Skip to main content

2023 Mosquito Surveillance

Adult Mosquitoes


Carbon-dioxide trap data

In August, the adult mosquito species that was most frequently collected in traps was Culex pipiens, averaging 9.742 per trap. This mosquito is present year-round, but peaks in abundance during late summer, as the warmer temperatures speed up its life cycle. Culex pipiens numbers are near average for this time of year. Culex erythrothorax, a mosquito that breeds among tules (cattails) in ponds and lakes, is far less prevalent this summer than typical with 0.975 per trap in August compared to an average August abundance of 17.851 per trap. This mosquito can emerge in massive numbers in the spring and summer but generally doesn’t fly far from its water source. In contrast, Culex tarsalis have been especially numerous this year, with 6.792 per trap compared to an average of 1.033. These mosquitoes breed in a variety of water sources that remain present from spring rains.

This table and graph show the average number of mosquitoes collected per CO2 trap per night during the month of August compared to the five-year average for the six most common mosquito species in San Mateo County.

SpeciesAugust 20235-year August average
Culex pipiens9.78.7
Culex tarsalis6.81.0
Culiseta incidens2.81.5
Culex erythrothorax1.017.9
Aedes sierrensis0.10.2
Ae. washinoi0.10.1
May contain: chart and line chart

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.

May contain: bow, weapon, chart, and line chart
May contain: soil, plant, tree, and vegetation
Mosquitoes are collected from the field using a carbon dioxide-baited trap .


Larval Mosquitoes

During the summer months, mosquito larval samples are usually collected from residential areas, such as backyard fountains and fishponds, water under buildings, storm drains, containers, and tree holes. Some are also collected from pockets of standing water along creeks. In addition, marshes and impounds of water continue to be sources of mosquito breeding of species such as Aedes dorsalis and Culex tarsalis. This August, 102 larval samples were collected in the field by vector control technicians and submitted to the lab. A District vector control technician uses a dipper to take a sample of the water and visually determines whether any mosquito larvae are present. The technician then transports the sample to the laboratory for the larvae to be counted and identified.

This August, eleven different mosquito species were identified in larval samples. The species most often found in samples was Culiseta incidens, present in 68 of the 102 samples (67%). This mosquito is active year-round in San Mateo County and is often collected from fishponds, containers holding water, and freshwater impounds. The high numbers of Culiseta incidens and larval samples from residential sources reflect an effort by the operations department to collect samples from residential areas that may be most impacted by mosquito presence. This data provides a record of the species present in San Mateo County and is helpful for early detection of introduced species.

Number and percentage of larval samples containing each mosquito species from water samples collected in August 2023. Larval samples commonly contain multiple species.


 Page last reviewed September 12, 2023

Join our mailing list