Abstract: Mosquito tolerance to pesticides is a complex and multi-layered phenomenon with genetic, metabolic, physiological, behavioral, and even physical mechanisms working together to produce a “resistant” phenotype. Vector Culex pipiens mosquitoes in the Chicago, Illinois region routinely demonstrate high levels of tolerance to pyrethroid control materials. Surveys utilizing the CDC bottle bioassay were conducted in 2018, 2019 and 2020 and demonstrated geographically widespread and temporally stable resistance to sumithrin and permethrin. Operational field trials in 2020 with Culex spp. mosquito populations and caged field trials in 2021 further corroborated a reduced susceptibility to ground-based ULV methodologies based on pyrethroid materials. Gravid Culex spp. mosquitoes, as measured by a BG Counter and modified CDC gravid traps, were especially poorly controlled in field trials. Further wind tunnel trials with resistant and susceptible mosquitoes demonstrated that resistant mosquitoes exhibit an enhanced resistance in the 48 hours after a blood meal that nearly halves their already reduced susceptibility. Metabolic and genetic resistance coupled with a transient post-blood meal resistance is likely to have important implications for the age-structure and viral prevalence of targeted vector mosquito populations.
