‘Eye-biting’ flies surge in San Gabriel Valley. How to protect yourself from their painful bites

The San Gabriel Valley foothills are seeing an increase in the number of tiny nuisance flies with a big bite.

The valley’s Mosquito and Vector Control District said black fly activity is high and will likely remain that way for a few weeks. The District is treating breeding sites near rivers to reduce the population of black flies, also called ‘eye-biting flies,’ but it takes time for treatments to take effect.

The flies earned their nickname because they tend to bite around the eyes and necklines of humans and domestic animals. The bites are painful, but do not transmit diseases in Los Angeles County.

“Their bites cause a lot of pain,” said Anais Medina Diaz, of the San Gabriel Valley Mosquito and Vector Control District. “A lot of the times they’re confused with mosquitos, but they’re actually smaller than mosquitos. Their bites are very different. Mosquitos will make you itchy, maybe after they’ve bitten you. Black flies, you’ll know right away.”

The District offered the following tips to protect yourself around black flies, which thrive near flowing water like rivers and streams in foothill communities like Altadena, Azusa, Bradbury, Duarte, Glendora, Monrovia, San Dimas, and Sierra Madre.

• Wear netting over your face. Black flies aim for the face and eyes
• Wear repellent on exposed skin. Only DEET works against black flies
• Turn off personal water features for 24 hours once a week

Sudden population spikes are sometimes caused by water releases from upstream dams. The releases are necessary for the region’s water management and usually occur in spring when mountain snow turns into warmer weather runoff.

The female deposits about 200-500 fertilized eggs in or on flowing water. The larvae emerge and attack to rocks or other surfaces under the water and feed, growing to about 1/4 inch. The process takes about 10 days to several months.

Eventually, they reach the adult stage and emerge from a pupal case, then float to the surface in a bubble of air.