TOXICITY OF SOME AGRO WASTE-DERIVED INSECTICIDES AGAINST MAIZE WEEVIL, Sitophilus zeamais (Motschulsky) [Coleoptera: Curculionidae]

Abstract

Stored product insect pests are problems throughout the world, as they diminish both the quantity and quality of grains. The resultant effect of this is an eminent threat to global food security which might negatively impact peoples’ well-being. Efforts by humans to curb the menace of these insect pests have brought about negative effects because of the use of synthetic insecticides. Recent development has been seen in the use of botanicals and agricultural wastes in the control of these pests. This research evaluated the toxicity of some agro-wastes (plantain peels, banana peels, cowpea seed coats, cassava peels and melon shells) on maize weevil, Sitophilus zeamais (Motshulcky). Powder of the wastes were tested at doses 0.2, 0.4, 0.6, 0.8, 1.0, 1.5 and 2.0g/ 20g while extract from the wastes were tested at concentrations 0.2, 0.4, 0.6, 0.8, 1.0, 1.5 and 2.0 ml /20g of maize seeds. Parameters that were evaluated in this study included adult mortality, numbers of adult emergence, % seed damage, % weight loss and weevil perforation index (WPI). Proximate and phytochemical composition of the agro-wastes were also investigated. Results showed that all the tested agro-waste powders and extracts significantly (p<0.05) reduced the population of adult S. zeamais on treated maize seeds. Melon shell powder at 2.0 g per 20 g maize seed caused 53.33% mortality of adult S. zeamais after 24 hours of treatment. This was followed by cassava peel powder that evoked 40.0% weevil mortality. The least toxic agro-wastes was cowpea seed coat powder that caused 20.0% mortality of adult weevil. The lethal dosage of banana peel, cassava peel, cowpea seed coat, melon shell, and plantain peel powder at which 50% (LD50) population of maize weevil died after the first day of exposure were 3.46 g, 2.74 g, 4.88 g, 1.89 g, and 4.38 g per 20 g maize seed, respectively. The same trends of results were recorded on the toxicity of agro-wastes extracts. Extracts were more toxic than powder of the tested agro- wastes. At concentration 2.0 ml, banana peel, cassava peel, melon shell, and plantain peel extract gave more than 50% mortality of maize weevil after the first day of exposure. All the concentrations of cassava peel and melon shell extract gave maximum seed protection against maize weevil. The phytochemical and antinutrient composition of all the agro-wastes used in this study showed that the wastes contain Tannin, Alkaloid, Saponin, Flavonoid, Oxalate, Phytate and Cyanide with different levels of composition in percentage and milligram per gram. The Tannin composition of the melon shell powder showed a strong positive correlation with the weevil adult emergence (r = 0.87) and % seed damage (r = 0.93). % weight loss caused by the weevil was moderate positively related with melon shell Tannin (r = 0.65). The relationship between maize weevil and adult emergence (r = 0.99), seed damage (r = 97), and weight loss (r = 0.98) indicate a strong positive correlation with melon shell alkaloid composition. Flavonoid composition of melon shell powder had perfectly strong negative correlation with the maize weevil biological activities, with the adult emergence and seed weight loss correlation coefficient (r) value of - 0.99 and – 0.88, respectively. The percentage seed damage was perfectly negatively significant with the melon shell flavonoid composition (r = -1.00). This research showed that cowpea seed coat, cassava peel, melon shell, banana and plantain peels powders and extracts had distinct toxicity effects on the survival of S. zeamais. The melon shell of all the examined agro-wastes powders had the lowest median lethal dosage (LD50) and concentration to the adult S. zeamais. The use of melon shell and cassava peel as bioinsecticides in the control of maize weevil in stored maize seeds among poor resource farmers and food merchants is advocated. The utilization of these wastes will also help reduce the problems associated with their management.