2025 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. We expect to see a 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 2025. The line graph that follows shows the five-year average of mosquito counts over the course of a year.
| Species | February 2025 | 5-year February average |
|---|---|---|
| _Culex pipiens_ | 0.6 | 2.8 |
| _Culex tarsalis_ | 0.2 | 0.4 |
| _Culiseta incidens_ | 0.9 | 0.4 |
| _Culex erythrothorax_ | 0.0 | 0.1 |
| _Aedes sierrensis_ | 0.0 | 0.0 |
| _Aedes washinoi_ | 0.7 | <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 February, larval surveillance focused on monitoring seasonal natural mosquito breeding sources such as freshwater impounds and marshes, while continuing to check backyard sources. District staff collected 126 larval samples in February.
The most frequently occurring species was Culiseta incidens, detected in 35% of February 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 Aedes squamiger and Aedes washinoi were also collected frequently. 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 22 % of larval samples this February.
Aedes washinoi is found in brackish waters and also fresher sources like ponds and impounds that are filled by rainwater this time of year. It is not a known disease vector but will bite humans. Aedes washinoi was found in 23% of larval samples in February.
Page last reviewed: March 6, 2025