Aerial Treatment of Bay Marshes to Reduce Population of Summer Salt Marsh Mosquitoes

Preventative treatment scheduled for June 20 in non-residential area in Palo Alto

SANTA CLARA COUNTY, CALIF. – The County of Santa Clara Vector Control District is scheduled to conduct an aerial mosquito treatment on Saturday, June 20 to prevent the spread of mosquitoes, specifically the summer salt marsh mosquito (Aedes dorsalis), currently breeding in the Palo Alto Flood Basin marsh. See map here. 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 salt marsh mosquitoes will become adults in a few days if left untreated. This species is a pest when present in large numbers and is also a secondary vector for Western Equine encephalitis. They are known to bite viciously during the day and can fly up to 30 miles from its breeding grounds to feed on humans and other large mammals. 

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 at The treatments are scheduled to start at approximately 7:30 a.m. and last a few hours. 

“Recent warm weather provided the ideal environment for mosquito larvae to develop in hard to reach areas of the marsh,” said Vector Control District Manager Dr. Nayer Zahiri. “These targeted treatments will help significantly reduce the adult mosquito populations in the area and lessen public disturbance.”

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. 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.

Commonly called the summer salt marsh mosquito, Aedes dorsalis 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-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.

Visit the County of Santa Clara at:

Like us on Facebook: 
Follow us on Twitter:


Media Contact: Beverly Perez, County of Santa Clara Vector Control District, 408-210-5774.


Posted: June 19, 2020​​




News Release


©2021 County of Santa Clara. All rights reserved.