2024 Mosquito Surveillance
Adult Mosquitoes (carbon-dioxide trap data)
During autumn, adult mosquito numbers are in decline, and remain low until spring weather triggers adult emergence of many species.
In October, mosquito counts remain low for Culex pipiens and Culex erythrothorax. These two species have had below average numbers throughout the summer and fall season. Although abundances are low, Culex pipiens has not yet had a seasonal decline. As temperatures continue to decrease in November, the numbers will likely drop, corresponding with an end to the West Nile season, and will remain low until warmer weather around May.
Culex erythrothorax, a mosquito that breeds in tule (cattail) ponds, was almost completely absent from traps this month. This is the second year in a row of especially low abundance for Culex erythrothorax, probably because of shifts in operational control methods from helicopter to drone treatments.
Other species counts are low and near average for this time of year.
The following table and graphs show the average number of adult mosquitoes collected per carbon dioxide-baited trap per night during October compared to the five year average for the six most common mosquito species in San Mateo County.
| Species | October 2024 | 5-year October average |
|---|---|---|
| _Culex pipiens_ | 4.6 | 9.2 |
| _Culex tarsalis_ | 0.2 | 0.2 |
| _Culiseta incidens_ | 0.6 | 0.5 |
| _Culex erythrothorax_ | <0.1 | 6.3 |
| _Aedes sierrensis_ | <0.1 | <0.1 |
| _Aedes washinoi_ | 0 | <0.1 |
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)
Mosquito larval samples are collected from sources of standing water such as marshes, impounds, backyard fountains, fishponds, water under buildings, storm drains, containers, creeks, and tree holes. 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 October, 57 larval samples were collected in the field by vector control technicians and submitted to the lab. Eight different mosquito species were identified in larval samples. The species most often sampled was Culiseta incidens, present in 41 of the 57 samples (72%). This mosquito is active year-round in San Mateo County and is often collected from fishponds, containers holding water, and freshwater impounds. Another commonly collected species was Culex pipiens (49% of samples). Culex pipiens breeds in underground sources such as storm drains as well as backyard sources like containers, small fountains, and bird baths. It is a vector of West Nile virus.
These numbers reflect an effort by the operations department to collect samples from high density residential areas that may be most impacted by mosquito presence. This data is a core part of the mosquito surveillance process and is helpful for early detection of introduced species. Controlling mosquitoes while they are in the larval stage is the best way to reduce the need for aerial spraying and prevent the spread of arboviral diseases.
Page last reviewed: November 4, 2024