Culex — the house mosquito
Culex spp.
Quick answer
Culex are the brownish "house mosquitoes" most people picture — they bite from dusk through the night, peaking in the first couple of hours after sunset. They breed in stagnant, organically-rich water (storm drains, ditches, neglected pools, rain barrels) rather than clean containers, feed heavily on birds and then mammals, and are the principal U.S. vector of West Nile virus. They respond well to CO₂, so a CO₂ + heat trap running through the dusk-to-night window targets them effectively.
Biology & identification
Medium-sized, dull brown, unbanded mosquitoes. Unlike container Aedes, Culex lay eggs in floating rafts on the surface of stagnant, organically polluted water — storm-water catch basins, clogged ditches, neglected swimming pools, septic seeps, rain barrels, and other "dirty water" sites.
This makes their breeding harder to eliminate by simple tip-and-toss — it is often public or infrastructure water, not a backyard saucer. Culex pipiens dominates in cooler northern climates; Cx. quinquefasciatus (southern house mosquito) in the warm South; Cx. tarsalis is a key West Nile vector in the West.
Adults overwinter as mated females in sheltered sites.
Peak activity — when it bites
Crepuscular to nocturnal — host-seeking begins around dusk and continues through the night, with a pronounced peak in the first 1–2 hours after sunset (the basis of SkeeterSwitch’s default dusk-through-night run window).
This is the genus the "mosquitoes bite at dusk" intuition actually fits.
Why it bites — host-seeking
Females need blood for eggs. Culex are ornithophilic-to-mammalophilic — they feed heavily on birds (amplifying West Nile virus in bird populations) and then shift to mammals, including humans, later in the season.
This bird-then-mammal "bridge" behavior is exactly what makes Culex the key West Nile bridge vector. They orient strongly to CO₂ and skin odor in low light.
How the CO₂ trap targets it
A strong trap target. Culex are CO₂-responsive and active in the dusk-to-night window the trap is scheduled to run, so a CO₂ + heat trap captures host-seeking females effectively (consistent with USDA-ARS trapping data).
Because much Culex breeding is in infrastructure water you cannot drain, the adult-trapping + Bti-in-standing-water combination is especially well-suited to this genus — you often cannot tip-and-toss a storm drain.
Range & vector status
The principal U.S. vector of West Nile virus, plus St. Louis encephalitis and (globally) Western equine encephalitis and lymphatic filariasis. Found across the entire continental U.S.
This is factual vector status: the trap reduces biting pressure, not disease risk per se.
Sources
- U.S. CDC, "West Nile Virus — Transmission" (Culex vector).link pending verification
- Farajollahi, A. et al. (2011), "Culex pipiens complex: a bridge vector," Infection, Genetics and Evolution 11(7):1577–1585.
- Kline, D.L. (2006), CO₂ trap response of Culex.link pending verification
Common questions
What time of day do Culex mosquitoes bite?
From dusk through the night, with a pronounced peak in the first one to two hours after sunset. This is the genus the "mosquitoes bite at dusk" intuition fits.
Why is Culex hard to control by dumping standing water?
Culex breed in stagnant, organically-rich water that is often public infrastructure — storm drains, catch basins, ditches — which you cannot simply tip and toss. Adult trapping plus Bti in standing water you can reach is the better-suited combination.
What disease does Culex spread?
Culex is the principal U.S. vector of West Nile virus (and St. Louis encephalitis). Stated factually — reducing biting pressure is not a disease-prevention claim.