28 Culex erythrothorax Dyar is a West Nile virus (WNV) vector that breeds in wetland habitats with 29 emergent vegetation. Urbanization and recreational activities near wetlands place humans and 30 birds in close proximity to Cx. erythrothorax, increasing the risk of WNV transmission. High 31 adult Cx. erythrothorax abundance peaked in a marsh bordering the San Francisco Bay of 32 California (USA) during the first 3 hours after sunset (5527 ± 4070 mosquitoes / trap night) 33 during which adult mosquito control efforts are likely most effective. These adult mosquitoes 34 were highly sensitive to permethrin, resmethrin, naled, and etofenprox insecticides when 35 tested in a bottle bioassay (LC50 = 0.35, 0.68, 0.71, and 4.1 µg/bottle, respectively). The 36 synergist piperonyl butoxide increased the sensitivity of the mosquitoes to a low concentration 37 of permethrin (0.5 µg/bottle) while also reducing knock down time, but did not have similar 38 effects with a higher permethrin concentration (2.0 µg/bottle). Biochemical enzyme assays 39 measuring bulk mixed-function oxidase, alpha-esterase, or beta-esterase activities in mosquito 40 homogenates were significantly higher in Cx. erythrothorax relative to a strain of Culex pipiens 41 that is sensitive to permethrin. Larvicide that was applied to the site had limited impact on 42 reducing the abundance of adult Cx. erythrothorax. Subsequent removal of emergent 43 vegetation in concert with reduced daily temperature in the environment substantially reduced 44 Cx. erythrothorax abundance. Land managers who have a need to control Cx. erythrothorax in 45 wetlands should consider vegetation removal over applying larvicide, while vector control 46 agencies are likely to successfully control viremic Cx. erythrothorax that enter nearby 47 neighborhoods by applying insecticides that target the adult stage of the mosquito within 3 h of 48 sunset. 3 49 Introduction 50 51 Culex erythrothorax Dyar, commonly known as the tule mosquito, is endemic to the 52 western southwestern states of the US [1]. The larvae breed in heavily vegetated regions of 53 shallow ponds and can be highly abundant in marsh habitats that contain dense clusters of 54 Schoenoplectus spp (common tule), Typha spp. (bulrush), or Myriophyllum aquaticum (parrot 55 feather) [2-4]. Unlike many species of mosquito, adult Cx. erythrothorax do not disperse 56 distantly from where they emerge [3, 5, 6]. The time of host-seeking is well documented for 57 Culex tarsalis Coquillett, another mosquito species found marsh habitats, which is reported to 58 be most active 1 -4 h after sunset [7, 8]. The time of day that Cx. erythrothorax is most likely 59 to seek a blood meal and would be best controlled by insecticides that target the adult 60 mosquito is not reported. Larvicide applications to constructed marsh habitats can markedly 61 reduce the abundance of adult Cx. erythrothorax [9]. However, the duration of reduction is 62 short, likely because adult mosquitoes immigrate from nearby sites that are not treated [5] and 63 d...