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Common Mosquitoes of Napa County

Napa County currently has 20 species of mosquitoes, ten of which are intensively monitored and controlled. Mosquitoes marked with an asterisk (*) are known carriers of West Nile Virus.

Banded Foul Water Mosquito (Culex stigmatosoma)*

This species is most commonly found breeding in high organic content water sources such as winery waste, sewage, log, and dairy ponds. Adult females prefer to feed on birds but will also bite humans.

Encephalitis Mosquito (Culex tarsalis). Photo: Joseph Berger, Bugwood.org

Encephalitis Mosquito (Culex tarsalis)*

This mosquito breeds in rain pools, marshes, ponds, and other fresh water sources. This mosquito causes few service requests, however, it does require a large part of our control effort to prevent the spread of the encephalitis virus that it carries. This species feeds primarily on birds and is only moderately aggressive towards humans.

Fish Pond Mosquito (Culiseta incidens)

This mosquito is usually found breeding in fishponds, creeks, and containers and is the second leading cause of service requests for the District. Small sources, such as buckets, cans and tires, can produce sufficient numbers to cause discomfort in a neighborhood. This mosquito is moderately aggressive and bites in the evening or in shady places during the day. It is easily noticed because of its large size and dark spotted wings.

Little House Mosquito (Culex pipiens)

Little House Mosquito (Culex pipiens)*

This mosquito is generally an urban problem. They can be found all year and breed in storm drains, catch basins, utility vaults, septic tanks, flooded basements, sumps, and in just about any water container found near human habitation. Adults readily enter homes and bite at night. It can take many hours to locate the breeding site for this mosquito due to the wide variety and types of habitats that it utilizes.

Summer Salt Marsh Mosquito (Aedes dorsalis)

This species is found between March and October in the tidal salt marsh areas and brackish seasonal wetlands of Napa County. The eggs are laid on plants and muddy areas of these wetlands and hatch when the breeding site is filled by high tides or spring rains. Adults are an aggressive daytime biting species capable of flying many miles from the marshes in search of a blood meal.

Summer Salt Marsh Mosquito (Aedes dorsalis)

Washino’s Willow Mosquito (Aedes washinoi)

This mosquito breeds in woodland depressions that fill with water, especially those areas dominated by willow trees and berry vines. Eggs are laid on the mud and organic material along the edges of the receding water in these areas. Adults are generally present in the early spring and are very aggressive.

Western Treehole Mosquito (Aedes sierrensis)

This species breeds in tree holes, which are water filled rot cavities or depressions found in many species of trees, especially oaks, bay laurel, eucalyptus, sycamore and elm. Any container near trees, that is partially filled with water and leafy debris, may also produce this pest. The eggs hatch when the tree hole or container fills with water. The adults emerge in March and remain in the area until early summer. This mosquito has a short flight range, is an aggressive biter, and is the primary vector of dog heartworm in Napa County.

Winter Marsh Mosquito (Culiseta inornata)

Females of this species rest during the summer and become active in the fall after the first rains. Eggs are laid on the surface of rain filled ponds in the fall. Many generations can be produced in one season. This mosquito bites at dusk in the fall and spring and can be aggressive. This is our largest mosquito, measuring approximately 3/8 of an inch in size.

Winter Salt Marsh Mosquito (Aedes squamiger)

This species breeds in the tidal and diked marshes of Napa County. The eggs are laid on plants and muddy areas in the marsh during the spring and hatch as soon as the marsh fills with rainwater in the fall. Adults emerge the following spring and are an aggressive daytime biting mosquito capable of flying more than 15 miles from their breeding site.

Woodland Malaria Mosquito (Anopheles punctipennis)

This mosquito is the primary vector of human malaria in the woodland and foothill areas of California. Clear algal-laden shaded pools of creeks and streams and the heavily vegetated margins of slow flowing streams and rivers are the primary breeding sites for this mosquito.