Both males and females are blood feeders that spend most of their time on the shoulders and backs of cattle. During extremely hot weather or when it rains, they may move to the protected underside of the animal. When disturbed, horn flies will fly up in a swarm but they will return to animals almost immediately. In addition, females leave occasionally to lay their eggs in fresh manure piles.
Bulls are more attractive to horn flies than are steers or cows.
Individual flies pierce the skin with their short, tube-like mouthparts 20 to 30 times per day to ingest a small amount of blood. Their feeding activity is painful and annoys the animals, as well as causing some blood loss. This stress is probably the cause of reduced weight gain during the summer. Horn flies can cause a 12 to 20 pound lighter calf at the end of the summer, as well as, greater weight loss per nursing cow. In addition, horn flies have been implicated in the spread of summer mastitis, a suppurative disease of non-lactating mammary glands.