The San Gabriel Valley is dealing with a ton of black flies that bite. Here’s why and what to do

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What are black flies? Black flies are tiny, pesky insects that often get mistaken for mosquitoes. The biting flies breed near foothill communities like Altadena, Azusa, San Dimas and Glendora. They also thrive near flowing water.

What you need to know: Black flies fly in large numbers and long distances. When they bite both humans and pets, they aim around the eyes and the neck. While the bites can be painful, they don’t transmit diseases in L.A. County.

A population spike: Anais Medina Diaz, director of communications at the SGV Mosquito and Vector Control District, told LAist that at this time last year, surveillance traps had single-digit counts of adult black flies, but this year those traps are collecting counts above 500.

So, why is the population growing? Diaz said the surge is unusual for this time of year.

“We are experiencing them now because of the warmer temperatures we've been having,” Diaz said. “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.”

What officials are doing: Officials say teams are identifying and treating public sources where black flies can thrive, but that many of these sites are influenced by natural or infrastructure conditions outside their control.

How to protect yourself: Black flies can be hard to avoid outside in dense vegetation, but you can reduce the chance of a bite by:

• Wearing loose-fitted clothing that covers the entire body. 
• Wearing a hat with netting on top. 
• Spraying on repellent, but check the label. For a repellent to be effective, it needs to have at least 15% DEET, the only active ingredient that works against black flies.
• Turning off any water features like fountains for at least 24 hours, especially in foothill communities.