Aerial Treatment of Bay Marshes to Reduce Population of Nuisance Mosquitoes

Annual preventative treatment scheduled for February 26, 2021

SANTA CLARA COUNTY, CALIF. – The County of Santa Clara Vector Control District is scheduled to begin its annual program to prevent the spread of mosquitoes, specifically the winter salt marsh mosquito (Aedes squamiger). To accomplish that, we will apply naturally occurring microbes and a mosquito-specific hormone on Friday, February 26. This treatment methodology has been safely and effectively used by the County annually since 1992.

The District has been closely monitoring the development of mosquito larvae in the areas to be treated. Current conditions create a high probability that a significant number of salt marsh mosquitoes will become adults in mid-February to mid-March if left untreated. This species is known to bite viciously during the day and can fly more than 15 miles from its breeding grounds to feed on humans and other mammals.

For the difficult-to-reach Palo Alto Flood Basin marsh and parts of the Zanker wetland, a helicopter will be used to cover large areas and minimize impact to the marsh habitat. Maps of the areas to be treated can be found below or at The treatments are scheduled to start at approximately 7:30 a.m. and last for a few hours.

“We follow the mosquito management best practices recommended by the Environmental Protection Agency,” said Vector Control District Manager Dr. Nayer Zahiri. “These efforts have been proven to be safe and effective for more than 25 years.”

The areas will be treated with a naturally occurring soil bacterium (Bacillus thuringiensis var. israelensis, or Bti) that activates when consumed by mosquito larvae, and a mosquito-specific treatment (methoprene) that prevents them from becoming adults. This eco-friendly application is short-lived in the environment and not harmful to birds, fish, other insects, wildlife, or humans. Aerial application of these treatments is used when mosquito breeding occurs over larger areas, a common practice by vector control districts throughout the Bay Area in their Integrated Pest Management (IPM) programs. More information about these products is available at

In the Palo Alto Flood Basin, the helicopter may make low altitude passes over trails surrounding the treatment area, so the public is advised to avoid areas where the helicopter is operating. Vector Control District staff and signage will be posted at various locations around the treatment area to notify visitors about the operation. The marsh trails will remain closed to the public for several hours during the treatment.

Commonly called the winter salt marsh mosquito, Aedes squamiger lays its eggs in the moist soil in late spring and early summer. These eggs can lay dormant for many years, even after repeated flooding. 

The District encourages the public to report mosquito-breeding sources and to take preventive measures, such as dumping standing water on your property, wearing long-sleeve shirts and long pants, and applying repellent when outdoors where mosquitoes are biting.

For free assistance on mosquito control, the public can contact the District office at (408) 918-4770 or fill out a service request online at

 2-26-2021 Mosquito Treatment Palo Alto Area

Palo Alto Mosquito Treatment Map 2.26.21


2-26-2021 Mosquito Treatment Zanker Wetland Area

Zanker Road Mosquito Treatment map 2.26.21



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Media Contact: María Leticia Gómez/Laurel Anderson, Office of Communications and Public Affairs, (408) 299-5119, [email protected]


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