There are over 2500 different species of mosquitoes throughout the world, of which, 150 species occur in the United States, 53 species occur in California, and 22 species occur in Alameda County. Eight of the species account for over 99% of complaints from the public and require most of the field control efforts by the District.
Aedes dorsalis (Salt marsh mosquito)
Aedes dorsalis is found primarily during the summer in tidal salt marsh areas. Eggs are laid in the marsh and hatch when the marsh is filled by high tides. Adults are very aggressive, fly moderate distances, and are noticeable near the marshes. This species is capable of producing very high numbers near marsh areas.
Aedes washinoi (Woodland pond Mosquito)
Aedes washinoi is produced in woodland depressions that fill with water. Eggs are laid on the mud and organic material along the edges of receding water in these areas. Adults, generally present in early spring, are very aggressive and may be found in large numbers
Aedes sierrensis (Tree hole mosquito)
Aedes sierrensis breeds in tree holes (rot cavities or depressions in trees which hold water). If near trees and partially filled with organic debris, artificial sources such as tires, buckets, and fishponds may produce these mosquitoes. Eggs hatch when the source fills with water. Adults emerge in March and remain in the area until early summer. They have a short flight range, are aggressive daytime biters, and are the primary vector of Canine Heartworm in Alameda County. This species is found anywhere suitable tree holes are found.
Culex pipiens (House mosquito)
Culex pipiens causes the largest number of service requests, and is an excellent carrier for West Nile virus. This mosquito is generally an urban problem. Adults can be found all year and breeds in cryptic sources such as storm drains, catch basins, utility vaults, septic tanks, flooded basements, sumps, etc. Adults readily enter homes, and bite at night. Due to the type and variety of breeding sources, locating the source of these mosquitoes can be difficult.
Culex tarsalis (Encephalitis mosquito)
Culex tarsalis is produced in rain pools, marshes, neglected swimming pools, ponds, and other fresh water sources. Although this species does not produce a large number of the District’s service requests, it requires a large part of the control effort to prevent the spread of encephalitis in Alameda County. It’s also an excellent carrier for West Nile virus. Adults feed primarily on birds and are only moderately aggressive towards man. This species is capable of reaching very high numbers and has the potential for mosquito-borne disease transmission.
Culiseta inornata (Winter marsh mosquito)
Culiseta inornata females rest during the summer and become active in the fall after the first rains. Eggs are laid on the surface of rain filled ponds. Many generations can be produced in one season. Females bite at dusk fall through spring and are moderately aggressive, quite large, and may reach very high numbers. It is very noticeable to the public because of its size and activity. This species is generally found close to temporary fresh water sources.
Culiseta incidens (Fish pond mosquito)
Culiseta incidens is produced in fresh water sources such as fish ponds, creeks, and containers. It is the second major cause of service requests for the District. Small sources can produce sufficient numbers to cause discomfort in a neighborhood. This mosquito is moderately aggressive, bites in the evening or shade, and is very noticeable because of its large size. It is primarily a problem of urban and suburban areas.