Changes in enzyme titres with age in four geographical strains of Aedes aegypti and their association with insecticide resistance.
The enzymes acetylcholinesterase, glutathione S-transferase (GST), glucose-6-phosphate dehydrogenase (G6PD), and general esterases were assayed in 4 strains of A. aegypti mosquitoes aged between 1 and 30 days. Microtitre plate methods were used to assay activity in the homogenates of individual mosquitoes. The levels of GST and G6PD declined with the age of the mosquitoes, while the activity for the other enzymes remained constant. Soluble protein content was also found to decline with mosquito age in all the strains. Insecticide bioassays showed that 2 strains (Trinidad and Virtudes) of A. aegypti were resistant to DDT, deltamethrin and malathion, whereas 2 other strains (Bangkok and Indian) were susceptible to all 4 classes of insecticides tested (which also included bendiocarb, propoxur and dicofol). Higher esterase activity levels in the resistant compared to the susceptible strains were assumed to be the cause of organophosphate resistance. The combination of DDT and deltamethrin resistance in 2 strains with normal GST and G6PD characteristics suggests that a kdr-type nerve insensitivity mechanism may be involved.