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

Abstract

Insecticidal activities of five pesticidal plant species,Tephrosia vogelii,Dysphania(Syn:Chenopodium)ambrosioides,Lippia javanica,Tithonia diversifoliaandVernonia amygdalina, which have been reported to control storage pests, were evaluated as leaf powders againstCallosobruchus maculatus(Fabricius 1775) in stored cowpea. Their efficacy was compared with the commercial pesticide Actellic dust (pirimiphos-methyl) at the recommended concentration (50 g/90 kg), and with untreated cowpea seeds as a negative control. The plant powders were applied at concentrations of 0.01, 0.1, 1 and 3 g/10 g of cowpea seeds in 250 ml plastic containers (to measure contact toxicity), or 0.005, 0.05, 0.5 and 5 g tied in small muslin cloth bags and hung in 500 ml plastic bottles containing 10 g 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 ofT. vogeliiat all concentrations,D. ambrosioidesat concentrations of 0.1, 1 and 3 g andL. javanicaat concentrations of 1 and 3 g; and fumigant toxicity usingD.ambrosioidesat concentrations of 0.5 and 5 g andL.javanicaat a concentration of 5 g. Head space analysis ofD.ambrosioidesandL.javanicaidentified 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.amygdalinaandT.diversifoliawere not effective, indicating that they should not be promoted for controlling bruchids in cowpea.