Egg -- Tan, yellow or white at first, the egg darkens to reddish-brown before hatching. It is oval-elongate in outline, flat or concave on one side, convex on the other and 1.2 mm long.
Larva -- The newly hatched maggot, about 1.5 mm long, develops through three instars to reach a mature length of 6.5 to 7.5 mm. Slender and white, it narrows to a point at the head.
Pupa -- Enclosed within the shrunken skin of the last larval instar, the pupa is 3.3 mm by 1.4 mm. Barrel-shaped and white at first, the outer covering (puparium) soon turns a dark reddish-brown.
Hosts -- Although this blood-sucking fly is primarily a nuisance to cattle, other animals such as sheep, goats, horses, mules and dogs are also bothered.
Damage -- Congregating on those areas of the body where they are not likely to be disturbed (base of horns, neck, throat, belly, thighs, back, etc.), horn flies suck blood from livestock through their needle-like mouthparts. Such feeding causes weight loss, reduced milk production, and reduced vitality. Furthermore, animals become so annoyed that they may injure themselves while attempting to dislodge the flies. Although infestations of 4,000 to 10,000 flies per animal are common in some parts of the country, horn flies rarely exceed 500 per animal in North Carolina.
Life History -- Favored by warm, moist weather, horn flies emerge in spring and seek out host animals. Although they locate hosts most successfully during the day, they usually disperse at night, sometimes traveling as far as 5 miles. Soon after initial feeding, females periodically leave the host animal and deposit 1 to 14 eggs in fresh cow manure. Both male and female horn flies apparently feed on the manure from time to time. Eggs hatch 16 to 24 hours later.
Over a 4- to 5-day period, the larvae feed in moist cattle dung and develop through three instars. They then either burrow about 4 cm (1.5 inches) into the soil or remain in the manure and pupate. During spring and summer months, a new brood of flies emerges 5 to 7 days later and repeats the cycle. As winter approaches, newly formed pupae overwinter giving rise to a new generation of flies the following spring. Although most prevalent in spring and summer, horn flies continue to produce a new generation approximately every 2 weeks well into autumn.
Chemical recommendations for horn fly control should be closely observed since some pesticides cleared for use on beef cattle cannot be used on dairy cows in order to avoid pesticide residues in milk.