Toxicity and biochemical mechanisms underlying the insecticidal efficacy of two plant extracts on Callosobruchus chinensis (Coleoptera: ‎Chrysomelidae) infesting cowpea seeds

The quest for new sources of cheap and eco-friendly insecticide for insect pest management remains a major challenge facing cowpea farmers in many developing countries. In this study, the toxicity and biochemical mechanisms associated with the insecticidal efficacy of acetone and n-hexane extracts of Xylopia aethiopica (Dunal) and Senna occidentalis (L.) on Callosobruchus chinensis (L.) were investigated. The insecticidal efficacy varied with concentration, exposure time and extracts type. Acetone extract of X. aethiopica was less toxic (LD50 = 2.47%) than its n-hexane extract (LD50 = 1.39%) but with S. occidentalis, acetone extract was more toxic (LD50 = 0.73%) than n-hexane extract (LD50 = 1.37%). Acetone extract of both plants evoked a significant reduction in egg-laying and eclosion ability of adult C. chinensis compared to n-hexane extract except on cowpea pre-treated with acetone extract of S. occidentalis. Only the extracts of X. aethiopica reduced protein concentration compared to control. The activity of glutathione reductase and glutathione peroxidase was significantly reduced by both extracts of S. occidentalis while only n-hexane extract of S. occidentalis elicited a significant reduction in the activity of glutathione S-transferase and trehalase compared to other treatment levels. GC-MS analysis depicted Diisoocotyl phthalate (50.37%) and isomers of Aromandendrene (19.22%) as the main compounds in S. occidentalis and X. aethiopica respectively. Both botanical extracts also contained other insecticidal and insectifuge compounds in differential amounts. Overall, the findings demonstrated the potential of both botanical extracts, particularly acetone extract of S. occidentalis as alternatives to synthetic insecticide for controlling adult C. chinensis.