A painful ‘eye biting’ fly is invading part of Southern California

Southern California residents are dealing with more than just pollen and allergies this spring as tiny “eye-biting” flies swarm people and pets across the San Gabriel Valley foothills, leaving itchy, red welts on faces, necks and arms.

The insects, identified as a type of black fly common to the region’s riverine foothills, are appearing in numbers far higher than usual for this season.

“These are very tiny black flies, smaller than a mosquito, and they usually breed in running water,” Anais Medina Diaz, director of communications for the San Gabriel Valley Mosquito and Vector Control District, told KTLA.

Communities including Azusa, Altadena, Glendora and San Dimas have reported daily encounters, with residents describing repeated bites throughout the day. Some have even experienced allergic reactions after prolonged exposure, ​​according to the San Gabriel Valley Mosquito and Vector Control District, the outlet reports.

Families in the Azusa area are growing increasingly concerned about the safety of children and pets spending time outdoors due to a surge in biting flies, according to the outlet. These insects leave large, itchy welts on the skin and their attacks occur even when people use protective clothing or apply insect repellent.

Vector control officials explain that the flies thrive in fast-moving water, such as that found in the San Gabriel River. The river provides an ideal environment for larval development, after which the adult flies emerge and spread into nearby foothill communities.

Typically, the insects appear later in the year, but unusually warm winter temperatures combined with increased river flow from early snowmelt have created favorable conditions for rapid population growth, officials said.

Medina Diaz said black fly populations have surged into the hundreds much earlier than usual, compared with only single-digit numbers at the same time last year. Luckily, Medina Diaz said, while black flies can be pesky and their bites can be painful, they do not transmit disease.

Still, experts recommend precautions when outdoors, including using DEET-based insect repellent, wearing light-colored, loose-fitting clothing, and avoiding peak activity times such as sunrise and afternoon.

Efforts to reduce black fly populations are already underway, focusing on treatments targeting larvae along local waterways, vector control officials report, per KTLA. While these measures aim to curb future infestations, adult flies that have already emerged may remain active for several weeks, meaning residents could continue to experience bites despite ongoing control efforts.

Officials also caution homeowners to keep an eye on decorative water features such as fountains or small streams on private property. If water in these features continues to flow, it can create additional breeding sites for black flies, potentially exacerbating local populations.