A surge of biting black flies is sweeping across the San Gabriel Valley foothills, with officials warning that the infestation could worsen. The increase in activity is due to warm weather and strong water flows creating ideal conditions for black fly breeding. The flies, often mistaken for mosquitoes, are particularly active in foothill communities such as Altadena, Azusa, Duarte, Glendora, Monrovia, San Dimas, and Sierra Madre. While the bites are painful, they are not known to transmit diseases in Los Angeles County. The district is urging residents to take precautions, including wearing protective clothing and using insect repellent containing DEET.