2026 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
In May, adult mosquito counts for the six most frequently collected species averaged below five per trap, which is typical for the month. Collection levels of Aedes washinoi dropped from April, from 9.3 to 5.0 mosquitoes per trap as water sources in Mills Field were successfully controlled or dried up. Culex pipiens and Culiseta incidens had a seasonal rise in abundance from April to May but counts remain low and near the five-year average.
The table below shows the average number of female adult mosquitoes collected per trap per night during May, compared to the five-year average. The line graphs on the next page show the average number of the six most common species of mosquitoes collected per trap per night throughout the months in 2026, and the five-year averages across the months of a year.
| Species | May 2026 | 5-year May average |
|---|---|---|
| _Aedes washinoi_ | 5.0 | 2.3 |
| _Culiseta pipiens_ | 3.0 | 2.7 |
| _Culiseta incidens_ | 2.1 | 1.8 |
| _Culex tarsalis_ | 0.5 | 1.0 |
| _Aedes sierrensis_ | 0.3 | 2.8 |
| _Culex erythrothorax_ | 0.2 | 0.5 |
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, larval samples were 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 133 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.
The collected samples contained larvae of eight different mosquito species. The most frequently collected mosquito in larval samples in May was Culiseta incidens (present in 80% of samples). This mosquito breeds year-round and is frequently collected from fishponds, containers holding water, and freshwater impounds. Technicians have been sampling these types of sources extensively with the hope of detecting invasive Aedes species, if present, as they often breed in suburban water sources.
Page last reviewed: June 4, 2026