Horn flies can be distinguished by their habit of congregating and resting head downwards on the back and shoulders of their host and by their delta-wing appearance. Horn flies tend to rest quietly until disturbed whereas other flies always seem to be on the move. Horn flies rarely leave their host except to lay eggs, to move to other cattle in the herd, or when cattle enter buildings. Infestations spread by the active dispersal of newly emerged adults to herds up to 8 km away in one season. Infestations are also spread by the movement of infested cattle, either on foot or by truck.
Horn flies feed by piercing the hide and sucking the blood.
Most feeding occurs along the underline of the animal and results in scabby, often bleeding, sores. Flies will continue to feed around the margins of these sores causing the sores to get larger with each successive year of attack. The hide gradually takes on a calloused, rough appearance. Each fly may feed from 10 to 38 times each day, causing annoyance and irritation to the host. Horn flies are more attracted to bulls than cows, with fly numbers reaching 4000 on bulls and up to 1500 on cows by mid-August.