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September 2021 Newsletter

Mosquito Repellent

Green spray bottle with the word "repellent" on it, as well as a white icon of a person on an orange shield next to a small black icon of a tick and a small black icon of a mosquito. A magnifying glass shows that the corner of the bottle says "DEET...%" on it.  Text in the image says "The active ingredient is usually listed on the front label of the repellent container."
Check the bottle to see the active ingredient, and apply according to label instructions.

When mosquitoes bite, they can spread viruses (like West Nile virus and Zika virus) that can make people sick. The best way to prevent diseases spread by mosquitoes is to prevent mosquito bites. EPA-registered insect repellents work to prevent mosquito bites, and they are safe for use on pregnant and breastfeeding women and children.

EPA-registered repellents include one of the following active ingredients:

  • DEET
  • Picaridin
  • IR3535
  • Oil of Lemon Eucalyptus (OLE)
  • Para-menthane-diol (PMD)

Learn more about how to choose a repellent here.


2021 West Nile virus data

Map of California with county outlines. Counties are colored blue (indicates WNV activity without human cases), green (WNV activity with human cases), or white (no WNV activity). San Mateo County is white (no WNV activity as of 8/5/21).
Image from the California Department of Public Health (

As of September 1, 2021, there have been 227 dead birds reported in San Mateo County. Of those, 60 have been suitable for testing and all have tested negative for West Nile virus (WNV). To date, there have been no West Nile virus detections in mosquitoes or sentinel chickens in San Mateo County in 2021.

Statewide, there have been 26 human cases of WNV in 2021. Throughout California, 3,990 dead birds have been reported and 1,241 have been tested with 155 (12%) dead birds positive for WNV. Additionally, 1,482 mosquito samples have tested positive for WNV. Statewide, 43 sentinel chickens and 5 horses have tested positive.

Read more about WNV here.


A report cover. The title is "Vector-Borne Disease Section Annual Report 2020." The California Department of Public Health logo (three small people icons) is in the lower left corner.  The cover background is dark purple. In the lower right corner, there are several green hexagons. Some have purple icons in them: a rodent, a person running, a flea, a tick, and a mosquito.

CDPH releases 2020 Vector-Borne Disease Section report

In August, the California Department of Public Health released their 2020 Vector-Borne Disease Section Annual Report, which can be found here.



2021 Mosquito Surveillance

Adult Mosquitoes

Adult mosquitoes in CO2 traps, updated Sept 2021 - please contact or 650-344-8592 for the data

Culex erythrothorax, a mosquito that breeds in lakes and ponds with tules, was the most frequently collected mosquito in August.  The high number of tule mosquitoes this month reflects a large seasonal emergence of this species in Pacifica. Adults of this species are usually present from around April through October. Seasonal helicopter treatments for this mosquito began in July but have not yet successfully suppressed the mosquito population. Additional helicopter treatments will occur in September and October. Numbers of Culex pipiens, the northern house mosquito and typically the most abundant mosquito across San Mateo County, remain below average for this time of year. Read more about mosquito surveillance here.

Larval Mosquitoes

Six mosquito larvae viewed under magnification. The larvae each have a oval-shaped dark head, a long thin lighter colored brown body, and appear to be covered with small thin hairs.
Magnified larval mosquitoes

The dry summer weather limits the number of mosquito larval samples that are collected. Summer samples are often collected from residential areas that contain water year-round, such as backyard fountains and fish ponds, water under buildings, storm drains, and other containers in yards. Some are also collected from pockets of still water along creeks. Read more about mosquito surveillance here.


2021 Service Requests

Service requests, updated Sept 2021 - please contact or 650-344-8592 for the data
Yellowjacket nest treatment was the most requested service in August 2021.

The total number of service requests is below average for this month of the year (480 compared to an average of 608). This is the lowest number of service requests in the month of August in the past five years. The number of yellowjacket and other wasp requests, while below average for the month of August, remains high. Yellowjacket and wasp requests are common in the summer and will likely remain numerous until the weather cools in autumn. The number of mosquito service requests is also lower than average, because of few Culex pipiens problems this year. Dead bird requests are lower than average as well, because of little West Nile virus activity this year throughout the coastal region. Read more about service requests here.

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