A surge of biting black flies is sweeping across the San Gabriel Valley foothills — and officials warn the infestation could get worse.
Activity is currently high, with biting expected to continue for the next several weeks, according to the San Gabriel Valley Mosquito and Vector Control District.
The sudden spike is fueled by warm weather and strong water flows, creating ideal conditions for black fly breeding.
Often mistaken for mosquitoes, black flies travel in large numbers and can cover long distances.
They’re especially active in foothill communities, including Altadena, Azusa, Duarte, Glendora, Monrovia, San Dimas, and Sierra Madre, where they breed in moving water, such as rivers and streams.
When they bite, the insects tend to target areas around the eyes and neck of both humans and animals.
While the bites can be painful, they are not known to transmit diseases in Los Angeles County, officials said.
This year’s outbreak is especially fierce. Traps that once caught just a handful of black flies are now overflowing with more than 500 adult black flies.
“We are experiencing them now because of the warmer temperatures we’ve been having,” Anais Medina Diaz, director of communications at the SGV Mosquito and Vector Control District, told LAist last year.
“And of course, all the water that’s going down through the river, we have a high flow of water that is not typical for this time of year.”
Crews are actively monitoring and treating affected waterways, though some breeding sites are influenced by natural and infrastructure conditions outside their control, according to the district.
In the meantime, residents are being urged to take precautions, including wearing protective clothing, covering exposed skin, and using insect repellent containing DEET.
Officials also recommend turning off water features, such as fountains, for at least 24 hours to disrupt breeding conditions.