Larval density mediates knockdown resistance to pyrethroid insecticides in adult Aedes aegypti

Abstract

BackgroundUnderstanding mechanisms driving insecticide resistance in vector populations remains a public health priority. To date, most research has focused on the genetic mechanisms underpinning resistance, yet it is unclear what role environmental drivers may play in shaping phenotypic expression. One of the key environmental drivers of Aedes aegypti mosquito population dynamics is resource-driven intraspecific competition at the larval stage. We experimentally investigated the role of density-dependent larval competition in mediating resistance evolution in Ae. aegypti, using knockdown resistance (kdr) as a marker of genotypic resistance and CDC bottle bioassays to determine phenotype. We reared first-instar larvae from susceptible and pyrethroid-resistant field-derived populations of Ae. aegypti at high and low density and measured the resulting phenotypic resistance and population kdr allele frequencies.ResultsAt low density, only 48.2% of the resistant population was knocked down, yet at high density, the population was no longer phenotypically resistant - 93% were knocked down when exposed to permethrin, which is considered susceptible according to WHO guidelines. Furthermore, the frequency of the C1534 kdr allele in the resistant population at high density decreased from 0.98 ± 0.04 to 0.69 ± 0.04 in only one generation of selection.ConclusionsOur results indicate that larval conditions, specifically density, can impact both phenotype and genotype of pyrethroid-resistant populations. Furthermore, phenotypic susceptibility to pyrethroids may be re-established in a resistant population through a gene x environment interaction, a finding that can lead to the development of novel resistance management strategies that capitalize on density-induced costs.

Experimental evaluation of the impact of household aerosolized insecticides on pyrethroid resistant Aedes aegypti

“…aegypti may be explained through multiple kdr mutations as well as other potential mechanisms of resistance 32 . Previous studies 27 , 33 and analyses of a random sample of 100 adult female Ae . aegypti from this study (50 survivors and 50 dead from the space spraying trial) showed that another kdr mutation (C1534) was found in ~100% of all tested mosquitoes.…”

“…In addition, as this study did not explicitly quantify selection, future studies should focus on the evolutionary consequences of repeated insecticide applications over multiple generations and on various pyrethroid resistance mechanisms, both in the field and the lab. Recent evidence shows that environmental conditions (e.g., larval habitats 33 ) or individual level variation (e.g., attributes of the gut microbiota 39 ) can greatly affect mosquito resistant phenotypes. Consequently, assessing the evolutionary implications of household insecticide use will require a comprehensive study integrating laboratory and field experimentation.…”

Widespread insecticide resistance in Aedes aegypti L. from New Mexico, U.S.A.

“…Each pan received approximately 4 pellets and the water was changed every 3 days. This regimen ensured that larvae were not crowded, as high larval densities can suppress insecticide resistance [45]. When pupae started to appear, they were removed daily and placed into small cups with water within BugDorm-1 Insect Rearing Cages (Bugdorm Store, Mega View Science, Taiwan).…”

The need for practical insecticide-resistance guidelines to effectively inform mosquito-borne disease control programs

“…Increased food availability during the aquatic immature stages of various mosquito species is known to increase insecticide resistance ( Grossman et al, 2018 ; Ong and Jaal, 2018 ; Owusu et al, 2017 ). Strikingly, the effect of high larval densities (which translates to fiercer competition for nutrients) can revert the resistance phenotype in genetically resistant larvae ( Grossman et al, 2018 ). Similar effects are found in adult female mosquitoes, where blood-fed females are generally more resistant than unfed females ( Machani et al, 2019 ).…”