Some species of black flies do not feed as adults, others require a blood meal in order to produce eggs. Females live from a few days to more than three months. They prefer calm sunny days, and will not fly at night or on windy days. Black flies are attracted to their animal or human victims by the carbon dioxide and moisture present in exhaled breath, dark colors, convection currents, perspiration, perfumes and toiletries. They often fly into eyes, nose, ears, and mouth.
Bites cause pain and dermatitis at the site of the bite because black fly saliva is toxic.
Flies can discourage people from remaining in or visiting certain areas. Losses to the livestock and poultry industries are difficult to evaluate, but there is usually a drop in milk, beef, and egg production where black flies are numerous. Black flies are known to be vectors of diseases. Black flies may prove to be transmitters of encephalitis. Some species of black flies feed on birds, and the encephalitis virus has been recovered from some species of birds.