Evaluation of the Insecticidal Potential of Leaf Oil Extracts of Acalypha wilkesiana (Muel.) Arg. against the Cowpea Weevil, Callosobbruchus maculatus Fabricius (Coleoptera: Chrysomelidae, Bruchidae)

Abstract

The extraction of Acalypha wilkesiana oil was carried out in the laboratory using a Soxhlet`s apparatus with six solvents, namely, ethanol, methanol, acetone, pet-ether, n-hexane and water. The biological activities of the different solvent extracts was investigated on Callosobruchus maculatus in the laboratory. Adult mortality, oviposition, adult emergence, long time storage ability and germinability were used as indices. The results obtained showed that all the different extracts were potent on C. maculatus. Ethanol extract was the most toxic because it recorded 100 % mortality within 96 hours of exposure. Aqueous extract was the least toxic among the extracts because it was only able to achieve 72.75 % mortality within 96 h. The extracts caused reduction in oviposition and adult emergence. Weevils in cowpea treated with ethanol and pet-ether extracts recorded the lowest number of eggs (12.25 and 12.45 respectively) which are not significantly (p < 0.05) different from the oviposition observed in methanol, acetone and pet-ether extracts. Adult emergence in ethanol and pet-ether extracts was totally suppressed while the aqueous extract recorded the highest adult emergence among the extracts used. Ethanol and pet-ether extracts totally prevented infestation and damage of cowpea seeds within the 90 days of storage. The percentage germination of all the treated seeds was generally high, ranging from 78.50 % to 95.25 %. The highest germination of 92.25 % was recorded in ethanol extract which is not significantly different from that of methanol extract. The results from this findings showed that extracts of A. wilkesiana were effective in protecting cowpea seeds against C. maculatus. This extract can be used as sustainable substitute to chemical insecticide in the control of C. maculatus.