Acute toxicity of organophosphorus and pyrethroid insecticides to Bombyx mori

Acute toxicities of two organophosphorus insecticides (dichlorvos and phoxim), four pyrethroid insecticides (permethrin, tetramethrin, bifenthrin, and ethofenprox), and their combined uses to the third instar of the silkworm, Bombyx mori (L.), were determined by feeding the insect with the insecticide-treated mulberry, Morus albus (L.), leaves. Twenty-four and 48 h after treatment, toxicity levels of all insecticides to the silkworm were in the very high or high range, and the LC50 values of permethrin, tetramethrin, bifenthrin, ethofenprox, dichlorvos, and phoxim were 1.60 and 0.75, 3.86 and 2.83, 0.09 and 0.06, 2.87 and 0.80, 6.63 and 4.11, and 1.05 and 0.45 mg liter(-1), respectively. The toxicity levels of 50:50 binary mixtures of organophosphorus and pyrethroid insecticides to the silkworm were in the high or middle range. Twenty-four and 48 h after treatment, the LC50 values of 50:50 binary mixtures permethrin + phoxim, permethrin + dichlorvos, tetramethrin + phoxim, tetramethrin + dichlorvos, bifenthrin + phoxim, bifenthrin + dichlorvos, ethofenprox + phoxim, and ethofenprox + dichlorvos to the silkworm were 1.49 and 0.85, 1.24 and 0.79, 2.20 and 1.08, 14.62 and 13.16, 0.33 and 0.13, 0.12 and 0.10, 2.81 and 1.37, and 4.82 and 3.00 mg liter(-1), respectively. Based on the combinations coefficient values, the toxicities of binary mixtures of organophosphorus and pyrethroid insecticides had additive effect except for the binary mixture of etramethrin + dichlorvos, which showed antagonism effect.