Insecticides for control of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae) in Pakistan and factors that affect their toxicity

The diamondback moth, Plutella xylostella (L.), is one of the most destructive insect pests of crucifer crops in Pakistan and other parts of the world where crucifers are grown. It has displayed an ability to develop resistance to most insecticides rapidly due to a range of biochemical and behavioral factors. Two factors affecting toxicity of insecticides, host plants and insecticide synergists, were assessed under laboratory conditions. The LC50 values of different insecticides varied significantly and feeding by P. xylostella on different host plants sometimes significantly affected their toxicity. Against P. xylostella collected in Pakistan, the insect growth regulator chlorfluazuron was the most toxic compound (LC50 of 0.0006 mg a.i. ml−1) and dimethoate was the least toxic (LC50 of 76.6 mg a.i. ml−1). Feeding on different hosts significantly affected toxicity of some insecticides. For example, when larvae were fed rocket plants, Eurica sativa, the LC50 of λ-cyhalothrin was 0.105 mg a.i. ml−1 whilst it was 0.035 a.i. ml−1 when larvae were fed cabbage, Brassica oleracea var. capitata. The LC50 values of lufenuron, profenofos, λ-cyhalothrin, spinosad and avermectin alone were 1.14, 8.67, 0.0418, 0.37, and 0.013 mg a.i. ml−1, respectively. With some, but not all insecticides, a low but sometimes significant level of synergism was recorded with use of the synergists piperonyl butoxide and S,S,S-tributyl phosphorotrithioate.