The impact of insecticide decay on the rate of insecticide resistance evolution for monotherapies and mixtures.

Background: The problem of insecticide decay following their deployment in public health applications is frequently highlighted as an issue for sustained disease control. There are additional concerns that it also increases selection for insecticide resistance. Despite these concerns insecticide decay is largely absent from models evaluating insecticide resistance management strategies.

Methodology: The impact of insecticide decay is investigated using a model which assumes a polygenic basis of insecticide resistance. Single generation evaluations are conducted that cover the insecticide efficacy and insecticide resistance space for insecticides when deployed as monotherapies or mixtures to mechanistically investigate how insecticide decay impacts selection for resistance. The outcome is the between generation change in bioassay survival to the insecticides. The monotherapy sequence and mixture strategies were compared in multi-generation simulations incorporating insecticide decay, with the outcome being the difference in strategy lifespan.

Results: The results demonstrate that as insecticides decay, they can apply a much greater selection pressure than that imposed by newly deployed, non-decayed insecticides; this applies to both monotherapies and mixtures. For mixtures, selection for resistance is highest when both insecticides have reduced decayed efficacies; this also occurs if reduced dosages are deliberately used in mixtures. Insecticide decay was found to reduce the benefit of mixtures compared to sequential monotherapies, especially when reduced-dose mixtures are used.

Conclusions: Insecticide decay is often highlighted as an important consideration for mixtures and these results indicate its absence in previous modelling studies may be over-inflating the performance of full-dose mixtures.

In summary: as insecticides decay, they can impose increasing selection pressures for resistance with reduced ability to control the target insect populations. The optimal frequency with which decaying insecticides should be replenished is an important policy consideration.