Mechanisms of DDT and Permethrin Resistance in Aedes aegypti from Chiang Mai, Thailand

Two strains of Aedes aegypti, one resistant to DDT but susceptible to permethrin(RdSp), and the other resistant to both DDT and permethrin (RdRp), were established in mosquitoes collected from Chiang Mai province, northern Thailand. Comparisons with a susceptible reference strain indicated that DDT resistance in both RdSp and RdRp strains was mainly due to an increase in DDTase activity. Similar moderate increases in cytochrome P450 levels were observed in the two resistant strains, hence this enzyme family may also play a role in DDT resistance. Glutathione S-transferase and esterase activities in the two resistant strains were similar and slightly higher than those of the susceptible strain, suggesting that neither enzyme group has a major role in permethrin resistance. The lack of an evident metabolic basis for the pyrethroid resistance in the RdRP strain suggests that nerve insensitivity may be present in this strain. The two mutations at residues reported to produce kdr resistance in other insects were not present, but some individuals from the permethrin resistant strain had an amino acid mutation at position 106 involving a valine to glycine mutation in the same segment 6 of domain II of the para sodium-channel gene, which may confer kdr-like resistance.