Mutation of the voltage-gated sodium channel genes or knockdown resistance (kdr) and metabolic resistance in Aedes aegypti (L.) (Diptera: Culicidae) are important resistance mechanisms against pyrethroids. The present study investigated the effect of relaxation of deltamethrin selection pressure on the level of mixed-function oxidases (MFO), when the allele frequency of S989P+V1016G mutations is fixed in a resistant Ae. aegypti strain (UPK-R) from Chiang Mai, Thailand. The mosquitoes were divided into two groups, exposure and nonexposure groups, and maintained for 12 generations in an insectary room. Adults of the exposure group (F3 to F12) were treated with 0.05% deltamethrin-impregnated papers. The median lethal concentrations (LC50) of deltamethrin of larvae were determined by World Health Organization (WHO) bioassay. MFO activity was determined in F0 and F12. The results revealed that there was a decreasing trend of adult mortality rates in the exposure group over time. The larval LC50 values of the exposure group were gradually increased, whereas those of the nonexposure group were gradually decreased. The level of MFO activity in the nonexposure group (F12) was lower than the parent and exposure groups (F12) by 1.5 and 4-fold in the larvae, respectively, and 1.5 and 2.5-fold in the adult females, respectively. However, the frequency of P989+G1016 alleles in both groups was 100% up to F12 when the experiment ended. This study indicates that there was a significant but small reduction in the activity levels of MFOs when pyrethroid selection pressure is relaxed in this kdr strain of Ae. aegypti.

Important insecticide resistance mechanisms in the dengue vector Aedes aegypti are mutations of voltage-gated sodium channel (VGSC) genes or knockdown resistance (kdr) and increased activity of metabolic enzymes. The objective of this study was to determine activity of mixed-function oxidases (MFO), esterases and glutathione-s-transferases (GSTs) in two strains of Ae. aegypti. The UPK-R strain, which harbors S989P and V1016G homozygous mutations in the VGSC, was compared with the wild-type PMD strain. Adult bioassays revealed that the UPK-R was resistant to DDT, permethrin, deltamethrin and malathion, whereas the PMD was resistant to only DDT. Enzyme activity in larvae, pupae and adults of the UPK-R strain was statistically higher than that observed in the PMD strain (mostly 1-2 fold). The current work supports previous studies which have suggested that increased MFO activity plays a partial role in pyrethroid resistance, whereas kdr is the major mechanism. Resistance to organophosphates and DDT is probably due to cross-resistance of MFO or increased activities of esterases and GSTs, respectively. Metabolic resistance combined with kdr may complicate insecticide-based control of dengue vectors in Thailand.