Residual efficacy of two synthetic insecticides in controlling the maize weevil, <i>Sitophilus zeamais</i> (Motschulsky) (Coleoptera: curculionidae) in storage

Abstract

The use of insecticides is an integral component among the strategies for effective management of stored product insect pests. This study was conducted to assess the toxicity of two synthetic insecticides, viz; Dichlorvos and Profenofos+Cypermethrin on Sitophilus zeamais in storage. Adult weevils were exposed to grains treated with the insecticides and filter papers impregnated with concentrations of 1.250mg/ml, 1.000mg/ml, 0.500mg/ml, 0.250mg/ml, 0.125mg/ ml and 0.000mg/ml Dichlorvos, and 2.750mg/ml, 2.200mg/ml, 1.100mg/ml, 0.550mg/ml, 0.275mg/ml and 0.000mg/ml of Profenofos+Cypermethrin. Mortality was recorded over a period of 3 to 24 hours for the filter paper bioassay while residual action of insecticide-treated grains was conducted 30 and 60 days after treatment. The filter paper bioassays revealed that mortalities of weevils exposed to the two insecticides ranged from 0.00-86.67 % and 13.33-100.00 % at 6 and 12 hours respectively. The LC₅₀ values from impregnated filter paper at 6 hours were 0.672mg/ml and 1.074mg/ml while LC₅₀ values at 12 hours were 0.291mg/ml and 0.027mg/ml for Dichlorvos and Profenofos+Cypermethrin, respectively. The LC₅₀ toxicity factor for Profenofos+Cypermethrin and Dichlorvos at 12 hours of exposure to S. zeamais was in a ratio of 10.78:1. The 30-days post-treatment residual efficacy of both insecticides gave 100.00% mortality of insects in 6 hours of exposure whereas at 60 days post-treatment, 100.00% mortality was achieved in 6 hours and 12 hours of exposure to Profenofos+Cypermethrin and Dichlorvos respectively. A mean number of 128.001.15 adult weevils emerged when 40.000.00 adults were introduced to untreated grains resulting in 5.00% weight loss of grains after 30 days. There was a significant direct relationship (r=1.00; p<0.05) between weight loss in untreated grains, and adult emergence. This study has revealed the efficacy of Dichlorvos and Profenofos+Cypermethrin as candidate insecticides for controlling S. zeamais infestation on maize.

Keywords: Profenofos+Cypermethrin; dichlorvos; toxicity; residual action; maize; weevils