Horn Fly

The horn fly is a small, dark fly closely associated with cattle, on which it spends nearly its entire adult life, leaving hosts only briefly to lay eggs. About half the size of a common house fly, it is easily overlooked individually but becomes conspicuous when hundreds gather in dense clusters on the backs, shoulders, and sides of grazing animals.

Unlike the face fly, which laps up secretions with sponging mouthparts, the horn fly has piercing mouthparts that project forward from the head, allowing it to take frequent, small blood meals directly from its host's skin throughout the day. This close, near-constant association with cattle distinguishes it from most other flies found in pastures.

Originally from Europe, the horn fly has become established across much of North America and other temperate cattle-raising regions since its introduction, and it remains one of the most abundant flies found directly on grazing livestock.

• Small size, roughly half the length of a common house fly
• Dark gray to black body with a stout build
• Piercing mouthparts that project forward from the head, visible at close range
• Typically found in dense clusters on the backs, shoulders, and sides of cattle
• Distinguished from face flies and stable flies by its small size and habit of remaining on the host almost continuously

Horn flies are found throughout temperate and subtropical cattle-raising regions worldwide, having spread widely from their native European range. They are almost always found directly on or very near grazing cattle in pastures rather than dispersed broadly across the landscape.

Populations peak during the warm grazing season and decline sharply with the onset of cold weather in temperate climates.

Adult horn flies remain on their host for most of their life, using piercing mouthparts to take frequent small blood meals throughout the day, and leaving the animal only briefly, mainly to lay eggs in fresh dung. Both males and females feed on blood, a trait less common among flies overall.

Because of their near-constant presence on cattle, horn flies form one of the most persistent insect associations found in pastures, with large numbers often clustering together on a single animal. Their larvae, in contrast, develop away from the host entirely, within fresh dung deposits.

Females briefly leave the host to lay eggs in very fresh cattle dung, often within minutes of it being deposited. Eggs hatch quickly, and larvae develop within the dung over about one to two weeks, passing through three instars.

Mature larvae pupate in the soil beneath the dung pat, and adults emerge within one to two weeks to seek out a host. Many generations occur during a single grazing season due to the short life cycle, and horn flies typically overwinter as pupae in the soil in temperate climates, with adults emerging the following spring.