Black flies, often called “gnats,” are part of the Simulium jenningsi species and become especially noticeable from late spring into early summer. In Pennsylvania, they have been classified as a pest of humans and livestock since the 1970s. Efforts to clean up pollutants in the state’s waterways created ideal breeding conditions for them to thrive, as their larvae spend much of their time in fast-flowing water feeding on miniature particles. Black flies are known for swarming around faces and for persistent biting, and they are attracted to carbon dioxide exhaled by humans and animals, perspiration, fragrances, dark clothing, and moving objects. Female black flies often enter the eyes, ears, nose, and mouth of humans and animals while seeking a host whose blood can support egg production and continue their life cycle. In the 1980s, Pennsylvania began the PA Black Fly Suppression Program, which uses helicopters to spray Bacillus thuringiensis israelensis, a naturally occurring soil bacterium, over affected areas to selectively target four black fly species that are especially troublesome for humans and livestock.