Contact and fumigant toxicity of five pesticidal plants against Callosobruchus maculatus (Coleoptera: Chrysomelidae) in stored cowpea (Vigna unguiculata)

Insecticidal activities of five pesticidal plant species, Tephrosia vogelii, Dysphania (Syn: Chenopodium) ambrosioides, Lippia javanica, Tithonia diversifolia and Vernonia amygdalina, which have been reported to control storage pests, were evaluated as leaf powders against Callosobruchus maculatus (Fabricius 1775) in stored cowpea. Their efficacy was compared with the commercial pesticide Actellic dust (pirimiphos-methyl) at the recommended concentration (50g/90kg), and with untreated cowpea seeds as a negative control. The plant powders were applied at concentrations of 0.01, 0.1, 1 and 3g/10g of cowpea seeds in 250ml plastic containers (to measure contact toxicity), or 0.005, 0.05, 0.5 and 5g tied in small muslin cloth bags and hung in 500ml plastic bottles containing 10g of cowpea seeds (to measure fumigant toxicity). Mortality of adults, oviposition deterrence, adult emergence, and percent seed damage were recorded. Complete protection of seeds and inhibition of adult emergence were achieved in Actellic dust-treated seeds; contact toxicity using leaf powders of T. vogelii at all concentrations, D. ambrosioides at concentrations of 0.1, 1 and 3g and L. javanica at concentrations of 1 and 3g; and fumigant toxicity using D. ambrosioides at concentrations of 0.5 and 5g and L. javanica at a concentration of 5g. Head space analysis of D. ambrosioides and L. javanica identified ascaridole and camphor, respectively, as components that could be responsible for the bioactivity of these plant species. These plants may, therefore, serve as effective but less harmful biopesticide alternatives to Actellic. Conversely, V. amygdalina and T. diversifolia were not effective, indicating that they should not be promoted for controlling bruchids in cowpea.