Residents in the San Gabriel Valley are contending with a dramatic surge in black flies, a painful little pest known for biting around the eyes and necks of people and pets.

The San Gabriel Valley Mosquito and Vector Control District issued a warning this week advising residents of a spike in black fly activity in foothill communities including Altadena, Azusa, Bradbury, Duarte, Glendora, Monrovia, San Dimas and Sierra Madre. The flies develop quickly in flowing waters, where females will lay 200 to 500 fertilized eggs at a time.

“Black flies are currently very active in the San Gabriel Valley, and many residents are feeling overwhelmed,” the district said in a statement. “Right now, populations are increasing due to favorable conditions, and black flies can travel up to five miles from where they emerge, which is why they’re being seen throughout the community.”

The district is treating river breeding sites to reduce populations, but warns this might take several weeks to take effect.

In the meantime, residents are advised to take protective measures such as wearing long pants and long sleeves and using protective netting over one’s face. People should also consider using DEET-containing repellent on exposed skin and turning off personal water features such as decorative fountains for 24 hours once a week, according to the district.

The bugs, measuring two to three millimeters, are so small they can be hard to see. Still, their bite can pack a painful punch.

Azusa resident Constance Yu described the persistent bugs as “like little demons but tiny,” while she swatted away the critters during an interview with CBS News this week.

Though the flies cause discomfort, they are not known to transmit diseases in L.A. County, according to the vector control district.

Spikes in black fly activity are often caused by scheduled water releases from upstream dams, which are necessary for the region’s water management but also create ideal breeding conditions for the pests.

The district monitors and sprays pesticides at breeding sites — including local rivers, streams and locations such as Morris Dam — and sets traps in foothill communities to track the population size and minimize the effects on residents.

This time last year, surveillance traps had single-digit counts of black flies. Now they are capturing more than 500 flies at a time, district spokesperson Anais Medina Diaz told LAist.

Diaz also said it is usual to see such so much fly activity this time of year, noting that the uptick is probably connected to the recent record-setting heat wave. Southern California is experiencing the hottest March on record, leading to a surge in snow runoff from the mountains.

“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.”

The black flies are not the only troublesome creature acting up during the unseasonably warm weather.

The Southland has also seen more rattlesnakes, with two recent human fatalities, as the early heat draws more serpents onto hiking trails. Toasty ocean temperatures have been linked to a great white shark sighting in Newport Beach that prompted a temporary beach closure Thursday.