COMPARATIVE TOXICITY OF BOTANICAL POWDERS, DIATOMACEOUS EARTH, PIRIMIPHOS METHYL, RICE HUSK (POWDER AND ASH) AGAINST TWO STORAGE BEETLES

Abstract

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 and rice husk (powder and ash) againstCallosobruchus maculatus (F.) and Sitophilus zeamais (Motsch) in stored cowpea and maize grains respectively. Toxicity test was carried out by exposing five pairs (male and female) each of freshly emerged insects to each of the substrate materials at the rates of 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5 g/20 grammes of grains. The experiment conducted at ambient temperature of 28 ± 2oC and relative humidity of 75±5% 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. Proximate analysis showed the presence of crude protein, fat content, crude fibre, moisture content and ash content. Results of the toxicity assay revealed that rice husk ash was the most toxic to adult C. maculatus and S. zeamais with 100% mortality (p≤0.05) within 72 hours of exposure at a concentration of 0.1g/20grammes grains. Pirimiphos-methyl gave a significantly high mortality of 76.67% and 90.00% at a concentration of 0.2 g/20 g of grains respectively in both cowpea and maize after 72 hours of application. The least efficacy was observed with rice husk powder evoking 3.33% and 33.33% at a dosage of 0.3g/20 grammes respectively in both cowpea and maize. The treated grains significantly reduced the oviposition and adult emergence of C. maculatus and were significantly different from the control across 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 0.5g/20g 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 significantly reduced the weight loss, damage and weevil perforation index caused by the beetle. Rice husk ash at 0.5g dosage recorded the lowest weight loss, grain damage and weevil perforation index of 0.40%, 1.95% and 5.80 % respectively.Rice husk ash was able to inhibit completely the emergence of adult S. zeamais at 0.1g dosage within 24 hours post treatment as well as reduce weight loss, in Sitophilus zeamais. The inhibition rate in Pirimiphos methyl (90.42%) was comparable with those of P. guineense (90.78% ) and diatomaceous earth (82.34%) at 0.1g dosage. Weight loss of 19.90% was highest in the control while there was no weight loss in the treated grains with rice husk ash for S. zeamais. The findings from this research showed that rice husk ash was most potent/toxic to the storage beetles, C. maculatus and S. zeamais.