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2022 Mosquito Surveillance

Adult Mosquitoes

The most frequently collected mosquito was Culex pipiens, which rose in numbers in October rather than experiencing a seasonal decline that typically begins in October. Culex pipiens abundance has been low throughout the summer season but was higher than average in October (13.352 mosquitoes per trap compared to a five-year average of 8.732) because of some sites that needed maintenance and were collecting standing water.

Culex erythrothorax was collected in below average numbers in October (0.134 per trap compared to a five-year average of 6.965). Unlike many years, there wasn’t a large emergence of these mosquitoes, except for a brief peak in April. The District treats larger tule marshes where this species breeds by drone or helicopter. The observed decrease in the species collection shows the treatments have succeeded in keeping adult mosquito numbers low.

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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 October compared to the five-year average for the six most common mosquito species in San Mateo County.

SpeciesOctober 20225-year October average
Culex pipiens13.48.7
Culiseta incidens0.40.6
Culex tarsalis0.2<0.1
Culex erythrothorax0.17.0
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 October, larval surveillance focused on backyard sources like fountains, fishponds, and containers in addition to catch basins and natural sources such as creeks and freshwater ponds. 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. This October, there were 104 samples submitted to the laboratory.  The most frequently occurring species were Culiseta incidens, the cool weather mosquito, present in 65 of the 104 larval samples. Despite the name, this mosquito is found year-round in a variety of sources, especially in backyard fountains and fishponds.  It is not known to transmit disease.

Number and percentage of larval samples containing each mosquito species from water samples collected in October 2022. Larval samples commonly contain multiple species.
 
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