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Background  The dramatic success of insecticide treated nets (ITNs) and long-lasting insecticidal nets (LLINs) in African countries has been countered by the rapid development of pyrethroid resistance in vector mosquitoes over the past decade. One advantage of the use of pyrethroids in ITNs is their excito-repellency. Use of the excito-repellency of pyrethroids might be biorational, since such repellency will not induce or delay the development of any physiological resistance. However, little is known about the relationship between the mode of insecticide resistance and excito-repellency in pyrethroid-resistant mosquitoes.    Methods  Differences in the reactions of 3 major malaria vectors in western Kenya to pyrethroids were compared in laboratory tests. Adult susceptibility tests were performed using World Health Organization (WHO) test tube kits for F1 progenies of field-collected  An. gambiae  s.s.,  An. arabiensis , and  An. funestus  s.s., and laboratory colonies of  An. gambiae  s.s. and  An. arabiensis . The contact repellency to pyrethroids or permethrin-impregnated LLINs (Olyset® Nets) was evaluated with a simple choice test modified by WHO test tubes and with the test modified by the WHO cone bioassay test.    Results  Field-collected  An. gambiae  s.s.,  An. arabiensis , and  An. funestus  s.s. showed high resistance to both permethrin and deltamethrin. The allelic frequency of the point mutation in the voltage-gated sodium channel (L1014S) in  An. gambiae  s.s. was 99.3–100%, while no point mutations were detected in the other 2 species. The frequency of takeoffs from the pyrethroid-treated surface and the flying times without contacting the surface increased significantly in pyrethroid-susceptible  An. gambiae  s.s. and  An. arabiensis  colonies and wild  An. arabiensis  and  An. funestus  s.s. colonies, while there was no significant increase in the frequency of takeoffs or flying time in the  An. gambiae  s.s. wild colony.    Conclusion  A different repellent reaction was observed in the field-collected  An. gambiae  s.s. than in  An. arabiensis  and  An. funestus  s.s. It might be that resistant mosquitoes governed by knockdown resistance ( kdr ) loose repellency to pyrethroids, whereas those lacking  kdr  maintain high repellency irrespective of their possessing metabolic resistance factors to pyrethroids. Further genetic evaluation is required for the demonstration of the above hypothesis.

Insecticidal and repellent activities of pyrethroids to the three major pyrethroid-resistant malaria vectors in western Kenya