The District’s aggressive campaign against mosquito larvae minimizes the need to use 'adulticides' that are targeted to control adult mosquitoes. However, it is not always possible to prevent all adult mosquitoes from emerging.

When adult mosquitoes infected with West Nile virus are found in an area, adult mosquito control treatments are used to quickly reduce the risk to human health. Adult mosquito control is always done in conjunction with intensified efforts to locate and reduce mosquito breeding sites so that more adult mosquitoes cannot emerge.

To learn more about West Nile virus, how it is spread, signs, and symptoms, visit the CDPH West Nile virus website.

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

Surveillance - The District monitors the abundance 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 presence of mosquito-borne diseases. The District also tests dead birds.

Mosquito management - The District focuses on finding sources of standing water where mosquitoes breed and eliminating 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 displays information and answers questions at community events to educate the public about mosquito-borne diseases and what residents can do to protect themselves and prevent mosquitoes from breeding.

West Nile virus can cause serious disease in humans, and it is spread by mosquitoes infected with the virus. If West Nile virus is detected in the community, the District’s initial response will be to intensify its efforts to locate and reduce mosquito breeding sites, increase its levels of larviciding in those areas in which West Nile virus has been found, and increase trapping of adult mosquitoes for disease testing.

Reducing the adult mosquito population with pesticides (adulticides) registered by the US Environmental Protection Agency (EPA) will be done when necessary to prevent human illness or to suppress a heavy nuisance infestation of mosquitoes. The decision to treat, using truck mounted equipment, will be based on surveillance information or the documentation of West Nile virus activity at a level that indicates a threat to human health.

Adulticide treatment, often referred to as 'fogging,' will be concentrated in areas most at risk for disease occurrence and will be conducted by state-certified vector control technicians. The District’s aggressive campaign against mosquito larvae is intended to minimize the need to use adulticides.

The District determines a radius from the location where a positive adult mosquito is detected, and this becomes the treatment area. 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 not intended to leave significant 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.

We use a variety of methods to notify residents that we are fogging. These include contacting local news media, posting a notice on our website, social media such as Nextdoor and Facebook, and sending email alerts to residents and organizations who have subscribed to our alert system. We also coordinate with cities to share information through their channels. The cities in our district also use their own notification systems; these vary by city.

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

You can also sign up for email alerts: click on 'Join our mailing list' in the lower right corner of your screen, or call 650-344-8592 to request to be added to the email notification list.

When necessary, we may use ground application of pesticides to kill adult mosquitoes that pose a health risk to the residents of San Mateo County. In the case of adulticiding, or targeting adult mosquitoes, we would use botanical insecticides (plant derived compounds) or synthetic versions that include pyrethrins and synthetic pyrethroids, as well as piperonyl butoxide.

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.

The product currently used by San Mateo County Mosquito and Vector Control District 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. This product contains no synergist, therefore no Piperonyl Butoxide (PBO). 

Our technicians apply adulticide product according to label instructions and use precisely-calibrated equipment. There is no need for residents to relocate during the treatment. Mosquito treatments pose minimal risk to people, pets, animals and the environment when applied by our licensed vector control professionals following label instructions.

Residents who would like to take extra precautions can keep their family and pets inside during the approximately four-hour 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.

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 1980.

The risks to the public and to the environment are very low. Mosquito adulticides are applied as ultra-low-volume (ULV) sprays that applications involve very small quantities of active ingredient in relation to the size of the area treated. Typically our technicians will use less than three ounces per acre, which minimizes exposure and risk to people and the environment. The product quickly breaks down harmlessly when exposed to sunlight. There is no need to relocate during the spraying.

The product used for adult mosquito control also does not affect pets, and the ultra low volume (ULV) application rates have a significant margin of safety for mammals, birds, fish, and reptiles.

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 county in mosquito control programs for decades, and there has been no evidence to suggesting risk to people or gardens.

At the ultra-low-volume (ULV) application rates applied late at night there should be no exposure or impact to local bees. The District works with the San Mateo County Beekeepers' Guild to further minimize any potential impacts on commercial and hobby beekeepers

Mosquito adulticides are applied as ultra-low-volume (ULV) sprays.  The risks to the public and to the environment from the ULV application are very low. ULV applications involve small quantities of active ingredient in relation to the size of the area treated, typically less than three ounces per acre, which minimizes exposure and risk to people and the environment.

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.

San Mateo County Mosquito and Vector Control District uses a variety of methods to control mosquito populations. Most of these methods do not involve pesticide use. You can learn more about integrated pest management here. Pesticides are just one part of our mosquito control program.

Prevention is the best approach, and we encourage residents to help prevent the spread of mosquitoes by eliminating standing water that can breed mosquitoes, such as flower pots, old ties, clogged rain gutters, and pet water bowls. Drain the water or change the water at least once a week to prevent mosquitoes.

While many species of bats, birds, and other animals eat mosquitoes as part of their diet, few species eat mosquitoes exclusively. As a result, they are not effective at controlling mosquito populations.

There is one animal that is very useful at controlling mosquito populations: mosquito fish. The District provides mosquito fish (Gambusia affinis) to residents free of charge. You can learn more about mosquito fish here.