Species: Culex tarsalis


Culex tarsalis is the most important mosquito vector of numerous encephalitic viruses in western North America. In Montana these viruses include St. Louis encephalitis (SLE), western equine encephalitis (WEE) and West Nile virus (WNV).


In the US, Cx. tarsalis is a common species west of the Mississippi River. In Montana it is commonly found in areas where irrigated agriculture occurs, the prairie landscape and mountain valleys. Cx. tarsalis has multiple generations during the summer. Egg laying sites include temporary or permanent pools often in riparian areas, wetlands and irrigated pastures. Habitats are characterized by clean water with surface sunlight pools between stands of emergent vegetation (cattails, bulrushes). Eggs are deposited individually, and they adhere to one another to form a raft of 50 to 150 eggs which floats on the water surface. Larvae hatch within a day or two following oviposition. Larvae tolerate a wide range of water conditions, including saline and secondary treated sewage effluent, but not water with excessive organic pollution. Larval and pupal development ranges from 7 days to <4 weeks and is dependent on water temperature and food availability (larvae only). In Montana, females overwinter as inseminated nullipars (never developed a batch of eggs). Overwintering sites might include attics, abandoned buildings, bridges, culverts and animal burrows. In the spring a blood meal is required to produce their first batch of eggs. Adult females readily feed on birds but mammals including rabbits, horses, cattle and man are also common blood meal sources. Analyses of blood from engorged Cx. tarsalis collected from Medicine Lake (Sheridan County, Mont.) showed feeding occurred on 16 bird species with the mourning dove the most common (Friesen and Johnson 2013). This species also blood fed on cattle, humans and white-tailed deer. Feeding activity usually begins at sunset and peaks around 11 PM.

Identifying Characteristics:

Rounded abdomen, pale band on proboscis, pale banding on apical and basal ends of hindtarsomeres, abdominal terga with pale bands along basal border.

Distribution Map:

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Mosquito and Vector Surveillance Program

MSU Extension - Department of Animal and Range Sciences

Montana State University
P.O. Box 172900
Bozeman, MT 50717-2900

Location: Marsh Labs, Room 59

Veterinary Entomology Research Associate:

Marni Rolston

Professor Emeritus of Veterinary Entomology:

Dr. Greg Johnson


Dr. Grant Hokit (Carroll College)