2026 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
In February, adult mosquito counts of all species were very low, which is typical for winter months. Particularly, Culex pipiens, the county’s most abundant mosquito, was well below average this February. We expect to see a seasonal rise in adult mosquito population levels in the spring. The following table and graph show the average number of mosquitoes collected per trap per night during February 2026. The line graph that follows shows the five-year average of mosquito counts over the course of a year.
This table and graph show the average number of the six most frequently trapped species of mosquitoes collected per trap per night during February.
| Species | February 2026 | 5-year February average |
|---|---|---|
| _Culex pipiens_ | 0.6 | 2.5 |
| _Culiseta erythrothorax_ | <0.1 | <0.1 |
| _Culiseta incidens_ | 0.3 | 0.5 |
| _Aedes sierrensis_ | 0.0 | 0.0 |
| _Culex tarsalis_ | 0.7 | 0.3 |
| _Aedes washinoi_ | <0.1 | 0.1 |
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 February, 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 244 larval samples in January. 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 eleven different mosquito species. The most frequently collected mosquito in larval samples in February was Culiseata 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 in the County before they become established.
Marsh-breeding species, such as Culiseta inornata and Aedes washinoi, were also collected frequently. The larvae of these mosquito species are abundant in winter months after rainfall fills their breeding sources. These mosquito species are not known to transmit disease in this region.
The District takes particular care to monitor the saline or brackish marsh areas that produce Aedes squamiger, as this species has the potential to emerge in huge numbers when the weather warms up. Although Ae. squamiger doesn’t vector disease, these large mosquitoes are known to be voracious daytime-biters. Fortunately, only 4% of larval samples contained Ae. squamiger, which is a low percentage for February.
Page last reviewed: March 17, 2026