Culex tarsalis (western encephalitis mosquito)

The short answer

Culex tarsalis is one of the most important disease-carrying mosquitoes in the western half of the country. It is a medium-sized, dark mosquito, and the feature that gives it away is a bright pale band across the middle of its dark proboscis (the long feeding tube), along with pale bands on its legs [1], [2]. It matters for public health because it is the main mosquito that spreads West Nile virus across western regions, and it is a principal carrier of western equine encephalitis and St. Louis encephalitis as well [3], [1], [2]. Its habit of feeding on both birds and people is what lets it move these viruses from wild birds into humans and livestock.

How to identify it

Culex tarsalis is a medium-sized, dark-bodied mosquito [2]. The most reliable field mark is a broad white or pale band around the middle of the dark proboscis, which is unusual and helps separate it from most other common Culex mosquitoes [1], [2]. Look also at the legs: it has pale bands that overlap the joints of the tarsi (the far leg segments), plus pale longitudinal stripes running along the middle and hind legs [1], [2]. On the underside of the abdomen it carries dark chevron (V-shaped) markings [1], [2].

Taken together, the pale-banded proboscis and the banded, striped legs are the quickest way to recognize this species. Plainer house mosquitoes such as Culex pipiens have a dark, unbanded proboscis, so the pale band is a useful point of difference.

Where it lives

Culex tarsalis is a western and plains species. Its range runs from northern Mexico and Baja California north to southern Canada, and from the Pacific coast eastward, though it becomes uncommon east of the Mississippi River [1]. It is most common in rural and agricultural country rather than dense city centers [3].

Its larvae grow in many kinds of standing ground water. Females lay eggs in newly flooded, sunlit surface pools, often fringed by grasses and annual vegetation, and the larvae develop in a wide range of sites including irrigation runoff from farmland, ditches, canals, wetlands, ground pools, hoofprints, catch basins, ornamental ponds, and even treated sewage effluent [1], [2]. Irrigated agriculture is a major reason the species thrives, because it turns otherwise dry land into productive breeding habitat.

Behavior

Culex tarsalis bites mainly around dusk and through the night, becoming most active after sunset [1], [2]. Early in the season, females feed largely on birds shortly after sundown. As populations build in late summer and birds start avoiding them, many females shift to feeding on mammals including rabbits, horses, cattle, and people [1], [2]. This switch from birds to mammals is exactly what makes the species dangerous, because a mosquito that fed on an infected bird can later bite a person or a horse.

It is also a capable flier. While females typically move only about 100 yards a day from their breeding sites, host-seeking and dispersal flights have been recorded reaching much longer distances, up to roughly 17 miles [1]. That range means breeding sites well outside a town can still supply mosquitoes that reach people.

Diseases it can carry

Culex tarsalis is described as the most important mosquito vector of arboviruses in western North America [1]. The three main concerns are West Nile virus, western equine encephalitis virus, and St. Louis encephalitis virus [3], [1], [2].

For West Nile virus, national surveillance guidance identifies Culex tarsalis as the primary vector across western states, alongside Culex pipiens in northern states and Culex quinquefasciatus in southern states [3]. The virus circulates in an enzootic cycle between Culex mosquitoes and birds, and people and horses are infected when a mosquito that fed on an infected bird later bites them [3]. Laboratory and field studies rank Culex tarsalis among the most efficient West Nile virus vectors known, and long-term surveillance has implicated it as the primary vector in parts of the West [4], [5]. Western equine encephalitis virus is likewise maintained in a cycle between Culex tarsalis and songbirds before spilling over to people and horses [1].

See the [West Nile virus], [western equine encephalitis], and [St. Louis encephalitis] disease pages for full detail on symptoms, testing, and risk.

How to reduce it

Because this mosquito breeds in standing ground water, the most effective control is water management: reduce or manage the standing water where larvae grow. On the household side, empty or turn over containers, keep ditches and gutters draining, and avoid letting irrigation and runoff pool for long stretches. Because much of the breeding happens in irrigated farmland, canals, and wetlands that individuals cannot simply drain, control of Culex tarsalis relies heavily on organized, area-wide mosquito control programs that monitor larvae and treat productive water with larvicides. For personal protection during the dusk and nighttime hours when this species bites, use an EPA-registered repellent, wear long sleeves and pants, and keep window and door screens in good repair.

Sources

1. Rutgers University, Center for Vector Biology. Culex tarsalis Coquillett. https://vectorbio.rutgers.edu/outreach/species/sp6.htm. Accessed 20 Jul 2026. (Identification: white median band on the proboscis, pale bands overlapping the tarsal joints, pale leg stripes, dark ventral abdominal chevrons; range from northern Mexico and Baja California to southern Canada, uncommon east of the Mississippi; larval habitat in newly flooded sunlit pools, agricultural runoff, wetlands, and treated sewage effluent; feeds on birds after sunset early in the season, then on mammals including rabbits, horses, cattle, and humans; flight up to about 17 miles and roughly 100 yards a day typically; the most important mosquito vector of arboviruses in western North America, transmitting St. Louis and western equine encephalitis.)
2. North Coast Media, Pest Management Professional. Western encephalitis mosquito: Culex tarsalis. https://www.mypmp.net/western-encephalitis-mosquito-culex-tarsalis/. Accessed 20 Jul 2026. (Medium-sized and dark with a broad white band around the middle of the proboscis and white leg bands; most active after sunset; feeds on birds then rabbits, horses, cattle, and humans; breeds in canals, ditches, ground pools, hoofprints, catch basins, ornamental ponds, and effluent; a principal carrier of western encephalitis, St. Louis encephalitis, and West Nile virus.)
3. CDC. Guidelines for West Nile Virus Surveillance and Control. https://www.cdc.gov/west-nile-virus/php/surveillance-and-control-guidelines/index.html. Accessed 20 Jul 2026 (page returned HTTP 403; claims verified from CDC search-result text). (Culex tarsalis is the primary West Nile virus vector in western states, with Cx. pipiens in northern states and Cx. quinquefasciatus in southern states; the virus is maintained in an enzootic cycle between Culex mosquitoes and birds; most common in rural habitats.)
4. CDC, Emerging Infectious Diseases. Vector Competence of California Mosquitoes for West Nile virus. https://wwwnc.cdc.gov/eid/article/8/12/02-0536_article. Accessed 20 Jul 2026. (Culex tarsalis was among the most efficient laboratory vectors of West Nile virus tested.)
5. Nature, Scientific Reports. Long-term surveillance defines spatial and temporal patterns implicating Culex tarsalis as the primary vector of West Nile virus. https://www.nature.com/articles/s41598-019-43246-y. Accessed 20 Jul 2026. (Long-term surveillance implicates Culex tarsalis as the primary West Nile virus vector in the studied western region.)

Related

• [West Nile virus]
• [western equine encephalitis]
• [St. Louis encephalitis]
• [Culex pipiens (northern house mosquito)]
• [How to manage standing water on farms and large properties]
• [What mosquito control districts do]

Educational note: This page is a general reference for identification and public health awareness. It is not pest-control or medical advice. For a mosquito problem in your area, contact your county or regional mosquito and vector control program, and if you have symptoms of a mosquito-borne illness, contact a healthcare provider.

Last reviewed 20 July 2026.