As defecation occurs, the fly heads toward the ground and is blanketed with fresh dung. The life cycle of the horn fly is 3 weeks or more. Egg laying usually occurs in the early morning. Each female fly lays up to 500 small, reddish brown eggs in clumps on grass and other vegetation under the manure. Horn fly maggots develop best in grass-type manure from cattle that are on pasture and poorly on the manure from cattle on concentrated feed.
Mature horn fly maggots transform to the pupal stage in or below the manure.
Upon emergence from the pupal stage, the adult flies immediately seek out cattle to begin feeding. In West Virginia, the horn fly is a pest during warmer months, usually April or May through September or October. Because adult flies are killed by frost with the onset of cold weather, many of the flies develop only to the pupal stage as fall approaches. The pupae enter a resting phase known as diapause and overwinter beneath manure pads or in the soil. The diapausing pupae produce adult flies the following spring.