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St. Tammany Parish Mosquito Abatement District relies on ultra-low volume (ULV) insecticides to manage adult Culex quinquefasciatus (Say 1823), the primary vector of West Nile virus in Louisiana, but abundance rarely declines post-treatment. Three field ULV applications each of adulticides containing deltamethrin, naled, or resmethrin were conducted from May through October 2022 to measure efficacy against free-flying Cx. quinquefasciatus. Female mosquitoes trapped before and after treatment were identified, counted, and age-graded by parity dissection. No insecticide applications reduced Cx. quinquefasciatus abundance, but trials with deltamethrin and naled significantly lowered average mosquito age in the area post-treatment. No trials with resmethrin reduced mosquito age, which corroborates laboratory susceptibility data indicating local resistance toward the pyrethroid. These trials establish parity dissections and relative age gradation as a means of measuring treatment success against Cx. quinquefasciatus., (© The Author(s) 2023. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)