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2022 Mosquito Surveillance

Adult Mosquitoes

The most frequently collected mosquitoes in December were Culex pipiens and Culiseta incidens. After an unusual rise in the number of Cx. pipiens in late fall, this species fell to below-average trap counts in November and December. No other species of mosquitoes were trapped in December, with the exception of a single Culiseta inornata trapped in Burlingame. Culiseta inornata is a large winter species that is relatively uncommon in traps and as a result it does not appear in the table or graph below.

Overall, fewer mosquitoes were trapped in 2022 compared to the prior 5-year averages, with early and late-season peaks of some Culex species bookending a summer of relatively low mosquito counts.

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Searsville Lake, in the Jasper Ridge Biological Preserve.  The overgrowth of aquatic vegetation shelters mosquito larvae from fish and other aquatic predators.  Aerial applications of a mosquito-controlling bacteria have been successful in limiting mosquito production in these areas.

Carbon-dioxide trap data

This table and graph show the average number of mosquitoes collected per CO2 trap per night during the month of December compared to the five-year average for the six most common mosquito species in San Mateo County.

SpeciesDecember 20225-year December average
Culex pipiens0.22.0
Culiseta incidens0.20.3
Culex tarsalis<0.1<0.1
Culex erythrothorax<0.10.5
Aedes sierrensis<0.1<0.1
Aedes washinoi<0.1<0.1

 This chart shows the average number of adult mosquitoes collected in CO2 traps per trap per night night during 2022. Data are shown for the six most common species of mosquitoes in San Mateo County.

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This chart shows the 5-year average number of mosquitoes collected per trap night by month for the previous 5 years. Data are shown for the six most common species found in San Mateo County.

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Larval Mosquitoes

Side view of a person with short hair and a tan colored baseball hat and blue surgical mask on. The person appears to be standing in a field, with a bright yellow vest and holding a metal stick that has a white plastic cup on the end of the stick. The person is looking in the cup.

 During November and December, larval surveillance transitioned from focusing on backyard sources like fountains, fishponds, and containers to monitoring seasonal natural sources such as freshwater impounds and marshes. District staff collected 105 larval samples in November, and 118 in December. Technicians use a dipper to take a sample of water and visually inspect it for mosquito larvae. If larvae are present, the sample is taken back to the District laboratory for species identification. The most frequently occurring species was Culiseta incidens, the cool weather mosquito, present in 75% of November samples and 44% of December samples. Despite the name, this mosquito is found year-round in San Mateo County in a variety of sources, especially in backyard fountains and fishponds.  It is not known to transmit disease.

Other winter marsh mosquito species are becoming more common towards the end of the year. These species include Culiseta inornata (the winter marsh mosquito) and Aedes squamiger (the winter salt marsh mosquito). The District takes particular care to monitor the saline or brackish marsh areas that breed Ae. squamiger, as this species has the potential to emerge in huge numbers when the weather warms up. Although these species do not currently vector any diseases in this region, they are known to be voracious day-biting mosquitoes.

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 Page last reviewed January 6, 2023

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