2025 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
In April, adult mosquito counts of all species were very low, but began a slight rise, which is typical in the spring. Culex erythrothorax, a mosquito that breeds in tule marshes and ponds, was not collected at all so far this spring, which is very unusual. Culex erythrothorax numbers have been low since 2022.
The following table and graph show the average number of mosquitoes collected per trap per night during April 2025. The graph that follows shows the five-year average of mosquito counts over the different months of a year.
| Species | April 2025 | 5-year April average |
|---|---|---|
| _Culex pipiens_ | 0.9 | 2.0 |
| _Culex tarsalis_ | 0.1 | 1.0 |
| _Culiseta incidens_ | 1.3 | 1.5 |
| _Culex erythrothorax_ | 0.0 | 3.2 |
| _Aedes sierrensis_ | 2.8 | 1.4 |
| _Aedes washinoi_ | 0.4 | 2.6 |
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 to be counted and identified by entomologists.
District staff collected 171 larval samples in April. The most frequently occurring species was Culiseta incidens - detected in 77% of April samples. This large percentage is because of a substantial effort to sample back yard standing water, such as in fishponds and fountains, that are preferred sources of Cs. incidens. Culex tarsalis was the next most collected species and was found in 9% of larval samples. This mosquito species is a West Nile virus vector that is found in a variety of sources, such as creeks, ponds, impounds and marshes. Early control of Culex larvae in the spring will help limit the risk of mosquito-borne disease in the upcoming summer months.
Page last reviewed: May 14, 2025