General information

Frequently Asked Questions about Adult Mosquito Control

Why are you fogging?

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 if necessary to prevent human illness or to suppress a heavy nuisance infestation of mosquitoes. The decision to spray, by truck mounted sprayers, will be based on surveillance information or the documentation of West Nile virus activity at a level that indicates a threat to human health. Spraying will be concentrated in areas most at risk for disease occurrence and will be conducted by certified and licensed applicators. The District’s aggressive campaign against mosquito larvae is intended to minimize the need to use adulticides.

How do you notify residents about fogging?

We use a variety of methods to notify residents that we are fogging, including contacting local media, posting a notice on our website, and sending email alerts to residents who have subscribed. The cities in our district also use their own notification systems; these vary by city.

How can I find out when and where you’re fogging?

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.

What products are used for fogging?

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 of 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 technicians. The product currently used by San Mateo County Mosquito and Vector Control District for adult mosquito control is called Zenivex E4.

Learn more about the adult mosquito control products.

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

There is nothing you need to do because we are spraying for adult mosquitoes. Pesticides used by the District for control of adult mosquitoes have a half-life of 1.5 days in water and 4.4 days in soil. The pesticide molecule rapidly degrades in sunlight at the soil and water surface into its constituent’s elements: carbon, hydrogen, and oxygen. It should not affect car paints or other painted surfaces.

Will the products you use for fogging hurt my family or my pets?

The risks to the public and to the environment are very low. Mosquito adulticides are applied as ultra-low-volume (ULV) sprays. ULV applications involve very small quantities of active ingredient in relation to the size of the area treated, typically less than 2 ounces per acre, which minimizes exposure and risk to people and the environment. There is no need to relocate during the spraying.

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

Will the products you use for fogging harm my garden?

All fruits and vegetables, whether grown at home or purchased, should be washed with water before consumption. However, the pesticides we use pose little risk to people or the environment and are labeled to spray over crops. These materials have been used extensively throughout the county in mosquito control programs for decades with no evidence to suggest harm to people or gardens.

How do you protect pollinators when fogging?

Although the pesticides used for adult mosquito control may be toxic to bees, 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.

Do the products you use for fogging hurt the environment?

The risks to the public and to the environment are very low. Mosquito adulticides are applied as ultra-low-volume (ULV) sprays. ULV applications involve small quantities of active ingredient in relation to the size of the area treated, typically less than 2 ounces per acre, which minimizes exposure and risk to people and the environment.

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.

Why can’t you use non-pesticide methods to control mosquitoes?

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 our integrated pest management program for mosquitoes here. Pesticides are just one part of our mosquito control program..

Can you use bats or birds to control 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.

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