2024 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
As summer temperatures increase, adult mosquito numbers also rise. The most frequently collected mosquito this June was Culex pipiens, at near average numbers. This is a common mosquito in San Mateo County and a vector of West Nile virus. Other species were also near average numbers, except for Culex erythrothorax, which is well below the 5-year average, with 0.23 per trap compared to an average of 5.05 per trap. This is the second year in a row that Cx. erythrothorax has been much less abundant than usual. This may be due to changes in mosquito control operations, such as application of materials by drone. Culex erythrothorax mosquitoes breed in tule marshes and ponds and usually peak in abundance in August.
The following table and graphs show the average number of adult mosquitoes collected per carbon dioxide-baited trap per night during June compared to the five year average for the six most common mosquito species in San Mateo County.
| Species | June 2024 | 5-year June average |
|---|---|---|
| _Culex pipiens_ | 6.5 | 7.9 |
| _Culex tarsalis_ | 0.8 | 1.5 |
| _Culiseta incidens_ | 2.3 | 1.8 |
| _Culex erythrothorax_ | 0.2 | 5.1 |
| _Aedes sierrensis_ | 1.0 | 1.2 |
| _Aedes washinoi_ | 0.6 | 2.1 |
This chart shows the average number of mosquitoes collected per trap during 2024
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 fish ponds, water under buildings, storm drains, containers in yards, and treeholes. Some were also collected from pockets along creeks. District staff collected 140 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 from eight different mosquito species. Larval samples in June were dominated by Culiseta incidens, present in 113 of the 140 samples (81%). 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 on the operations department to collect samples from residential sources. This data provides a record of the species present from a given source and is helpful in detection of introduced species.
Other commonly collected species were Culex pipiens (26% of samples), and Culex tarsalis (11% 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. tarsalis breeds in a variety of freshwater sources such as marshes, ponds, and ditches. Both species of Culex mosquitoes are important vectors of West Nile virus. Controlling mosquitoes while they are in the larval stage is the best way to prevent disease during the summer months.
Page last reviewed: July 8, 2024