2025 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
In December, adult mosquito counts were very low, which is typical for this time of year. Culex pipiens was the most abundant mosquito in December, but numbers were slightly below average (1.63 per trap compared to a five-year average of 2.34). Aedes sierrensis and Aedes washinoi are two seasonal mosquitoes that weren’t detected at all in December. These mosquito species will likely be seen again as adults in March or April.
The following table and graph show the average number of mosquitoes collected per trap per night during December. The second graph shows the five-year average number of mosquitoes collected in traps over the different months of the year.
| Species | December 2025 | 5-year December 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_ | 0.1 | 0.2 |
| _Aedes washinoi_ | 0.0 | 0.0 |
This chart shows the 5-year average number of mosquitoes collected per trap night by month in 2025. Data are shown for the six most common species found in San Mateo County.
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 December, 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. District staff collected 166 larval samples in December. 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. Larval samples in December were dominated by Culiseta incidens, present in 110 of the 166 samples (66%). This mosquito is present year-round in San Mateo County and is frequently collected from fishponds, containers holding water, and freshwater impounds. The high number of Cs. incidens sampled reflects the focused effort of District technicians to collect samples from residential sources. This data provides records of species found from individual water sources and is helpful in early detection of introduced species.
The November and December rains filled impounds and ponds that are breeding sources for marsh mosquitoes. This December we detected Aedes squamiger (10% of samples), Aedes washinoi (7% of samples) and Culiseta inornata (5% of samples). These marsh species will continue to be found during larval inspections until the seasonal sources dry up during warmer spring weather. Although these species do not currently vector any diseases in this region, they will eagerly bite mammals, including humans.
This graph shows the proportion of each mosquito species collected in larval samples in December 2025.
Page last reviewed: January 7, 2026