Evaluation of the effect of mosquito age and prior exposure to insecticideon pyrethroid tolerance in Anopheles mosquitoes (Diptera : Culicidae)

The times for knockdown of resistant and susceptible strains of both Anophe les stephensi Listen (DUB234 and BEECH) and A. gambiae Giles (RSP and KWA), on exposure to a permethrin impregnated net were determined to evaluate th e effect of age on susceptibility to permethrin. The mean knockdown times o f pyrethroid resistant strains of A. stephensi and A. gambiae were signific antly shorter if they were 10 days old (either fed or unfed), as compared w ith those of newly emerged mosquitoes. Based on KT50 values, the resistance ratios between resistant and susceptible A. stephensi were 3.5 and 2.2 for the newly emerged and 10-day-old mosquitoes, respectively. After Ih exposu re to paper impregnated with 0.25% permethrin, mortality of newly emerged i ndividuals of the susceptible and resistant strains of A. stephensi was sig nificantly less than when they were 10 days old. Irritability by permethrin was less with the resistant strain of A. stephensi. In the other strains i rritability declined significantly with age. Pre-exposure of susceptible mo squitoes of BEECH and IRAQ strains of A. stephensi for a short time to perm ethrin did not increase tolerance when the mosquitoes were re-exposed 24 h later. Because of the age effect and the possibility of an effect of pre-ex posure under some circumstances, confirmatory tests on newly emerged F-1 in dividuals reared from wild collected mosquitoes are recommended in monitori ng for resistance in wild populations.