Resistance to dieldrin plus fipronil assorts with chromosome inversion 2Lain the malaria vector Anopheles gambiae

Cyclodiene insecticide resistance in malaria vector mosquitoes of the Anoph eles gambiae species complex (Diptera: Culicidae) has been reported previou sly from several parts of Africa. We report resistance to dieldrin, a cyclo diene, in two laboratory strains of An. gambiae Giles sensu stricto code-na med Ian P20 from Nigeria (1979) and CIG from Cote d'Ivoire (1997). Dieldrin resistance levels were high in adult female mosquitoes (40-75% survived ex posure to 4% dieldrin for 1 h) and was closely linked with chromosomal para centric inversion 2La. This inversion did not occur in Hardy-Weinberg propo rtions, but showed an excess of heterozygotes in both strains, which may ac count for the high levels of resistance. This linkage also suggests that di eldrin resistance in Ian P20 is dominant. After subsamples of strain Ian P2 0 were exposed for 1 h to dieldrin 4% or fipronil 2% (discriminating concen trations), the resultant mortality-rates (61% and 65%) were not significant ly different. Most survivors after fipronil treatment also survived subsequ ent exposure to dieldrin (46/50 = 92%). This apparent cross-resistance betw een insecticides of two classes (cyclodiene and phenyl pyrazole) has implic ations for the management of insecticide resistance in wild populations of the An. gambiae complex.