Preferred habitats are wooded coves along fast flowing streams. The larvae are aquatic and feed on microscopic organic material in streams. Black flies swarm mainly in the early morning and evenings oftentimes completely surrounding people who are unfortunate enough to be where they are. Long distance host finding begins with flies detecting odors upwind. As they near the host, they orient to carbon dioxide emissions and within six feet they use vision.
Black flies can discriminate between colors often preferring black, blue and green.
These flies are more common in hilly and mountainous areas where shallow, fast flowing streams occur. The bites from black flies can be very irritating. Many bites are the rule and not the exception. Headaches, nausea and swelling of lymph nodes are common reactions. Severe systemic reactions may occur.