Potential of four tropical plant powders as grain protectants against Callosobruchus maculatus (Coleoptera: Chrysomelidae)

Effects of four tropical plant (Aframomum melegueta, Zanthoxylum zanthoxyloides, Piper guineense and Eugenia aromatica) were investigated under tropical laboratory storage conditions for the protection of cowpea seeds against insect infestation. The plant materials were pulverised into fine powder after air drying and admixed with 20 g of cowpea seeds at the rates of 0.2, 0.3, 0.4 and 0.5 g in 125 ml plastic containers for contact toxicity experiment and 0.5 g of each plant powder to 50 g cowpea seeds for the fumigant toxicity experiment. Ten unsexed Callosobruchus maculatus were used for contact toxicity experiment, while two copulating pairs of C. maculatus were used for fumigant toxicity experiment. Callosobruchus maculatus response to the plant powders was recorded at 24 and 48 hrs post treatment for contact toxicity bioassay, while observation for fumigant activity was recorded at 24, 48, 72 and 96 hrs after treatment. All plant powders significantly (P < 0.05) exerted adult mortality in relations to dosage and exposure time. Cowpea seeds treated with 0.5g of E. aromatica had highest mortality of 90% and 100% at 24 and 48 hrs after exposure respectively for contact toxicity, while A. melegueta recorded the highest adult mortality for all exposure periods for the fumigant toxicity. The insects’ reactions to the plant powders admixed with cowpea seeds were restlessness, loss of coordination, knock-down and eventual death. The study indicated that the plant powders could be used as suitable alternative to synthetic insecticides to suppress C. maculatus infestation in stored cowpea seeds among the resource-poor farmers.