Association Between Pyrethrum Knockdown Time and Sodium Channel Genotypes in California Aedes Aegypti

Abstract

Background: Since their detection in 2013, Aedes aegypti has become a widespread urban pest in California. The availability of cryptic larval breeding sites in residential areas and resistance to insecticides pose significant challenges to control efforts. Resistance to pyrethroids is largely attributed to mutations in the voltage gated sodium channels (VGSC), the pyrethroid site of action. However, past studies have indicated that VGSC mutations may not be entirely predictive of the observed resistance phenotype. Methods: To investigate the frequencies of VGSC mutations and the relationship with pyrethroid insecticide resistance in California, we sampled Ae. aegypti from four locations in the Central Valley, and the Greater Los Angeles area. Mosquitoes from each location were subjected to an individual pyrethrum bottle bioassay to determine knockdown times. A subset of assayed mosquitoes from each location was then analyzed to determine the composition of 8 single nucleotide polymorphism (SNP) loci within the VGSC gene. Results: The distribution of knockdown times for each of the five Californian populations sampled was non-parametric with bimodal distributions. One group succumbs to insecticidal effects around 35-45 minutes and the second group lasts up to and beyond the termination of the assay (120+ minutes). We detected 5 SNPs polymorphic within California populations. One is potentially new and alternatively spliced (I915K), and four are known and associated with resistance: F1534C, V1016I, V410L and S723T. The Central Valley populations (Clovis, Dinuba, Sanger and Kingsburg) are fairly homogenous with only 5% of the mosquitoes showing heterozygosity at any given position. In the Greater LA mosquitoes 55% had at least one susceptible allele at any of the five SNPs. The known resistance allele F1534C was detected in almost all sampled mosquitoes (99.4%). We observe significant heterogeneity in individuals with the same genotypes, confirming that the resistance SNPs alone cannot. Conclusions: Resistance associated VGSC SNPs are prevalent, particularly in the Central Valley. Interestingly, among mosquitoes with all 4 resistance associated SNPs, we observe heterogeneity in bottle bioassay profiles suggesting that other mechanisms are important to the individual resistance of Ae. aegypti in California.