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2024 Mosquito Surveillance

Adult Mosquitoes (carbon-dioxide trap data)

A blue water cooler and cup dispenser attached to a tree trunk in an outdoor setting.
Mosquitoes are collected from throughout the County using carbon dioxide-baited traps.

In March 2024, counts for all adult mosquito species are very low, less than two per trap, as is expected during the winter. The abundance of adult mosquitoes will gradually increase as the weather warms up in spring. This March was the first detection in 2024 of adult Aedes washinoi, Aedes sierrensis, and Culex erythrothorax in traps. Ae. washinoi breed in freshwater and brackish marshes, impounds, and natural ponds and can be an aggressive human biter. Ae. sierrensis breed in treeholes or other small containers in wooded areas. They emerge as adults in the spring and can be a nuisance to campers, hikers, and those who live on woodsy properties. Cx. erythrothorax breed in tule (cattail) ponds, and typically stay near their water source. They prefer to feed on birds, but might play a role in circulating West Nile virus in the area.

The following table and graphs show the average number of adult mosquitoes collected per carbon dioxide-baited trap per night during March compared to the five year average for the six most common mosquito species in San Mateo County.

SpeciesMarch 20245-year March average
_Culex pipiens_0.61.8
_Culex tarsalis_0.70.3
_Culiseta incidens_1.50.5
_Culex erythrothorax_<0.10.4
_Aedes sierrensis_<0.1<0.1
_Aedes washinoi_0.7<0.2

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)

Image of a magnified sample that has three mosquito larvae in it. They are in a lit circle with a black border. The larvae have small thin hairs and dark brown heads.
Early-stage Culiseta incidens larvae, viewed through a microscope.

During March, larval surveillance focused on monitoring seasonal natural sources, such as freshwater impounds and marshes, while continuing to check backyard sources. Rainy days in March kept water sources abundant throughout the county. District staff collected 258 larval samples in March. 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 most frequently occurring species, in 65% of samples, was Culiseta incidens. This mosquito is found year-round in San Mateo County in a variety of sources, especially backyard fountains and fishponds.  These mosquito species are not known to transmit human disease. Other commonly collected species in March were Culiseta inornata, a marsh breeding mosquito, and Culex tarsalis, which breeds in a variety of freshwater sources such as marshes, ponds, and ditches. Culex tarsalis is a vector of West Nile virus and controlling breeding sources for this species while it is in the larval stage is the best way to prevent disease during the summer months.

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Page last reviewed: April 3 2024

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