Trailing vegetation or rotted aquatic plants also are attractive to black flies, providing sites for the larvae to attach for feeding. Breeding may also occur in rivulets formed by the flooding of fields. The black fly life cycle can be rapid, taking about three weeks from egg laying to maturation of the adult. Only the female bites, the blood meal being used to provide protein for egg maturation. Adults live about two weeks.
Populations can grow very rapidly.
Two to four generations may be produced annually. Individual females may lay several hundred eggs. Adult black flies are migratory, commonly flying many miles from larval breeding sites. As an extreme example, migrations of more than 90 miles are reported in Canada. Black fly attacks on people, cattle, horses and pigs tend to be concentrated around the ears and head.