g cowpea seeds. These results indicate that the plant materials are toxic to the cowpea bruchids especially the mixed proportions 60L: 40W, which was the most effective with the lowest LC50 and LT50.

Key words: Callosobruchus maculates, Cymbopogon citratus, infestation, mortality, Ocimum suave.