2026 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
In January, adult mosquito counts were very low, which is typical for this time of year. Culex pipiens was the most abundant mosquito in January with numbers near average. Aedes washinoi is a seasonal mosquito that is often detected as larvae in freshwater marshes. This mosquito will typically peak in numbers in April but will sometimes fly in winter if there is a period of warm temperatures. Aedes sierrensis is another seasonal mosquito. It hasn’t been found in the adult stage this winter, but the larvae are commonly collected during the colder months. They will emerge as adults in the spring.
This table and graph show the average number of the six most frequently trapped species of mosquitoes collected per trap per night during January.
| Species | January 2026 | 5-year January average |
|---|---|---|
| _Culex pipiens_ | 1.6 | 2.3 |
| _Culiseta erythrothorax_ | <0.1 | 0.5 |
| _Culiseta incidens_ | 0.1 | 0.2 |
| _Aedes sierrensis_ | 0.0 | 0.0 |
| _Culex tarsalis_ | 0.2 | 0.1 |
| _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 January, 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 238 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 ten different mosquito species. The most frequently collected mosquito in larval samples in January was Culiseata incidens (38%). This is a lower percentage than in most months, as marshes were heavily sampled in January. This mosquito is present year-round and is frequently collected from fishponds, containers holding water, and freshwater impounds.
Marsh-breeding species, such as Culiseta inornata, Aedes squamiger 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.
This graph shows the proportion of each mosquito species collected in larval samples in January 2026.
Page last reviewed: February 6, 2026