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County to Spray for Winter Salt Marsh Mosquitoes

thuringiensis var. israelensis

SANTA CLARA COUNTY, CALIF. – Weather permitting, the Santa Clara County Vector Control District (SCCVCD) will be spraying by helicopter to reduce the emergence of adult mosquitoes on Friday, February 11, 2011. The spraying will begin at approximately 8:00 a.m. and last three to five hours. The areas to be treated include the Alviso Marsh, Smith Yard Marsh, New Chicago Marsh, and Zanker Marsh. Aerial treatment is necessary because the areas are extensive and difficult to be treated from the ground. No residences or businesses fall within the area to be treated. The aerial treatment is intended to minimize the number of mosquitoes and reduce the risk of mosquito bites to residents in the surrounding communities.

Commonly called the “Winter Salt Marsh Mosquito,” Aedes squamiger lays eggs in the moist soil in late spring and early summer. These eggs can lay dormant for many years, even after repeated flooding. High tides and seasonal rains, together with the short days and cooler temperatures of winter, cause the eggs to hatch when they are submerged in water. Although this species has not yet been shown to transmit West Nile Virus, it is known to bite viciously during the day and can fly over 20 miles from its breeding grounds to feed on humans and other mammals.

SCCVCD has been closely monitoring the development of mosquito larvae in the areas to be treated. Current field conditions and mosquito growth trends indicate a high probability that a significant number of Salt Marsh Mosquitoes will become adults in early to mid March if left untreated. The mosquito fly-off may affect residents from the north coastal areas of the county to the southernmost part of the City of San José and east to Milpitas.

Approximately 408 acres will be treated with environmentally safe products: methoprene, an insect growth regulator, and Bacillus thuringiensis var. israelensis (Bti). Bti is a natural bacteria that, when consumed by mosquito larvae, activates an insecticidal protein that kills the larvae. These products are mosquito-specific and short-lived in the environment: they effectively control the immature (aquatic stage) mosquitoes, but are not harmful to birds, fish, other insects, wildlife, or humans. More information about these products is available at

Access to the areas will be restricted during the aerial spray, but open to the public immediately afterward.

SCCVCD continues to encourage residents to report mosquito-breeding sources and take preventive measures such as wearing long-sleeved shirts and long pants and applying repellent when outdoors where mosquitoes are active.

For more information about mosquito prevention, go to or call 408.918.4770.

Media Contact: Russ Parman 408-593-6176; Noor Tietze 408-210-5773; Victor Romano 408-593-6193
Released: February 10, 2011

Winter Mosquito Treatment Zone

Winter Mosquito Treatment Zone

2011 Helicopter Spray Area