Hobbs, N. P.; Hastings, I.

The issue of insecticide decay in the public health deployments of insecticides is frequently highlighted as an issue for disease control. There are additional concerns insecticide decay also impacts the selection for insecticide resistance. Despite these concerns insecticide decay is lacking from models evaluating insecticide resistance management strategies. The impact of insecticide decay is modelled using a model which assumes a polygenic basis of insecticide resistance. Single generation selection events covering the insecticide efficacy and insecticide resistance space for both monotherapies and mixtures are conducted. With the outcome being the between generation change in the bioassay survival to the insecticides. The monotherapy sequence strategy and mixture strategy were compared against each other when including insecticide decay, with the outcome being the difference in strategy lifespan. The results demonstrate that as insecticides decay, they can apply a greater selection pressure than newly deployed insecticides, a process which can occur for both monotherapies and mixtures. For mixtures, it is seen that the rate of selection is highest when both insecticides are at reduced efficacies which would occur if reduced dose mixtures were used. Inclusion of insecticide decay in simulations was found to reduce the benefit of mixtures against monotherapy sequences, and this is especially so when reduced-dose mixtures are used. Insecticide decay is often highlighted as an important consideration for mixtures. The inclusion of insecticide decay in models is often lacking, and these results indicate this is absence may be over-inflating the performance of full-dose mixtures. As insecticides decay, they still provide selection pressures with reduced ability to control transmission, replenishing worn-out insecticides more frequently should be considered.