Black flies get into the nose, eyes, ears, and mouth and crawl into clothing when abundant. On cattle and horses, the ears seem to be the favorite feeding location. Flies lacerate the skin and suck blood. A strong anticoagulant in the saliva facilitates bleeding of the host. A lesion may form at the bite and the bitten area itches.
Black flies serve as vectors of human and animal diseases in some areas of the world.
An important protozoan parasite of poultry, Leucocytozoon simondi , is transmitted by black flies in the United States. Black fly adults are active throughout most of the spring, summer, and fall and even fly during warm spells in winter. Females lay their eggs after a blood meal. Eggs are deposited in masses on objects near water or under water, particularly at the crests of falls and spillways of impoundments. Some species lay eggs in water.