Aerial Treatment near Alviso Sewage Lagoons to Reduce Breeding of Mosquitoes Potentially Carrying West Nile Virus

Targeted treatments scheduled for June 14 in non-residential areas

SANTA CLARA COUNTY, CALIF. – The County of Santa Clara Vector Control District is scheduled to conduct treatment to reduce the larval population of the Western Encephalitis mosquito near Alviso sewage lagoons, a non-residential area. This species, Culex tarsalis, is known to carry diseases such as West Nile virus, Western Equine encephalitis, and Saint Louis encephalitis.

Due to the high volume of West Nile virus activity nearby, the possibility of massive breeding, and working in areas that are hard to reach, an aerial treatment has been scheduled for Sunday, June 14. The treatment, which involves the application of naturally occurring microbes, 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 Western Encephalitis mosquitoes will become adults in a few days if left untreated.

For the difficult to reach sewage lagoons in Alviso, a helicopter will be used to cover large areas that are otherwise hard to reach with regular equipment. A map of the area to be treated can be found here or at The treatments are scheduled to start approximately after 7:30 a.m. on June 14 and last a few hours. 

“As weather gets warmer, mosquito populations and the risk of disease starts to increase,” said Vector Control District Manager, Dr. Nayer Zahiri. “These treatments will help reduce adult mosquito populations in the area and prevent human cases of West Nile virus.”

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

Posted: June 12, 2020​


News Release


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