Only the female black flies are bloodsuckers. Their bite is extremely painful, and the injection of a venom into the skin causes intense itching, local swelling and soreness. All exposed parts of the body are subject to attack, tough they favor the head, just beneath the hat rim. In the United States, black flies are not known to transmit diseases to humans. Livestock, pets, poultry and wildlife are also severely irritated by these flies.
Black flies live as larvae in shallow, clear, fast-running water in rivers and streams.
The black, spindle-shaped larvae live on the river bottom attached to rocks and other submerged objects and feed on tiny bits of organic matter, algae and protozoa. Larvae transform in the water to adult flies that rise to the water surface in a bubble of gas. The adult flies are usually present for about 3 weeks before they die. Eliminating black flies is not practical. Fogging for adult control, as a municipality might do against mosquitoes, is a possibility, though past experiences have shown limited success because of the continuous emergence of new adults and the long distance movements of flies.