DOSAGE DIFFERENTIAL-EFFECTS OF PERMETHRIN-IMPREGNATED INTO BEDNETS ONPYRETHROID-RESISTANT AND SUSCEPTIBLE GENOTYPES OF THE MOSQUITO ANOPHELES-STEPHENSI

Effects of bednets impregnated with permethrin 200mg and 500mg/m(2) on pyrethroid resistant and susceptible strains of Anopheles stephensi a nd their F-1 hybrid progeny were studied, using free-flying female mos quitoes 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 n etting. Unexpectedly an apparently higher feeding rate, but lower knoc kdown and mortality rates, of mosquitoes were obtained for each of the three genotypes with the higher concentration of 500 mg/m(2) compared with the lower dose of 200 mg/m(2). At the lower dose there was 100% mortality 24h after exposure of all three genotypes, suggesting that t here would not be selection for resistance at this dose. However, at t he higher dose there was significantly higher mortality of the suscept ible strain than of the F-1 hybrids, suggesting incomplete recessivene ss of this resistance and that there would therefore be effective sele ction for resistance by this dose.When female mosquitoes were confined in bioassay cones on treated netting, the resistant strain of An.step hensi 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 F-1 hybrids. The higher dose provoked more irr itation of each genotype; this could explain the greater knockdown and mortality rates of mosquitoes exposed to the lower dose which was les s irritating and hence more effectively insecticidal. Thus a dose of 2 00 mg/m(2) is preferable to 500 mg/m(2) for malaria vector control.