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Invasive Aedes aegypti Mosquitoes Have Been Detected Again in the County

For more information on this invasive mosquito visit our Aedes aegypti informational page. For scheduled treatments regarding this mosquito, visit Wide Area Larvicide Spraying page.

 

Press Release - March 17, 2025

FOR IMMEDIATE RELEASE
March 17, 2025

Vector Control District to Conduct Aerial Treatment of Bay Marshes to Reduce Population of Nuisance Mosquitoes

Preventative treatment is tentatively scheduled for Thursday, March 20, in a nonresidential area in Palo Alto, weather permitting

SANTA CLARA COUNTY, CALIF. —  The County of Santa Clara Vector Control District is scheduled to conduct an aerial mosquito-larva-control treatment on Thursday, March 20, in the Palo Alto flood basin, a nonresidential area along San Francisco Bay. 

The treatment will prevent the growth of mosquito larvae currently breeding in the flood basin, specifically, those of the winter salt marsh mosquito (Aedes squamiger). The treatment uses naturally occurring microbes and a mosquito-hormone regulator to reduce the mosquito population.  

The district actively monitors mosquito breeding and the development of mosquito larvae in the flood basin. Winter salt marsh mosquitoes are known to travel up to 20 miles in search of a blood meal. They are opportunistic feeders and aggressive biters.  

The district will use a helicopter for the treatment, because marsh is difficult to reach by land. The district has used this safe and effective method nearly every year since 1992, helping to minimize the impact on marsh habitat and the community’s quality of life.

Depending on weather conditions, the treatment will start at approximately 7:30 a.m. and last a few hours. If the weather does not allow for a successful treatment, the district will post updates to its website and social media platforms (Instagram, Facebook and X).

“Our team regularly inspects these marshes and have done treatments by hand. However, the recent rain has created pockets of water that we can’t access by foot. If we don’t act soon, the mosquito larvae in the marsh will soon mature into biting adult mosquitoes.” said Vector Control District Manager, Dr. Nayer Zahiri. “This treatment will significantly reduce the mosquito population and prevent adult mosquitoes from flying off into neighboring communities.”

The area will be treated with a naturally occurring soil bacterium (Bacillus thuringiensis subspecies israelensis) and a mosquito hormone regulator (methoprene) that prevents immature mosquitoes from becoming adults. These pesticides do not last long in the environment and, at the low doses used in the treatment of mosquito larvae, are not harmful to birds, fish, beneficial insects, wildlife or humans. The district uses aerial application when mosquito breeding occurs over larger areas, a common practice among vector control districts in the Bay Area. For more information on the materials used in these treatments, visit the district’s mosquito-larva-control webpage.

The helicopter may make low-altitude passes over trails surrounding the treatment area, so the district recommends that members of the public avoid areas where the helicopter is operating. The district will post signs at various locations around the treatment area to alert visitors. Marsh trails will be closed to the public during the treatment. There is no risk of exposure to pesticides on open trails during the treatment, and the public will be able to access all areas after the treatment is complete.

The district encourages the public to report day-time biting mosquito activity and to take preventive measures such as dumping standing water on their property; wearing long-sleeved shirts and long pants in the morning and evening, when mosquitoes are most active; and applying insect repellent when spending time outdoors. For more information on mosquitoes and mosquito prevention, visit the district’s mosquitoes webpage. 

For free assistance with mosquito control, residents may contact the district office at (408) 918-4770 between 7:30 a.m. and 3:00 p.m. or submit an online service request.

3/20/25

Mosquito Control Treatment: Palo Alto Flood Basin

Map of the Palo Alto Flood Basin area scheduled for a mosquito control treatment
 
 ABOUT THE COUNTY OF SANTA CLARA, CALIFORNIA

The County of Santa Clara government serves a diverse, multicultural population of 1.9 million residents in Santa Clara County, Calif., making it more populous than 14 states in the United States. 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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