resistance as the probability of a simultaneous appearance of resistance to two insecticides with different modes of action would be less likely (Curtis, 1985). The kdr gene being semirecessive, the heterozygous mosquitoes (RS) behave approximately in the same way as the susceptible homozygous mosquitoes (SS) with respect to mortality (Chandre et al., 2000). The synergistic effect obtained from bifenthrin and chlorpyrifos-methyl may facilitate the elimination of the heterozygous individuals (RS), thus preventing the selection of the kdr mutant gene that confers tolerance to pyrethroids. On the other hand, since the combination tested on the strains of An. gambiae highly resistant to pyrethroids does not induce any synergistic effect, the concept of mixture could prove less advantageous in locations where resistant populations already exit. The pyrethroid insecticide having only little or no effectiveness, only the organophosphorous insecticide kills the resistant mosquitoes, which then simply results in an added effect to the mor- tality rate induced by the two insecticides.
The evaluation of mosquito nets impregnated with a mixture of a pyrethroid and chlorpyrifos-methyl must therefore now be conducted in experimental huts, not only in the areas where An. gambiae is susceptible to pyrethroids, but also in the areas where this mosquito is resistant (Darriet et al., 2000).