Culex erythrothorax: Insecticide Susceptibility and Mosquito Control

Culex erythrothorax Dyar is a West Nile virus (WNV) vector that breeds in wetland habitats with emergent vegetation. Urbanization and recreational activities near wetlands place humans and birds in close proximity to Cx. erythrothorax , increasing the risk of WNV transmission. High adult Cx. erythrothorax abundance peaked in a marsh bordering the San Francisco Bay of California (USA) during the first 3 hours after sunset (5527 ± 4070 mosquitoes / trap night) during which adult mosquito control efforts are likely most effective. These adult mosquitoes were highly sensitive to permethrin, resmethrin, naled, and etofenprox insecticides when tested in a bottle bioassay (LC50 = 0.35, 0.68, 0.71, and 4.1 µg/bottle, respectively). The synergist piperonyl butoxide increased the sensitivity of the mosquitoes to a low concentration of permethrin (0.5 µg/bottle) while also reducing knock down time, but did not have similar effects with a higher permethrin concentration (2.0 µg/bottle). Biochemical enzyme assays measuring bulk mixed-function oxidase, alpha-esterase, or beta-esterase activities in mosquito homogenates were significantly higher in Cx. erythrothorax relative to a strain of Culex pipiens that is sensitive to permethrin. Larvicide that was applied to the site had limited impact on reducing the abundance of adult Cx. erythrothorax . Subsequent removal of emergent vegetation in concert with reduced daily temperature in the environment substantially reduced Cx. erythrothorax abundance. Land managers who have a need to control Cx. erythrothorax in wetlands should consider vegetation removal over applying larvicide, while vector control agencies are likely to successfully control viremic Cx. erythrothorax that enter nearby neighborhoods by applying insecticides that target the adult stage of the mosquito within 3 h of sunset.