Pyrethroids and organophosphate resistance in Aedes aegypti (Diptera: Culicidae) and their underlying mechanisms

BACKGROUND: For decades, insecticides have been central to controlling the yellow fever mosquito, Aedes aegypti (L.), but extensive use has driven resistance development. This study investigates resistance of Ae. aegypti to pyrethroids (permethrin, deltamethrin) and organophosphates (malathion, pirimiphos-methyl) and their underlying mechanisms across Malaysia, Thailand, Indonesia, and the USA. METHODS: pirimiphos-methyl. Each assay included four replicates of 25 mosquitoes, with mortality assessed at 24 h post-exposure. Genomic DNA was extracted from 10 resistant individuals per population, and two coding regions of the voltage-gated sodium channel (VGSC) gene (domains II and III) were amplified and sequenced to detect known and novel kdr mutations. For biochemical analysis, 40 newly emerged, non-blood-fed females per strain were individually homogenized to quantify mixed-function oxidase (MFO), esterase (α- and β-EST), glutathione S-transferase (GST), and acetylcholinesterase (AChE) activity. RESULTS: High resistance levels were recorded in Malaysian and US Ae. aegypti strains, with low mortality ranging between 9% and 22% for pyrethroids. New mutations T1520I (8-15%) and I1011M (10-15%) were identified in Malaysian populations, the first detection of T1520I in the country, while V1016I (10%) was newly detected in Indonesian strains. Malaysian mosquitoes had multiple kdr mutations (S989P, V1016G, F1534C, and T1520I) in triple- and quadruple-haplotype combinations. The US Riverside strain showed a nine- to 10-fold increase in β-EST and three- to fivefold increase in MFO and GST activity compared to the VCRU susceptible strain, indicating strong metabolic resistance. In contrast, the highly resistant Malaysian Hamna strain exhibited no significant upregulation (P > 0.05) in detoxifying enzymes, suggesting resistance was driven primarily by kdr mutations. Thai strains lacked kdr mutations but exhibited altered AChE (20-35% remaining activity) and elevated GST (2-3 times higher than control). CONCLUSIONS: The detection of novel kdr mutations and diverse resistance mechanisms underscores the adaptability of Ae. aegypti to insecticide pressure and highlights the urgent need for continuous monitoring and integrated resistance management strategies.