Adult Mosquitoes (carbon-dioxide trap data)

Adult mosquito counts rose in June, particularly the species Culex erythrothorax, which has numbers about twice 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 typical for this time of year.

The table below shows the average number of female adult mosquitoes collected per trap per night during June, 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.

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 June, 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 229 larval samples in June. 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 May was Culiseta incidens (present in 76% 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 with the hope of detecting invasive Aedes species, if present, as they often breed in suburban water sources.

Culex stigmatosoma, was also often collected in June (29% of samples). This mosquito is not an aggressive biter of humans but can play a role in maintaining West Nile virus in the bird population. 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: July 2, 2026