<b>Pyrethroid Insecticide Resistance in Aedes aegypti Field Populations Across Dengue Hotspots in Islamabad</b>

Background: Aedes aegypti is the principal vector of dengue in Pakistan, and insecticide-based control remains a primary intervention. However, localized resistance patterns in Islamabad are insufficiently characterized. Objective: To assess the susceptibility of Ae. aegypti populations across Islamabad to commonly used pyrethroids and to evaluate spatial patterns of resistance intensity. Methods: A cross-sectional entomological study was conducted across ten sectors of Islamabad (March–August 2024). Adult susceptibility to cypermethrin, deltamethrin, and lambda-cyhalothrin was assessed using WHO tube bioassays, while larval susceptibility to permethrin was evaluated through concentration-response assays to determine LC₅₀, LC₉₀, and resistance ratios relative to a susceptible laboratory strain. Results: Lambda-cyhalothrin demonstrated full susceptibility across all sites (≥98% mortality). Deltamethrin showed confirmed resistance at one site (F-10 Markaz), while cypermethrin exhibited resistance at two sites (F-10 Markaz, I-8/4). Permethrin showed reduced larval susceptibility across all sites, with resistance ratios ranging from 4.2 to 18.4 and moderate resistance observed in five sectors. Resistance intensity was positively correlated with proximity to agricultural land (r = 0.72, p < 0.01). Conclusion: Insecticide susceptibility in Islamabad is compound-specific and spatially heterogeneous. Evidence-based, site-specific resistance management strategies are essential for effective vector control