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A helicopter will spray wetlands in Palo Alto and San Jose on Wednesday as part of a mosquito abatement program.

Trails in the areas will be closed to the public during the spraying, scheduled to start around 7:30 a.m. and last a few hours, Santa Clara County’s Vector Control District announced.

The substance used in the aerial spraying is not harmful to humans and other mammals, birds or fish but it prevents mosquito larvae from maturing to adults, the district said. It is composed of a soil bacterium (Bacillus thuringiensis) and the insect growth regulator methoprene, which is used in household flea and tick sprays.

The areas to be sprayed are shown on the map above. The Palo Alto area covers part of the Baylands Nature Preserve, including the Adobe Creek Loop Trail. The San Jose area is at the north end of Zanker Road, near Alviso.

More information can be found at the vector control district’s website, www.sccvector.org.