Topical applications of pyrethroid and organophosphate mixtures revealed positive interactions against pyrethroid-resistant Anopheles gambiae.

The efficacy of a binary mixture of bifenthrin (pyrethroid) and chlorpyrifos-methyl (organophosphate) has been tested against susceptible and pyrethroid-resistant strains of Anopheles gambiae Giles, the major malaria vector in sub-Saharan Africa. Dose-mortality regression lines were determined for each insecticide alone and in mixtures by topical applications on adult female mosquitoes. A combination index (CI) was used to quantify the interactions occurring between the pyrethroid and organophosphate insecticides. The results revealed synergism at high doses against both susceptible (0.7 > CI > 0.3) and pyrethroid-resistant (0.9 > CI > 0.7) An. gambiae. These results suggest that insecticide mixtures may be an alternative strategy for vector control, especially in areas where mosquitoes are resistant to pyrethroids.