2025 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
In May, adult mosquito counts of all species are below five per trap per night, which is typical for this time of year. However, Culex pipiens and Aedes sierrensis numbers had a seasonal increase. Aedes sierrensis typically peaks in May, so the abundance might soon drop to lower summer levels. Culex pipiens is likely to continue to increase in number, as it is usually most abundant from June to October. T his 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 May. The graph that follows shows the five-year average of mosquito counts over the different months of a year.
| Species | May 2025 | 5-year May average |
|---|---|---|
| _Culex pipiens_ | 2.9 | 2.8 |
| _Culex tarsalis_ | 0.3 | 1.0 |
| _Culiseta incidens_ | 1.3 | 1.8 |
| _Culex erythrothorax_ | <0.1 | 1.0 |
| _Aedes sierrensis_ | 4.9 | 2.1 |
| _Aedes washinoi_ | 0.6 | 2.2 |
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 May, vector control technicians continued inspections of seasonal natural mosquito sources, such as freshwater impounds and marshes that are still holding water, while also checking backyard sources. District staff collected 177 larval samples in May. 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.
As usual, the most frequently occurring species, in 84% of samples, was Culiseta incidens. This mosquito is found year-round in San Mateo County in many urban sources, especially fountains and fishponds. Another commonly collected species was Culex pipiens (21% of samples), which breeds in similar sources as Cs. incidens, including storm drains and neglected swimming pools. Culex stigmatosoma was found in 9% of samples and breeds in freshwater sources with high organic content, such as horse troughs and sewers. Culex stigmatosoma is not an aggressive biter of humans or other mammals but can play a role in maintaining the West Nile virus cycle in birds. Controlling mosquitoes while they are in the larval stage is the best way to prevent disease during the summer months.
Page last reviewed: June 5, 2025