What is a mosquito adulticide?

Mosquito adulticides are a type of insecticide that kill adult mosquitoes.

 

Why is adult mosquito control necessary?

The District uses mosquito adulticide when there is a public health risk or higher-than-expected numbers of mosquitoes. For example, the District uses mosquito adulticides when West Nile virus is found in adult mosquitoes in an area - this reduces the chance of a person being bitten by and getting sick from a mosquito carrying West Nile virus.

Adulticiding is the only method we have of directly reducing the number of adult mosquitoes active in an area.  This can be very important in certain situations - ranging from stopping the spread of West Nile virus, to controlling a dangerous invasive species, to protecting occupants of a building with a broken pipe from mosquitoes.

It’s important to remember that adult mosquito control is a valuable, if sparingly used, part of our larger integrated pest management program, which includes surveillance of mosquitoes, controlling larvae, and public education.

 

How does adult mosquito control ('fogging') work?

The District uses truck-mounted equipment to release a very fine mist (ultra-low volume, or ULV) that targets flying mosquitoes. The droplets are so small that most break down or evaporate before settling on surfaces. Mosquitoes that fly through the mist are killed on contact.

 

How do you notify residents about fogging?

You can sign up for email alerts or call 650-344-8592 to request to be added to the email notification list.

Our latest fogging information will always be posted on the Adult Mosquito Control Update section of our website.

We will also contact local news media, post on social media such as Nextdoor and Facebook, and coordinate with cities to share information through their channels.

 

What products are used for fogging?

The products we use are registered with the EPA and applied according to label directions by our trained and certified vector control technicians using calibrated equipment. You can always call the District office if you have questions about the products used for mosquito control. Additional information about pesticides can be found on the EPA website or by contacting the National Pesticide Information Center (you can also call them at 1-800-858-7378).

 

What should I do if you're fogging in my neighborhood?

It is not necessary to take any special precautions when an adult mosquito control treatment occurs in your neighborhood. The products are applied at extremely low volumes, and there is no need to leave the area or change your normal activities during the treatment. If you would prefer to avoid contact with any pesticide droplets, you may choose to stay indoors and close doors and windows until the treatment is complete.

The type and amounts of mosquito adulticide product we use will not affect painted surfaces, unlike the malathion the State of California used to control the Mediterranean fruit fly in the late 1980s.

Beekeepers may want to cover their hives during the treatment.

Residents with fish ponds may want to cover their ponds during the treatment.

 

How do you protect pollinators when fogging?

At the ultra-low-volume (ULV) application rates applied late at night, there should be no or extremely limited exposure or impact to local bees. The District works with the San Mateo County Department of Agriculture (beekeepers are required to register their hives annually with the Department) and the San Mateo County Beekeepers' Guild to further minimize any potential impacts on commercial and hobby beekeepers.

 

 

 

Page last reviewed: October 1, 2026