Pyrethroid insecticides are commonly used to control Aedes aegypti, an important human disease vector, however, insecticide resistance is spreading rapidly amongst this mosquito species. Interactions between the two major mechanisms of pyrethroid resistance, cytochrome-P450-monooxygenases (CYPs)-mediated detoxification and voltage-sensitive sodium channel (Vssc) target gene modifications (known as kdr), can influence the rate that pyrethroid resistance evolves and therefore, influence the level of resistance in a population. Little is known about the evolution of the individual resistance mechanisms, and how the interaction between the mechanisms affects fitness or pathogen transmission. Our goal was to determine how the mechanisms interact both in the presence of insecticides (i.e. what levels of resistance do they individually and collectively give) and in the absence of insecticides (i.e. what fitness costs do they individually and collectively induce). We used three congenic strains containing the different resistance mechanisms alone and in combination: CYP:ROCK (CR) that contains CYP-mediated detoxification but no kdr; KDR:ROCK (KR) that contains only kdr; and CYP+KDR:ROCK (CKR) that contains both the kdr and CYP-mediated resistance mechanisms. We then assessed the insecticide resistance and cross-resistance of several pyrethroids and evaluated the fitness costs based on both specific fitness parameters (i.e. developmental time, survival, fecundity, fertility, and body size [life table study]), and by measuring overall fitness via an allele-competition study. From our results, it appears that kdr has a fitness cost that is not due to fertility, fecundity, body size, developmental time, or survival, but due to other factors (e.g. mating or food competition). Comparing the insecticide resistance with the fitness costs, it appears that kdr provides a high fitness advantage to Ae. aegypti in the presence of insecticides yet has only a modest fitness cost in the absence of insecticides. CYP-mediated resistance, however, provides only low to moderate fitness advantage, while inducing a high fitness cost.

Fitness advantages and disadvantages of pyrethroid resistance mechanisms in Aedes aegypti, and their potential consequences to human health

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