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Aerial Treatment of Bay Marshes to Reduce Population of Nuisance Mosquitoes

Preventative treatment scheduled for Wednesday, March 24 in non-residential area

SANTA CLARA COUNTY, CALIF. – The County of Santa Clara Vector Control District is scheduled to conduct an aerial mosquito treatment on Wednesday, March 24 to prevent the spread of mosquitoes currently breeding in the Palo Alto Flood Basin, specifically the Winter salt marsh mosquito (Aedes squamiger) and the Western encephalitis mosquito (Culex tarsalis). The application uses naturally occurring microbes and a mosquito-specific hormone. 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 mosquitoes will become adults in a few days if left untreated. Winter salt marsh mosquito species are a pest when present in large numbers and are also a secondary vector for Western Equine encephalitis, while the Western encephalitis mosquito is the primary vector for Western Equine encephalitis and St. Louis encephalitis.

For the difficult-to-reach areas in the Palo Alto Flood Basin marsh, a helicopter will be used to cover large areas and minimize impact to the marsh habitat. A map of the area to be treated can be found below or at The treatment is scheduled to start at approximately 7:30 a.m. and last a few hours.

“These mosquito species tend to breed in seasonal wetlands and marshes, usually during Spring,” said Vector Control District Manager Dr. Nayer Zahiri. “This treatment will significantly help reduce the mosquito population by allowing us to treat those hard-to-reach areas that we would otherwise not able to reach by foot.”

The area 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

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. Signage will be posted at various locations around the treatment area to notify visitors about the treatment operation. The marsh trails are not opened to the public during treatment that is expected to last several hours.

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-sleeved 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 by calling (408) 918-4770, or filling out a service request online at


The County of Santa Clara government serves a diverse, multi-cultural population of 1.9 million residents in Santa Clara County, California, making it more populous than 14 states in the U.S. The County provides essential services to its residents, including public health protection, environmental stewardship, medical services through the County of Santa Clara Health System, child and adult protection services, homelessness prevention and solutions, roads, park services, libraries, emergency response to disasters, protection of minority communities and those under threat, access to a fair criminal justice system, and many other public benefits.

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03-24-2021 Mosquito Treatment

Palo Alto Flood Basin

Palo Alto Map


Media Contact: Beverly Perez, Vector Control District, (408) 918-4602, [email protected]

Posted: March 23, 2021​