Phytochemical compositions and insecticidal efficacy of four agro-waste used as biological control of cowpea beetle, Callosobruchus maculatus (Fab.) [Coleoptera: Bruchidae

Abstract

BackgroundThe cowpea bruchid, Callosobruchus maculatus is the major postharvest insect pest of cowpea seeds in storage. This had led to huge losses and quality deterioration of stored cowpea seeds that serve as poor man’s meat in lieu of expensive meat source in developing countries. This research was carried out to evaluate the bioefficacy of rice husk, maize cob, groundnut, and cowpea pods against C. maculatus in the laboratory. Adult insect mortality, eggs laid, adult emergence, damaged seeds, weight loss and beetle perforation index (BPI) were evaluated. Phytochemicals of the wastes were investigated.ResultsThe result showed that the agricultural wastes contained alkaloids (1.56–2.77 mg/g), saponin (1.51–3.38 mg/g), phytate (7.00–17.76 mg/g), oxalate (0.32–1.13 mg/g). All agricultural wastes showed a high mortality effect on C. maculatus, and their effects increased as the exposure time and concentration/ dosage increased. Beetle mortality was highest in cowpea pod with 80% mortality for powder after 3 and 4 days when applied at 0.5 g/20 g cowpea seed. This showed that cowpea pod powder has the greatest insecticidal activity while the least was observed in maize cob powder (73.33%). Fewer adults emerged with maize cob having the least emergence when applied at 0.2–0.5 g dosage. Extracts of all agricultural wastes tested against C. maculatus were able to affect 70–100% mortality after 4 days of application at concentration 0.5 ml with cowpea pod extract causing 100% mortality of beetle after 4 days of application. The calculated lethal dose (LD50 and LD90) and concentrations (LC50 and LC90) of wastes powders and extracts cowpea pod was observed to have the lowest lethal dose while maize cob wastes were the highest across all period of exposure.ConclusionBase on the results obtained, cowpea pod waste was the most toxic in biocontrol of C. maculatus.