Residents in the San Gabriel Valley are facing a notable increase in black fly activity, spurred by the recent record-breaking heat wave in Southern California. The San Gabriel Valley Mosquito and Vector Control District has issued a warning, highlighting the painful bites inflicted by these pests, particularly around the eyes and neck. Black flies thrive in flowing waters, where they lay hundreds of eggs, and can travel up to five miles from their breeding sites. The heat wave has also intensified snow runoff, creating favorable conditions for black fly proliferation. The district is actively treating river breeding sites to control the population, but results may take weeks to materialize.