Insecticide resistance profiles and mechanisms in field populations of <i>Aedes aegypti</i> and <i>Aedes albopictus</i> (Diptera: Culicidae) in Taiwan

Insecticide resistance in Aedes aegypti (Linn., 1762) and Aedes albopictus (Skuse, 1894) has hindered efforts to control dengue outbreaks. Understanding resistance profiles and underlying mechanisms is essential to guide the selection of insecticides and synergists for operational use in northern and southern Taiwan. In this study, we conducted WHO-standard insecticide susceptibility assays, synergist-insecticide bioassays, and molecular diagnostics on field-collected Aedes populations to characterize resistance phenotypes and elucidate their mechanisms. Three pyrethroid insecticides: deltamethrin, permethrin, and lambda-cyhalothrin, and two non-pyrethroids: pirimiphos-methyl (organophosphate) and bendiocarb (carbamate), were tested. Aedes aegypti exhibited resistance to all three pyrethroids and pirimiphos-methyl. High frequencies of multiple simultaneous kdr mutations (S989P, V1016G, F1534C, and D1763Y), along with cytochrome P450 monooxygenase, significantly conferred the pyrethroid resistance in Ae. aegypti. All northern Ae. albopictus populations showed resistance to pyrethroids, although susceptibility to pirimiphos-methyl and bendiocarb varied among populations. The I1532V mutation was detected at low frequency and was not associated with pyrethroid resistance in these populations. Piperonyl butoxide (PBO) restored susceptibility in most Ae. albopictus populations, indicating a significant role of cytochrome P450 monooxygenases in conferring pyrethroid resistance. Furthermore, a significant correlation was found between the frequency of insecticide applications in local areas and reduced mosquito mortality, suggesting that chemical control practices exert selective pressure on Ae. albopictus -populations. This study reveals substantial variation in both resistance phenotypes and mechanisms between Ae. aegypti and Ae. albopictus in Taiwan. These findings underscore the need for adaptive vector control strategies to mitigate resistance development and sustain the efficacy of chemical interventions.