2026 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
Adult mosquito counts in August were near average, except for Culex erythrothorax, which declined in average per trap from July despite high numbers at Mills Field, near the San Francisco airport. This mosquito breeds in tule marshes and ponds. Technicians are working to access areas where it is difficult to apply pesticides because of thick vegetation surrounding standing water. The District is working with aerial applicators (drones and/or helicopters) to treat these areas.
The table below shows the average number of female adult mosquitoes collected per trap per night during August, 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 | August 2026 | 5-year August average |
|---|---|---|
| _Culiseta pipiens_ | 6.5 | 6.6 |
| _Culiseta erythrothorax_ | 4.5 | 11.2 |
| _Culiseta incidens_ | 1.5 | 1.5 |
| _Culex tarsalis_ | 0.6 | 1.9 |
| _Aedes washinoi_ | <0.1 | 0.1 |
| _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 August, summer mosquito larval samples were collected from a variety of areas, such as backyard fountains and fishponds, water under buildings, storm drains, containers in yards, and other residential sources. District staff collected 83 larval samples in August. 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 seven different mosquito species. The most frequently collected mosquito in larval samples in August was Culiseta incidens (present in 73% 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 pipiens was common in August, appearing in 42% of samples. Although it prefers feeding on birds, it can transmit West Nile virus to humans after feeding on an infected bird. Residents should eliminate standing water and maintain pools, gutters, and drains to reduce mosquito breeding and disease risk.
Culex boharti was collected from one larval sample. This mosquito is rarely collected in adult mosquito traps. It is also infrequently collected as larvae but is sometimes found in shaded pools or pockets of streams. This species is known only to feed on amphibians, with no records of it biting humans.
Page last reviewed: September 22, 2026