Assessing permethrin resistance for Culex tarsalis in southern California

In the western United States, Culex tarsalis is the most important vector of West Nile virus. Due to the wide range of environments where Cx. tarsalis are found, this species is under insecticide pressure from both agricultural spraying and vector control. Resistant mosquitoes may display resistance mechanisms such as target site insensitivity. The most common target site insensitivity in Cx. tarsalis is a knockdown resistance (kdr) mutation. Cx. tarsalis were collected from five southern California locations with varying environmental and habitat characteristics to determine prevalence of permethrin resistance. Permethrin resistance was evaluated through CDC bottle bioassays to assess a resistance phenotype and by qPCR and sequencing to detect the presence of a kdr mutation. Mosquitoes were placed within glass bottles coated with 43 µg of permethrin with a diagnostic survival time of 20 minutes indicating resistance. Individual mosquitoes were classified as either resistant or susceptible according to their survival time. The resistance genotype for phenotypically resistant or susceptible individuals were examined using qPCR and sequencing. Phenotypic permethrin resistance was evident in Cx. tarsalis populations collected from the southern California desert communities (Coachella Valley), while Cx. tarsalis from the southern California inland valley region (western Riverside County) were all susceptible. Individual Cx. tarsalis from the Coachella Valley were found to have homozygous or heterozygous kdr mutations. In contrast, individual Cx. tarsalis collected from western Riverside County were found to have a mix of homozygous or heterozygous wild type alleles.