'Mosquito adulticides' are products used to control adult mosquitoes. We use only EPA-registered adulticides, which are only applied by licensed applicators and only in areas with high numbers of adult mosquitoes, areas where mosquito-borne pathogens have been detected, or other cases where public health is at risk.

The District currently uses two products to control adult mosquitoes in San Mateo County. One is Zenivex E4, which contains 4% etofenprox. The other is DeltaGard, which contains 2% deltamethrin. Other materials may occasionally be used as outlined in the District's Programmatic Environmental Impact Report.

Adult mosquito control products are applied at an ultra-low volume (ULV) from either hand-held or truck mounted equipment. Hand-held equipment is used to treat small contained areas, for example, when a broken pipe under a building creates a large puddle that produces thousands of mosquitoes. On some occasions, we may need to apply mosquito adulticides to a larger area using truck-mounted equipment. Truck-mounted adult mosquito treatments are generally conducted at night.

The products used by the District for adult mosquito control are applied at very low concentrations in a very fine mist. Adult mosquitoes die when they come into contact with the fine mist. Because of the extremely small size of the droplets in the mist, most of the material evaporates or breaks down before settling on surfaces.

It is not necessary to take any special precautions when an adult mosquito control treatment occurs in your neighborhood (except for beekeepers and people with fishponds - see below). 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.

Beekeepers should cover their hives during the treatment.

Residents with fish ponds should cover their ponds during the treatment.

If you have questions about the products used for adult mosquito control, you can call the District office at 650-344-8592. You can also call the National Pesticide Information Center at 800-858-7378.

To learn more about Zenivex E4, visit the manufacturer's website.

To learn more about DeltaGard, visit the manufacturer's website.

 

 

Page last reviewed: October 13, 2026