Fitness cost of insecticide resistance in overwintering conditions in Culex pipiens pallens

Culex pipiens pallens, a vector of filariasis and West Nile Virus, has developed resistance to pyrethroid and other insecticides used for mosquito control. However, the frequency of the kdr (knockdown resistance) and rdl (resistance to dieldrin) traits, which result in resistance to pyrethroids and fipronil, respectively, exhibits seasonal fluctuation between 0% to 50% level, despite continuous use of these insecticides. To assess the fitness costs of the kdr and rdl traits in Cx. pipiens pallens under overwintering conditions, susceptible (SUS), pyrethroid-resistant (PY-R), and fipronil-resistant (FP-R) strains of mosquitoes were exposed to winter-like conditions (4℃ and 9L:15D) for five weeks. The results revealed that the PY-R strain had a 28% lower survivorship rate (65.3%) and significantly reduced blood-feeding rates (68.6% to 7.9%) compared to the SUS strain (83.3% to 50%), whereas FP-R strain showed no significant differences. However, both the PY-R and FP-R strains showed reduced oviposition rates than the SUS strain in both conditions. Moreover, gene expression analysis revealed significant differences in the expression of metabolism-related genes such as FABP and P5C reductase, HSP70, and zinc-carboxypeptidase, indicating potential metabolic costs associated with lipid accumulation and desiccation resistance. This was supported by the differences of water, lipid, and glucose content among the three strains. Overall, these findings suggest that both pyrethroid and fipronil resistance in Cx pipiens pallens have some fitness costs in winter conditions, while pyrethroid resistance appears to have an additional cost on survivorship.