Santa Clara County pest control officials on Thursday used helicopters to spray biological substances meant to stop vicious-biting mosquitoes from hatching in a national wildlife refuge near Milpitas.
The county's Vector Control District will drop an insect control substance and bacteria to destroy the larvae of winter salt marsh mosquitoes, said district spokesman Noor Tietze.
The helicopters began applying the substances at 8 a.m. onto the Alviso, Smith Yard, New Chicago and Zanker marshes covering about 728 acres in the Don Edwards San Francisco Bay Natural Wildlife Refuge, Tietze said.
The spraying is expected to last about four hours and typically reduces the population of the mosquitoes in the South Bay Area by about 90 percent, Tietze said.
The winter salt marsh mosquito lays eggs in the soil of the marshes in the late spring to early summer that if left to grow hatch into larvae in the winter to become flying adults by early to mid-March, Tietze said.
The district oversees the annual control of the mosquitoes, which can fly more than 20 miles from breading grounds and pack a vicious bite on humans that can pierce blue jeans, he said.
"Each year, we have to keep watch," Tietze said. "They are aggressive in day (time) biting."
The eggs the insects lay can remain dormant for years in the marsh soil, then come back to life and hatch when water from winter rains seeps down.
The substances the district's scientists use are methoprene, a growth regulator that alters hormones in the larvae to keep them from evolving into flying adults and a natural bacteria called Bti that when consumed by larvae produces a protein that kills it.
Both of the substances are harmless to humans, animals, birds, fish, and other wildlife including other insects, Tietze said.
The mosquitoes are known as a "nuisance species" with a bite that might cause itching in humans but does not transmit diseases such as West Nile virus or malaria, Tietze said.
For humans, "everyone has a different reaction" to the bite, however it is best to avoid scratching bites, Tietze said. The mosquitoes also attack horses and cattle and can bite through cowhide, he said.
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