A helicopter assault against the summer salt-marsh mosquito in the Palo Alto Baylands began Thursday, Aug. 23, at about 7 a.m., the Santa Clara County Vector Control District has announced.

The district is trying to control breeding of the aggressive mosquitoes, which have plagued residents in Palo Alto and Menlo Park for several weeks.

Officials said spraying by helicopter is necessary because the areas where the insects breed are extensive and difficult to treat from the ground. The chemicals pose little danger to people, and there are no residences or businesses within the area to be treated, vector control officials said.

The mosquito -- Aedes dorsalis -- lays its eggs in the moist soil just above the water line. The eggs can lie dormant for a decade, 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, according to officials.

The salt-marsh mosquito has not been shown to transmit West Nile Virus, but it is known to bite viciously during the day and can fly up to five miles from its breeding grounds to feed on humans and other mammals, officials said.

A broken tide wall in the Palo Alto Baylands has allowed water levels to rise and fall in the tidal basin, creating ideal conditions for mosquito breeding. Vector control officials have been closely monitoring the development of mosquito larvae, and current conditions are ideal for eggs to hatch.

Recent adult "fly-offs" have created considerable discomfort for residents and workers in nearby areas, according to officials.

Approximately 150-400 acres are being 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 act specifically on mosquitoes and are short-lived in the environment, officials said. The products control the immature (aquatic stage) mosquitoes but are not harmful to birds, fish, other insects, wildlife or humans.

Thursday's spraying is different from recent fogging operations that took place in Los Altos. Unlike the very low-volume aerosol that focused on adult mosquitoes, these applications are being applied at 10 gallons of water per acre, to thoroughly cover the marshes, officials said.

Access to the Baylands is restricted during the spraying, which could last until noon, but the area will open to the public immediately afterward, officials said.

Next week, the district will again take to the air but with a different, granular form of chemical. That form should be longer lasting and may obviate the need for weekly sprayings, said Russ Parman, Santa Clara County Vector Control acting district manager.

Santa Clara Valley Water District, which is in charge of the broken tidal gate, is awaiting permits and is preparing environmental documents prior to fixing the gate, spokesman Marty Grimes said. The district hopes to have a final permit from the state Regional Quality Control Board by the end of next week, he said.

But Sept. 4 would be the earliest that a temporary fix would be in place because repairs cannot be made until after the nesting season under the Migratory Bird Act.

"The good news is it will take one day to do a temporary fix," he said, adding the hole would be filled by pressure grouting.

Plans for a permanent fix are not yet in place but would likely involve digging out the old area and filling it in with concrete, he said.

Vector Control is encouraging residents to report places where mosquitoes are breeding and take steps to avoid getting bitten, such as wearing long-sleeved shirts and long pants and applying repellent when outdoors where mosquitoes are biting.

More information about mosquito prevention or the products used in spraying is available at Vector Control or by calling 408-918-4770.

Click here to view the map of the area being sprayed.