Aerial Treatment of Bay Marshes to Reduce Population of Nuisance Mosquitoes

Preventative treatment scheduled for Wednesday, January 26, 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 control treatment on Wednesday, January 26, in the Palo Alto Flood Basin, a non-residential area. This treatment will prevent the growth of mosquitoes currently breeding in the flood basin, specifically the Winter salt marsh mosquito (Aedes squamiger). The control treatment uses naturally occurring microbes and a mosquito-specific hormone to reduce the mosquito population. This application method has been safely and effectively used by the County every year since 1992.

The District has been closely monitoring the development of mosquito larvae in the areas to be treated. Winter salt marsh mosquito species are a pest when present in large numbers and are a secondary vector for Western equine encephalitis – a disease that can be transmitted to humans through the bite of infected mosquitoes.

Map of the Palo Alto Flood Basin showing mosquito control treatment area
An aerial mosquito control treatment has been scheduled for Wednesday, January 26, 2022 in the Palo Alto Flood Basin, a non-residential area.

A helicopter will be used to apply the mosquito control materials to both the large areas and areas that are difficult to reach. This method also minimizes impact to the marsh habitat.  The control treatment is scheduled to start at approximately 7:30 a.m. and last a few hours. A map of the area to be treated can be found below or at

“Current environmental conditions create a high probability that a significant number of mosquitoes will become adults in a few days if left untreated,” said Vector Control District Manager Dr. Nayer Zahiri. “This treatment will significantly help reduce the mosquito population and eliminate a fly off of adult mosquitoes.”

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 the immature mosquitoes from becoming adults. This eco-friendly application is short-lived in the environment and is 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. For more information on the control materials used for these treatments, please visit

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 control treatment. Marsh trails will not be opened to the public during the treatment.

The District encourages the public to report mosquito activity and to take preventive measures, such as dumping standing water around the home, wearing long-sleeved shirts and long pants, and applying insect repellent when spending time outdoors. For more information on mosquitoes and mosquito prevention, visit

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


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Media Contact: Beverly Perez, Vector Control District, 408-210-5774, [email protected]

Posted: January 25, 2022


News Release


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