ORGANOPHOSPHORUS INSECTICIDE RESISTANCE IN A NEW STRAIN OF CULEX-QUINQUEFASCIATUS (DIPTERA, CULICIDAE) FROM TANGA, TANZANIA

Fourth instar larvae of a new strain of Culex quinquefasciatus Sav from Tanzania (TANGA) were tested for insecticide resistance. Initially, the resistance ratio (RR) to chlorpyrifos was 41.8, to temephos 30.8, to propoxur 3.7. After 2-3 years of laboratory culture, resistance to chlorpyrifos and propoxur had declined (chlorpyrifos 5.7, 3.8; propoxur 1.9, permethrin 1.9). Significant synergism was found between s,s,s-tributyl trithiophosphate (DEF) and chlorphyrifos, reducing the RR from 8.0 to 2.5. Synergism between piperonyl butoxide and permethrin was less than in a susceptible control strain. Twelve esterase isozymes of different relative mobilities (Rm) on polyacrylamide gel electrophoresis were identified, ten of which remained when the strain was reinvestigated two years (approximately 32 generations) later. Null activity for all but one of these bands was observed in some larvae. Four esterase bands (Rm 0.25, 0.27, 0.31, ().34, designated A2, A3, B2, B3) showed polymorphism in activitv, with very intense bands in some larvae. The mean frequency of bands with activitv greater than standard, declined as organophosphorus (OP) resistance declined, but resistance was unconnected with the frequency of nulls at these positions. In mass larval assays of in vitro sensitivity of acetylcholinesterase (AChE) to propoxur, the I50 exceeded 10 x 10(-4) M, compared with 0.1 x 10(-4)M in a reverted resistant strain (RANGOON). Single larval assays revealed heterogeneity, which was interpreted on the basis of an AChE resistance allele (Ace(R)) with a frequency of 0.23.