Biochemical detection of temephos resistance in Aedes (Stegomyia) aegypti (Linnaeus) from dengue-endemic areas of Selangor State, Malaysia.

A aegypti larvae, from 6 consecutive ovitrap surveillances in two dengue-endemic areas, Taman Samudera and Kg Banjar, Selangor, were evaluated for susceptibility to an operational dosage of temephos (1 mg/L). Resistance levels were also detected by biochemical testing. Larval bioassays were conducted according to modified WHO standard methods. Biochemical enzyme assays of Ae. aegypti were conducted to detect the emergence of insecticide resistance and to define the mechanisms involved in temephos resistance. Four enzymes, ie non-specific esterases, mixed function oxidases, glutathione-S-transferases, and insensitive acetylcholinesterases, were studied. The range of 50% mortality lethal times (LT50) for all strains of Ae. aegypti tested against temephos was 36.07-75.69 minutes, with resistance ratios ranging between 0.85-1.79. Esterases played a major role in the Ae. aegypti resistance mechanism to temephos, while mixed function oxidases, glutathione-S-transferases, and insensitive acetylcholinesterases, were not involved. However, high-mixedfunction oxidase and glutathione-S-transferase levels indicated that Ae. aegypti might also develop resistance to other groups of insecticides. Resistance studies showed weekly variations in resistance against temephos and variations in enzyme levels in Ae. aegypti, indicating that the insecticide resistance of field populations of Ae. aegypti was heterogeneous. This study suggested that regular and continuous monitoring of resistance should be conducted in all dengue-endemic sites in Malaysia, to confirm the continued efficacy of insecticides, and to help choose the most effective insecticides for dengue control.