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Insecticide resistance and the underlying resistance mechanisms were studied in seven species of insect vegetable pests (aphids Aphis gossypii, Myzus persicae, Aphis cracciuora, Toxoptera citricidus and Lipaphis erysirni; diamond-back-moth Plutella xylostella; leafminer Liriomyza huidobrensis) and two predatory ladybird beetles (Coccinella sexrnaculatus and Thea cincta). Insects, collected from the Central Province, Sri Lanka were subjected to bioassays with malathion, chlorpyrifos, propoxur and permethrin by topical application and LC,, and LC,, values were obtained. M. persicae, P. xylostella and L. huidobrensis had high resistance to all the insecticides tested with the exception of M. persicae to propoxur. The aphid T. citricidus was the least tolerant pest to all four insecticides tested. Compared to the pest species, the ladybird beetle C. sexmaculatus had a moderate level of insecticide tolerance whereas T. cincta was more susceptible. DDT resistance, as determined by a discriminating concentration, was highest in P. xylostella and least in L. huidobrensis. Biochemical analysis of insect metabolic enzymes showed that elevation of esterases was the major resistance mechanism in aphids and P. xylostella. Highest esterase activity was present in M. persicae. Native polyacrylamide gel electrophoresis resolved elevated esterase isoenzymes in all species except in T. citricidus and T. cincta. Malathion carboxylesterase activity was found in P. xylostella, M. persicae, and L. huidobrensis. Higher glutathione S-transferase activities of P. xylostella and L. erysirni were correlated with their higher DDT resistance. Increased mono-oxygenase titres occurred in A. gossypii, M. persicae and T. citricidus. Insecticide inhibition of the organophosphate and carbamate target site acetylcholinesterases indicated that target site insensitivity is a major mechanism in L. huidobrensis.

Biochemical Characterization of Insecticide Resistance in Insect Pests of Vegetables and Predatory Ladybird Beetles