Culex tarsalis

Geographic Range

Culex tarsalis is a North American species of mosquito that occupies a large swath of territory between northern Mexico and southern Canada, spreading from the Pacific to the Atlantic coast. It is most commonly seen in California, at elevations ranging as high as 3000 meters. The species is much less abundant in areas east of the Mississippi River, although there have been several reported cases of C. tarsalis occurrences as far as New Jersey, which were found exploiting freshwater impoundments constructed to eliminate the salt marsh species Aedes sollicitans. (Crans and McCuiston, 1987; Reisen, 1993)


This species can be found in almost every environment in the U.S. The spread of C. tarsalis across the U.S. has been facilitated by human activity, with irrigation making formerly inhospitable arid regions available for breeding, which can occur in any sunlit standing freshwater. The species has been found at elevations ranging from sea level to over 3000 meters. Larval habitats are often shared with Culiseta inornata, Culex quinquefasciatus, Culex salinarius, Culex pipiens, and Culex stigmatosoma, as well as a number of species of Aedes and Anopheles. Although a widespread and hardy species, C. tarsalis was not able to establish a large presence in the Northeast due to competition from C. salinarius. (Crans and McCuiston, 1987; Reisen, 1993; Rejmankova, et al., 1988)

  • Aquatic Biomes
  • lakes and ponds
  • Range elevation
    0 to 3000 m
    0.00 to 9842.52 ft

Physical Description

Culex tarsalis is a black mosquito distinguished by a white band on its proboscis, as well as white bands on its tarsal joints. It also has white longitudinal stripes extending along the middle and hind legs, and dark chevron patterns along the underside of its abdominal segments.


Developmental periods range from 7 days to almost four weeks, depending upon temperature and food availability. Larvea and pupae live in the water, and emerge as adults into the air. (Reisen, 1993; Walton, et al., 1990)


Females oviposit egg rafts conataining an avaerage of about 190 eggs in newly flooded, freshwater substrates, larvae exploiting microfloral blooms produced by the release of nutrients from decomposing vegetation. It was found that larval populations declined markedly 2-3 weeks after habitat flooding due to a reduction in these blooms. Typical developmental sites vary in size from small man-made containers to large ponds. In general, C. tarsalis will breed in water in any sunny location, and will tolerate a wide range of water conditions. Developmental periods range from 7 days to almost four weeks, depending upon temperature and food availability. Females spend the winter carrying inseminated nullipars (undeveloped eggs) which require a blood meal to mature in the spring. However, in a phenomenon termed autogeny, some females are able to mature their initial egg batch without a blood meal, and oviposit 4-5 days after emergence. (Reisen, 1993; Walton, et al., 1990)

Females lay eggs in a "raft" that floats on top of the water. Each raft contains about 190 eggs. Females prefer to lay their eggs in newly flooded areas, so nutrients are available for larvae to eat. Females will lay their eggs in all types of wet areas, from small containers of standing water to large ponds. Females spend the winter carrying undeveloped eggs. Once they feed on blood the next spring, their eggs can finish developing, and can be laid. (Reisen, 1993; Walton, et al., 1990)

Beyond developing eggs, and laying them in an appropriate habitat, females do not provide parental care.

  • Parental Investment
  • pre-fertilization
    • provisioning


Culex tarsalis is most active in the few hours after sunset, parasitizing both bird and mammal hosts. Host specificity is quite catholic, and adjusts to changes in host availability. During the daytime, adults can be found resting in shaded areas such as tree cavities and animal burrows. Culex tarsalis has been recorded to disperse up to 27 kilometers on host-seeking flights, averaging about 90 meters per day. (Reisen, 1993)

Communication and Perception

These mosquitoes find hosts by detecting the sweat and carbon dioxide exhaled by mammamls or birds.

Food Habits

Culex tarsalis exhibits relatively generalized (catholic) blood feeding patterns, feeding on both birds and mammals, depending on host availability and distribution. When populations are low in the spring, most females tend to feed on birds. In the late summer when abundances are higher, birds will learn to avoid these mosquitoes, causing C. tarsalis to seek mammalian hosts, including rabbits, horses, cattle, and humans. This seasonal shift may be a significant factor in zoonotic viral transmission. In general, C. tarsalis shows a preference for bird hosts over mammals at a ratio of approximately 3:1.

