DORAL, Fla. — The head of the U.S. Centers for Disease Control and Prevention says aerial spraying of the insecticide naled is killing many mosquitoes in a Miami neighborhood where the insects apparently transmitted Zika to 15 people.

Dr. Tom Frieden says aerial spraying that began Thursday quickly killed adult mosquitoes that pesticides on the ground couldn't reach.

Puerto Rico's governor wouldn't authorize naled's use because of environmental concerns, despite widespread Zika infections there. Frieden says minuscule amounts sprayed at dawn and dusk have no effect on people.

Frieden says Miami's infections were limited to a small area within a 1-square-mile zone that encompassed the Wynwood neighborhood. Gov. Rick Scott says a 10-block corner of that area has been cleared of active infections.

Florida's Department of Health says there's no evidence of mosquitoes transmitting Zika elsewhere in Miami.