2025 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
In June, adult mosquito counts of the top six most commonly-trapped species are all below five per trap per night, which is typical for this time of year.
Culex erythrothorax was trapped at substantially lower numbers than average. This mosquito species has been suppressed the last three summers by District control efforts, but historically had very high abundances in the summer. Culex pipiens is likely to continue to increase in number, as it is usually most abundant from June to October. This mosquito is a West Nile vector and the main target of the seasonal catch basin control program.
The following table and graph show the average number of mosquitoes collected per trap per night during June. The graph that follows shows the five-year average of mosquito counts over the different months of a year.
| Species | June 2025 | 5-year June average |
|---|---|---|
| _Culex pipiens_ | 3.9 | 4.8 |
| _Culex tarsalis_ | 0.3 | 1.2 |
| _Culiseta incidens_ | 1.5 | 2.0 |
| _Culex erythrothorax_ | 0.1 | 4.8 |
| _Aedes sierrensis_ | 0.7 | 1.3 |
| _Aedes washinoi_ | 0.2 | 1.5 |
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 June, summer mosquito larval samples were collected from residential areas, such as backyard fountains and fishponds, water under buildings, storm drains, containers in yards, and tree holes. Some were also collected from pockets along creeks. District staff collected 129 larval samples in June. 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 June were dominated by Culiseta incidens, present in 101 of the 129 samples (78%). This mosquito is present year-round in San Mateo County and is frequently collected from fishponds, containers holding water, and freshwater impounds. The high numbers of Culiseta incidens samples reflect an effort of the operations department to collect samples from residential sources. This data provides records of species found from individual water sources and is helpful in detection of introduced species.
Other commonly collected species were Culex pipiens (25% of samples), and Culex stigmatosoma (16% of samples). Culex pipiens breeds in underground sources such as storm drains and backyard sources including containers holding water and small fountains or bird baths, whereas Cx. stigmatosoma breeds in similar sources but prefers high organic content in the water. Culex pipiens is an important vector of West Nile virus. Cx. stigmatosoma does not often bite humans but can play a role in West Nile virus transmission among birds. Controlling mosquitoes while they are in the larval stage is the best way to prevent disease during the summer months.
Page last reviewed: July 10, 2025