The larvae are found in clear or foul water in a variety of habitats including ditches, irrigation systems, ground pools, marshes, pools in stream beds, rain barrels, hoofprints, and ornamental pools. Foul water in corrals and around slaughter yards appear to be favorite larval habitats in many localities. Cx. tarsalis are biters, attacking at dusk and after dark, and readily entering dwellings for blood meals. Domestic and wild birds seem to be the preferred hosts. and man, cows, and horses are generally incidental hosts. (Carpenter and LaCasse 1955:296)
Culex tarsalis is one of the most important vectors of arboviruses in the United States. This mosquitoes is the most common species known for transmitting West Nile virus in North Dakota. Cx. tarsalis are also believed to be the primary vector of St. Louis encephalitis and western equine encephalitis in the Dakotas.