Comparative toxicity of botanical powders, diatomaceous earth, pirimiphos methyl, rice husk (powder and ash) against Callosobruchus maculatus (Fab.)

This study was carried out to investigate the comparative toxicity of seeds of Piper guineense Schum & Thonn, Capsicum annum L., diatomaceous earth, pirimiphos-methyl dust, rice husk (powder and ash) against Callosobruchus maculatus (F.) in stored cowpea seeds. Toxicity test was carried out by exposing five pairs each of freshly emerged beetles to the substrate materials at the rates of 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 g/20 g of grains. The experiment conducted at ambient temperature of 28 °C ± 2 °C and 75% ± 5% relative humidity was replicated three times and arranged in a completely randomized design. Adult mortality was observed for four days. The following phytochemicals were observed in the materials: alkaloids, saponins, tannin, flavonoid and glycosides. Results of the toxicity assay revealed that rice husk ash was the most toxic to adult C. maculatus with 100% mortality (p ≤ 0.05) within 72 h of exposure at a concentration of 0.1 g/20 g grains. Pirimiphos methyl gave a significantly high mortality of 76.67% at a concentration of 0.2 g/20 g of grains in cowpea after 72 h of application. The least efficacy was observed with rice husk powder evoking 3.33% at a dosage of 0.3 g/20 g in cowpea. The treated grains significantly reduced the oviposition and adult emergence of C. maculatus and were significantly different from the control in all the different dosages. Oviposition and adult emergence were lowest in rice husk ash with 8 eggs and 24.62% adult emergence in C. maculatus, at 0.5 g/20 g dosage. The percentage adult emergence in the untreated grains was significantly different (higher) (p ˃ 0.05) from the emergence in the treated cowpea grains across the dosages. At all levels of dosages, the powders and ash significantly reduced weight loss, damage and weevil perforation index caused by the beetle. Rice husk ash at 0.5 g dosage recorded the lowest weight loss, grain damage and weevil perforation index of 0.40%, 1.95% and 5.80%, respectively. The findings from this research showed that rice husk ash was most potent/toxic to C. maculatus and its use could be encouraged especially in small scale storage. Received