Adult Mosquitoes (carbon-dioxide trap data)

In April, adult mosquito counts for several species continued their expected spring increase. Collection levels of Aedes washinoi were well above average, with 9.3 mosquitoes per trap compared to a five-year average of 2.7. A series of late rains led to increased breeding of this species in difficult-to-reach impounds and ponds in Mills Field, near San Francisco International Airport. April is usually the peak month for adult Aedes washinoi. Other species were all less than 5 per trap per night, which is typical for April.

The table below shows the average number of female adult mosquitoes collected per trap per night during April, 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.

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 April, 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 175 larval samples in April. 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 April was Culiseta incidens (79% of samples).  This mosquito is present 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.

Culex pipiens, another backyard mosquito, was also collected often (18% of samples). This mosquito is the most frequently trapped species in the adult stage in San Mateo County and is a vector of West Nile virus. Residents should dump standing water sources like buckets, bird baths, and yard toys in addition to maintaining swimming pools, gutters, and drains to keep this mosquito from becoming a nuisance and a disease risk around their homes.

Page last reviewed: May 11, 2026