A multiple basis for insecticide resistance in a strain ofCulex quinquefasciatus(Diptera: Culicidae) from Muheza, Tanzania, studied as resistance declined

Abstract

Evidence from observing resistance in decline in fourth instar larvae of the multiple resistant MUHEZA strain ofCulex quinquefasciatusSay from Tanzania (MUHEZA), showed that the major mechanisms for chlorpyrifos and propoxur resistance were different. Resistance to chlorpyrifos declined more than 400-fold (from an initial resistance ratio (RR) of 14285) while resistance to propoxur remained stable for at least 30 generations of laboratory culture. Significant synergism was found between propoxur and piperonyl butoxide (PB), propoxur and s,s,s-tributyl trithiophosphate (DEF), and permethrin and PB, but antagonism occurred between chlorpyrifos and PB, and no synergism between chlorpyrifos and DEF. Nine esterase isozymes active against naphthyl acetates on polyacrylamide gel electrophoresis (PAGE), were identified, four (A2, A3, B2, B3) showing polymorphism in activity, with very intense expression at one or other position in more than 32% of larvae, and null expression in less than 30%. The frequency of intense bands and of nulls both declined as resistance declined. A selected substrain (MUHEZA-fb), breeding true for A2, A3, B2 and B3 at the standard level of activity, showed almost stable chlorpyrifos resistance (RR=1428–1785) for approximately 35 generations. In mass larval assays ofin vitrosensitivity of acetylcholinesterase (AChE) to propoxur, the I₅₀in MUHEZA was 950-fold greater than in a reverted resistant strain (RANGOON). Single larval assays indicated an AChE resistance allele (Ace_R) at frequency 0.43. PAGE of AChE revealed nine isozymes in MUHEZA and five in RANGOON, three of which were in common. It is concluded that propoxur resistance was due principally toAce^Rwith a minor influence from oxidases and non-specific esterases, while chlorpyrifos resistance was due to an interaction betweenAce^Rand non-specific esterases, the latter exerting the dominant effect.