Dosage differential effects of permethrin impregnated into bednets on pyrethroid resistant and susceptible genotypes of the mosquito Anopheles stephensi.

Effects of bednets impregnated with permethrin 200 mg and 500 mg/m2 on pyrethroid resistant and susceptible strains of Anopheles stephensi and their F1 hybrid progeny were studied, using free-flying female mosquitoes of these three genotypes, in a room with a human subject under a polyester net, having one of his arms in contact with the treated netting. Unexpectedly an apparently higher feeding rate, but lower knockdown and mortality rates, of mosquitoes were obtained for each of the three genotypes with the higher concentration of 500 mg/m2 compared with the lower dose of 200 mg/m2. At the lower dose there was 100% mortality 24 h after exposure of all three genotypes, suggesting that there would not be selection for resistance at this dose. However, at the higher dose there was significantly higher mortality of the susceptible strain than of the F1 hybrids, suggesting incomplete recessiveness of this resistance and that there would therefore be effective selection for resistance by this dose. When female mosquitoes were confined in bioassay cones on treated netting, the resistant strain of An. stephensi showed significantly less irritability (scored as the time until first flight take-off) in response to each dose, as compared with the susceptible strain and F1 hybrids. The higher dose provoked more irritation of each genotype; this could explain the greater knockdown and mortality rates of mosquitoes exposed to the lower dose which was less irritating and hence more effectively insecticidal. Thus a dose of 200 mg/m2 is preferable to 500 mg/m2 for malaria vector control.