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.

District staff are always checking stagnant water throughout the County for larval mosquitoes. Whenever possible, staff control mosquito larvae in the water using larvacides (insecticides that kill larval mosquitoes). However, sometimes there is a new water source, or a water source that is difficult for staff to access. In these cases, using mosquito adulticides can help keep people in the area safe from mosquito bites and the diseases they can spread.

The public plays a key role in helping control mosquitoes in their larval stage (and making it less likely that the District will need to do a mosquito adulticiding treatment). Everyone should:

• Dump standing water at least once a week
• Report biting mosquitoes so that District staff can check your and neighboring properties for standing water
• Let District staff access your yard when needed so they can check for standing water. Mosquitoes only need a very small amount of water to grow, and District staff are very well-trained in finding even the tiniest, oddest water sources.

 

What does the District do year-round to prevent mosquitoes? 

The District uses an integrated mosquito management strategy that focuses on surveillance, mosquito management, and education.

Surveillance - The District tracks the numbers of mosquitoes throughout the county year-round using several kinds of mosquito traps. Carbon dioxide traps use dry ice as bait to attract hungry female mosquitoes. The trap has a fan that sucks in approaching mosquitoes. The District then tests the collected mosquitoes for the disease pathogens, like West Nile virus. The District also tests dead birds.

Mosquito management - The District focuses on finding sources of standing water where mosquitoes breed and controlling immature mosquitoes (in areas like marshes, creeks, catch basins, and storm drains). The goal is to prevent mosquitoes from becoming adults, when they can bite and potentially infect humans. If a West Nile virus-positive adult mosquito is detected, the District takes the additional step of conducting adult mosquito control treatments to reduce the adult mosquito population in the area and lower the potential risk of a West Nile virus human infection.

Education - The District visits schools, community events, and community organizations to teach people how to protect themselves and prevent mosquitoes from breeding.

 

How does fogging work?

The District determines the treatment area based on location of positive mosquitoes or where trap numbers are higher-then-expect. The District uses truck-mounted equipment that emits a fine mist of microscopic droplets containing an insecticide approved by both the U.S. and California environmental protection agencies. This application method is known as ultra-low volume (ULV). The microscopic droplets are airborne and should not leave any residues. Flying mosquitoes are killed by contact with the pesticide. Adult mosquito control is done only at night when mosquitoes are flying and when vehicle and human foot traffic are minimal.

To measure the effectiveness of the treatment, the District traps mosquitoes in the treatment area before and after a treatment.

 

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?

All of 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.

The main product the District currently uses for adult mosquito control is Zenivex® E4 RTU (active ingredient etofenprox 4%, EPA Registration Number 2724-807). Zenivex E4 RTU mosquito adulticide is a high efficacy, reduced risk adulticide that provides quick, permanent knockdown and reliable control of adult mosquito populations in any mosquito habitat flying at the time of application.

 

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

There is no need for you to relocate during the treatment. Residents who would like to take extra precautions can keep their family and pets inside during the treatment, with windows and doors shut. By sunrise, the insecticide will quickly break down with the sunlight.

The District applies insecticides at ultra-low volume (ULV), which means residents aren’t likely to contact anything that has enough insecticide on it to be harmful. All control materials used in our mosquito control program are approved by the Environmental Protection Agency for use in public areas and are widely used by vector control agencies throughout California and the United States. Our technicians apply adulticide product according to label instructions and use precisely-calibrated equipment. Mosquito treatments pose minimal risk to people, pets, animals and the environment when applied by our licensed vector control professionals following label instructions. Typically our technicians will use less than three ounces per acre, which minimizes exposure and risk to people and the environment.

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.

 

Will the products you use for fogging harm my garden?

In general, all fruits and vegetables should always be washed before consumption, whether they are grown at home or purchased. The pesticides we use pose little risk to people or the environment and are labeled in accordance with EPA regulations to spray over crops as well as other locations where adult mosquito control is needed. These materials have been used extensively throughout the country in mosquito control programs for decades, and there has been no evidence to suggesting risk to people or gardens.

 

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.

 

Where can I find more information about the products you use for fogging?

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 (http://www.epa.gov) or by contacting the National Pesticide Information Center (NPIC) (http://npic.orst.edu) or toll free 1-800-858-7378.

 

 

 

Page last reviewed: September 18, 2026