Monitoring of resistance is a recommended part of a complete integrated management plan as insecticide resistance can reduce the effectiveness of mosquito control. The challenge for mosquito control programs is to implement laboratory data into operational activities; at what point does the resistance seen in testing show up as reduced kill in the field? The answer to this question is not clear. To better understand the relationship between laboratory data and adulticide application in the field, we used recently colonized Aedes aegypti (FL) and Culex quinquefasciatus (LA). We established distinct substrains based on knockdown resistance (kdr) mutations of the voltage gated sodium channel and then conducted extensive laboratory resistance characterization using Type I and Type II pyrethroids as well as the organophosphate malathion. All field strains were “resistant” to pyrethroids in the CDC bottle bioassay although the mortality curves differed. We quantified the resistance levels and mechanistic contributions by topical application (with and without the synergist piperonyl butoxide) and enzymatic activity testing. We observed the primary contributor to resistance in these field Ae. aegypti was the specific kdr genotype and that enzymatic activity was not different than laboratory strains. In contrast, in the Culex substrains, enzymatic resistance was the major contributor and kdr played a much smaller role. Having uncovered differing resistance profiles in the laboratory, we then conducted wind tunnel spray tests using formulated, commercially available products. All were effective against the laboratory strains but the efficacy against the field strains was varied. The unsynergized pyrethroid product produced minimal knockdown and mortality. The synergized pyrethroid gave little knockdown after 1 hour but was effective within 24 hours against 5 of the 6 Ae. aegypti kdr substrains but not against any of the Culex field substrains. The malathion product was effective against the Aedes substrains but was not effective against the Culex strains. Exposure to permethrin treated uniforms also showed reduced efficacy with field strains and, in some cases, complete loss of protective efficacy. These studies demonstrate that predicting operational efficacy based on only laboratory assessment is difficult and not all resistance detected in the lab results in field failure. Operational efficacy clearly varies based on the contributions of the underlying mechanisms.

Relating laboratory insecticide resistance testing to operational efficacy in mosquitoes

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