The horn fly is considered one of the most important pests of pasture cattle. These flies spend most of their adult life on the body of their cattle host. Horn flies have long bayonet-type mouthparts called a proboscis, which contain rasping teeth used to tear through the skin of cattle causing blood to pool at the skin surface. Both sexes of the horn fly feed on blood. Horn flies take 20-30 small blood meals from their host throughout the day. It is not uncommon for cattle to harbor over a thousand horn flies on a single animal. Imagine the irritation associated with tens of thousands of painful bites per day!
Prior to their introduction into North America in the late 1800's, horn flies were common on cattle in Europe where they were noted to cluster at the base of the horns; giving them their common name. In North America, horn flies are most commonly seen clustered along the back of cattle, but will move in masses to cluster along the sides and belly of cattle on sunny days when daytime temperature is high. Horn flies peak during midsummer in cooler northern climates, while in hotter southern regions of North America they peak in early and late summer.