Efficacy of extracts of the different plant parts of Piper nigrum against bruchid beetle (Callosobruchus maculatus) in stored cowpea

The Cowpea (Vigna unguiculata) is an important legume crop because of its protein constituent. Though it is highly consumed by Sri Lankan, the production is restricted due to the insect pest infestation. Callosobruchus maculatus is the one among the insect pests causing severe damage to the cowpea seeds at field and storage. The management of C. maculatus using risk-free substances is inevitable for the steady supply of cowpea grains and to ensure the safety of consumers. Thus, a laboratory study aims to investigate the efficacy of different plant parts of Piper nigrum, namely seed, leaf, and stem against C. maculatus, by comparing with the control and the composition of secondary metabolites in such plant parts. Four treatments, viz., pepper seed methanol extract (10% w/v), pepper leaf methanol extract (10% w/v), pepper stem methanol extract (10% w/v), and methanol as control, were evaluated against C. maculatus. The effect of the extracts of the different plant parts of P. nigrum, in terms of percentage of mortality, fecundity, newly emerged adults of C. maculatus at its F1 generation, weight loss in cowpea seeds, number of damaged cowpea seeds, and the germination percentage of cowpea seeds were evaluated under laboratory condition at 28-30 °C and 75±5% RH. The phytochemical qualitative analysis and repellent effect were also performed for different pepper extraction. The results revealed that the per cent mortality was 100% in the cowpea seed lots treated with methanol extraction of pepper seed followed by pepper stem methanol extraction, whereas zero eggs were laid on the cowpea seeds treated with pepper seed methanol extract as the treatment recorded with 100% mortality at 48 hours of the experiment. The study also indicated zero adult emergences, damaged seeds, and weight loss in such treatment with maximum viability of cowpea seeds. Further, the highest repellency effect was also recorded in the cowpea seeds treated with methanol extraction of pepper seeds. Thus, methanol extraction of pepper seed was recommended as the best among the checked treatments. The phytochemical analysis confirmed the presence of alkaloids, glycosides, saponin, phenol, steroids, flavonoids, terpenoids, and tannins in the seed and stem of the pepper plants. Although similar secondary metabolites were present in the seed and stem of pepper plants, the methanol extract of pepper seed exhibited maximum efficacy against C. maculatus. Thus, further studies are vital to confirm the presence of different components in each group of secondary plant metabolites.