Both sexes also feed on nectar and plant fluids. (Reisen, 1993; Wekesa, et al., 1997)

  • Animal Foods
  • blood
  • Plant Foods
  • nectar
  • sap or other plant fluids


Bats and birds are known to eat mosquitoes.

Ecosystem Roles

These mosquitoes transmit St. Louis Encephalitis and Western Equine Encephalitis. They are a nuisance to many species of birds and mammals. They also may be prey for birds and bats. Because adults eat nectar, they may pollinate some plant species.

Species Used as Host

Economic Importance for Humans: Negative

Culex tarsalis is one of the most important vectors of arboviruses in North America, most noted for the transmission of St. Louis Encephalitis (SLE) and Western Equine Encephalitis (WEE). Both are zoonotic diseases maintained in natural bird reservoirs. Some mammals, particularly jackrabbits, may be involved in transmission cycles in certain areas. SLE, sometimes called "sleeping sickness" or "summer flu," is associated with aseptic meningitis and inflammation of brain tissue and can be fatal in humans, usually among children or the elderly, with a mortality rate of up to 23%. WEE is less virulent in humans, with a mortality rate of about 4%; however, it is a serious disease in horses and can impose a heavy economic cost in lost livestock and vaccination and mosquito control programs. Human infections are most common in the warmer months, especially toward the end of summer as C. tarsalis begins to seek mammalian hosts. Transmission rates are also affected by temperature.

Culex tarsalis has also been observed to be a vector for the Llano Seco, Turlock, Gay Lodge, and Hart Park viruses, as well as several species of avian malaria. In laboratory experiments, C. tarsalis was found to be a competent vector of Japanese and Venezuelan Equine Encephalitis viruses. (Noden, et al., 1995)

Conservation Status

This species is extremely abundant in many places and in no need of special protections.


Matthew Wund (editor), University of Michigan-Ann Arbor.

Raymond Pahk (author), University of Michigan-Ann Arbor, Solomon David (editor), University of Michigan-Ann Arbor.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal


uses smells or other chemicals to communicate


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


mainly lives in water that is not salty.


An animal that eats mainly plants or parts of plants.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.


the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

internal fertilization

fertilization takes place within the female's body


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.


This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.


an animal that mainly eats nectar from flowers


active during the night


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


an animal that mainly eats blood


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


uses sight to communicate


Crans, W., L. McCuiston. 1987. Rare New Jersey mosquito species collected in 1986. Proc. N. J. Mosquito Control Assoc., p: 89-94.

Noden, B., C. Pumpuni, J. Vaughan, J. Beier. 1995. Noninfectious sporozoites in the salivary glands of a minimally susceptible anopheline mosquito. Journal of Parasitology, 81: 912-915.

Reisen, W. 1993. The Western Encephalitis mosquito, Culex tarsalis. Wing Beats, 4 (2): 16.

Reisen, W., R. Meyer, S. Presser, J. Hardy. 1993. Effect of temperature on the transmission of western equine encephalomyelitis and St. Louis encephalitis viruses by Culex tarsalis (Diptera: Culicidae). Journal of Medical Entomology, 30, no. 1: 151-160.

Rejmankova, E., M. Rejmanek, M. Pitcairn, R. Washino. 1988. Aquatic vegetation in rice fields as a habitat for Culex tarsalis and Anopheles freeborni. Proceedings and papers of the annual conference of the California Mosquito and Vector Control Association, 56: 160-163.

Walton, W., N. Tietze, M. Mulla. 1990. Ecology of Culex tarsalis (Diptera: Culicidae): factors influencing larval abundance in mesocosms in southern California. Journal of Medical Entomology, 27: 57-67.

Wekesa, J., B. Yuval, R. Washino, A. Vasquez. 1997. Blood feeding patterns of Anopheles freeborni and Culex tarsalis (Diptera: Culicidae): effects of habitat and host abundance. Bulletin of Entomological Research, 87: 633-641.