2026 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
Adult mosquito counts remained steady from June to July, except for the species Culex erythrothorax, which has numbers about five timeas as high as average. This mosquito breeds in tule marshes and ponds and the high numbers are from sites in Mills Field, near the San Francisco airport. Technicians are working to access areas where it is difficult to apply pesticides because of thick vegetation surrounding standing water. Other mosquito species averaged below five per trap, which is a little lower than typical for this time of year.
The table below shows the average number of female adult mosquitoes collected per trap per night during July, 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 | July 2026 | 5-year July average |
|---|---|---|
| _Culiseta erythrothorax_ | 15.4 | 3.1 |
| _Culiseta pipiens_ | 4.7 | 5.6 |
| _Culiseta incidens_ | 1.9 | 1.7 |
| _Culex tarsalis_ | 1.4 | 1.5 |
| _Aedes washinoi_ | 0.1 | 0.2 |
| _Aedes sierrensis_ | 0.1 | 0.2 |
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 July, 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 163 larval samples in July. 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 nine different mosquito species. The most frequently collected mosquito in larval samples in July was Culiseta incidens (present in 66% of samples). This mosquito breeds year-round and is frequently collected from fishponds, containers holding water, and freshwater impounds. Technicians have been sampling these types of sources extensively to survey for invasive Aedes species, if present, as they often breed in suburban water sources.
Culex pipiens, was also often collected in July (44% of samples). This mosquito is a primary vector of West Nile virus in San Mateo County. 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: August 7, 2026