Crews spent the night spraying pesticides targeting aggressive Asian tiger mosquitoes in parts of the San Gabriel Valley northeast of Los Angeles.
Spraying began after midnight Wednesday and wrapped up around 5 a.m. in infested neighborhoods of Arcadia, South El Monte and an unincorporated area north of El Monte.
The invasive species, distinctive for its black and white stripes, can transmit viruses including dengue and chikungunya.
Residents are urged to reduce standing water in yards and report any black and white mosquitoes or any mosquitoes that are biting during the day.
Spraying was done by the Greater Los Angeles County Vector Control District and the San Gabriel Valley Mosquito & Vector Control District.