2025 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
During winter, adult mosquito numbers are in decline, and remain low until spring weather triggers adult emergence of many species. In January, all mosquito species averaged less than one per trap per night.
The following table and graphs show the average number of adult mosquitoes collected per carbon dioxide-baited trap per night during January compared to the five year average for the six most common mosquito species in San Mateo County.
| Species | January 2025 | 5-year January average |
|---|---|---|
| _Culex pipiens_ | 0.5 | 0.5 |
| _Culex tarsalis_ | 0.1 | 0.1 |
| _Culiseta incidens_ | <0.1 | 0.3 |
| _Culex erythrothorax_ | <0.1 | <0.1 |
| _Aedes sierrensis_ | 0.0 | 0.0 |
| _Aedes washinoi_ | 0.0 | <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)
Mosquito larval samples are collected from sources of standing water such as marshes, impounds, backyard fountains, fishponds, water under buildings, storm drains, containers, creeks, and tree holes. A District vector control technician uses a dipper to take a sample of the water and visually determines whether any mosquito larvae are present. The technician then transports the sample to the laboratory for the larvae to be counted and identified.
During January, larval surveillance focused on monitoring seasonal natural mosquito breeding sources such as freshwater impounds and marshes, while continuing to check backyard sources. Additionally, water from January rains can collect under houses and provide a sheltered place for larvae to develop. District staff collected 144 larval samples in January.
The most frequently occurring species was Culiseta incidens, detected in 52% of January larval samples. This mosquito is found year-round in San Mateo County in a variety of sources, especially backyard fountains, and fishponds. 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 human disease.
The District takes particular care to monitor the saline or brackish marsh areas that breed Aedes squamiger, as this species has the potential to emerge in huge numbers when the weather warms up. Although this species does not currently vector any diseases in this region, they are known to be voracious day-biting mosquitoes. Aedes squamiger occurred in 13% of larval samples this January.
Page last reviewed: February 7, 2025