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 November, mosquito counts were low for all species, as is typical this time of year. Culex pipiens, was the only mosquito species detected in any substantial amount, with an average of 3.43 per trap, which is lower than the five-year average of 5.27 per trap. November was the first month since February that species Culex erythrothorax and Aedes sierrensis were not present in traps. These mosquito species will likely return in early spring.
The following table and graphs show the average number of adult mosquitoes collected per carbon dioxide-baited trap per night during November compared to the five year average for the six most common mosquito species in San Mateo County.
| Species | November 2024 | 5-year November average |
|---|---|---|
| _Culex pipiens_ | 3.4 | 5.3 |
| _Culex tarsalis_ | <0.1 | 1.1 |
| _Culiseta incidens_ | 0.2 | 0.3 |
| _Culex erythrothorax_ | 0 | 1.1 |
| _Aedes sierrensis_ | 0 | <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 November, 59 larval samples were collected in the field by vector control technicians and submitted to the lab. Six different mosquito species were identified in larval samples. The species most often sampled was Culiseta incidens, present in 46 of the 59 samples (78%). 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 (39% 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: December 16, 2024