2026 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
In March, adult mosquito counts of all species remained low, but started to rise as winter transitioned into spring. March was the first detection this year of adult Aedes sierrensis, the Western tree hole mosquito. As the common name suggests, this mosquito usually breeds in water collected in holes in trees. In San Mateo County, the abundance of adult Ae. sierrensis usually peaks in May.
The table below shows the average number of female adult mosquitoes collected per trap per night during March, compared to the five-year average. The line graphs on the next page show the average number of the six most common species of mosquitoes collected per trap per night throughout the months in 2026, and the five-year averages across the months of a year.
| Species | March 2026 | 5-year March average |
|---|---|---|
| _Culex pipiens_ | 1.5 | 1.3 |
| _Culiseta erythrothorax_ | <0.1 | 0.3 |
| _Culiseta incidens_ | 0.8 | 0.9 |
| _Aedes sierrensis_ | 1.0 | <0.1 |
| _Culex tarsalis_ | 1.4 | 0.3 |
| _Aedes washinoi_ | 0.5 | 0.3 |
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.
Larval Mosquitoes (collections from water sources)
During March, larval samples were collected from sources of standing water such as marshes, impounds, backyard fountains, fishponds, water under buildings, storm drains, containers, creeks, and tree holes. District staff collected 288 larval samples in March. 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 collected samples contained larvae of twelve different mosquito species. The most frequently collected mosquito in larval samples in March was Culiseta incidens (65%). This mosquito is present year-round and is frequently collected from fishponds, containers holding water, and freshwater impounds. Technicians have been sampling these types of sources extensively with the hope of detecting invasive Aedes species, if present, as they often breed in suburban water sources.
Culex pipiens, another backyard mosquito, was also collected often (18% of samples). This mosquito is the most frequently collected species as an adult in San Mateo County and is a vector of West Nile virus. Residents should dump standing water sources like buckets, bird baths, and yard toys in addition to maintaining swimming pools, gutters, and drains to keep this mosquito from becoming a nuisance and a disease risk around their homes.
Page last reviewed: April 2, 2026