Identification of Molecular Determinants of Resistance to Pyrethroid Insecticides in Aedes aegypti (Diptera: Culicidae) Populations in California, USA

Abstract

The first breeding populations of Aedes aegypti (Linnaeus) were identified in California in 2013, and have since been detected in 13 counties. Recent studies suggest two introductions likely occurred, with genetically distinct populations in the central and southern regions of the state. Given the threat of dengue, chikungunya, and Zika virus transmission, it is imperative to understand if these populations harbor genes that could confer resistance to pyrethrin-based insecticides, known as pyrethroids, the most commonly used class of adulticides in the state. In 2017, the California Department of Public Health initiated a pesticide resistance screening program for Ae. aegypti to assess the presence of specific mutations on the sodium channel gene (V1016I and F1534C) associated with knockdown resistance to pyrethroids. Mosquitoes collected between 2015 and 2017 from 11 counties were screened for mutations using real-time polymerase chain reaction assays. Results revealed distinctly different resistance profiles between the central and southern regions. The central population displayed nearly fixed resistant mutations at both loci, whereas the southern population was more variable. The relative proportion of resistant alleles observed in sampled mosquitoes collected in southern California increased each year from 2015 through 2017, indicating potential increases in resistance across this region. The presence of these mutations indicates that these mosquitoes may be predisposed to surviving pyrethroid treatments. Additional biological and biochemical assays will help better elucidate the mechanisms underlying insecticide resistance in California Ae. aegypti and prompt the use of pesticides that are most effective at controlling these mosquitoes.

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

“…4A). This is consistent with previous reports showing little to no polymorphism in VGSC fragments in central California populations (7,32). The quantitatively different genetic variations of the Greater LA population from the Central Valley population is also consistent with a previous study monitoring both Central Valley and southern California locations (7).…”

“…This is consistent with previous reports showing little to no polymorphism in VGSC fragments in central California populations (7,32). The quantitatively different genetic variations of the Greater LA population from the Central Valley population is also consistent with a previous study monitoring both Central Valley and southern California locations (7). It is noteworthy that independent studies utilizing whole genome sequence (33), SNP chip (34), and microsatellite (34) indicated different genetic makeup separating southern CA and Central CA.…”

“…These surveillance efforts found that between 2015 and 2017 resistant alleles were largely xed in the Central Valley population, and less abundant (> 80% for F1534C and > 61% for V1016I from 2015-2017) but increasing in prevalence in the Southern population of Ae. aegypti (7). Studies in Mexico have identi ed similar trends (17).…”

“…The VGSC mutations V1016I, F1534C and V410L are prevalent in Ae. aegypti in the Americas and have been closely monitored by the California Department of Public Health (CDPH) (7,13,15,17). These surveillance efforts found that between 2015 and 2017 resistant alleles were largely xed in the Central Valley population, and less abundant (> 80% for F1534C and > 61% for V1016I from 2015-2017) but increasing in prevalence in the Southern population of Ae.…”

“…However, the invasive populations of Ae. aegypti in California possess genetic mutations conferring resistance to the pyrethroid class of insecticides typically applied for vector control (6,7). Pyrethroids act on insect voltage gated sodium channels (VGSC) by binding to open channels and blocking the channel in the open conformation.…”