THE MERCK VETERINARY MANUAL
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Horn Flies

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The common name of Haematobia irritans comes from the fact that these flies often cluster in the hundreds around the base of the horns of cattle. This major pest of cattle is found in most cattle-producing areas of the world. Populations are common in Europe, North Africa, Asia Minor, and the Americas. Throughout North America, horn flies are found almost exclusively on cattle, but they will feed on horses, sheep, goats, and wildlife. Horn flies are found in much larger numbers and for longer periods of time in the southern and southwestern USA.

Adult horn flies spend their entire life on their host, and females leave only to oviposit eggs on fresh cow feces, where larval and pupal development occurs. In the southern USA, the life cycle can be as short as 1 wk, but in cooler climates and in the spring or fall, development can take 2–3 wk. In some warmer areas (south Florida and southernmost Texas), horn flies reproduce throughout the year.

When the air temperature is <70°F (21°C), horn flies cluster around the base of the horns of cattle. In warmer climates, the flies often cluster in large numbers on the shoulders, back, and sides; these areas are least disturbed by tail switching. On hot, sunny days, horn flies accumulate on the ventral abdomen.

Newly emerged flies seeking their host may travel 7–10 miles (11–15 km) but usually find a host in much shorter distances. Migration seldom occurs over any great distance. In the southern USA, fly populations on individual animals may be in the thousands, especially on bulls not receiving chemical treatment; in the north, they may not exceed 100, although the damage inflicted is similar.

Horn flies feed frequently (as many as 20 times/day), sucking blood and other fluids; female flies are more aggressive than males. Feeding causes pain, annoyance, and blood loss in cattle. Irritated animals also lose weight because of their less efficient use of feed. Heavy infestations cause lesions along the ventral midline of the animal. Horn flies cause great economic losses annually in the USA; 14% reductions in weight gains on range cattle and losses of 12–14 lb (5–6 kg/head) in weaned calves are common. In dairy cattle, milk production may be reduced 10%–20%. These flies also serve as the intermediate host for Stephanofilaria stilesi (see Stephanofilariasis), a filarial parasite that produces plaquelike lesions on the ventral abdomen of cattle.

Horn flies can be easily identified by their dark color, size (~3–6 mm long, approximately half the size of a stable fly), and bayonet-like proboscis that protrudes forward from the head.

Horn flies are relatively easy to control with whole-animal chemical sprays and with self-treating devices (eg, dust bags or back rubbers) in a forced-use manner. Dust bags are most effective when cattle are required to pass under them daily to reach water or mineral supplements. Dust bags leave a deposit of insecticides along the dorsum, the areas where horn flies spend most of their time. Back rubbers allow cattle to treat themselves as they scratch. The insecticide should be diluted with a good grade of mineral oil according to label instructions. Feed additives pass through the animal to kill larval stages that develop in fresh cow feces. All animals must consume a minimum dose of a feed additive regularly. Insect growth regulators also prevent development of larvae in cow feces. When used according to label directions, insecticide-impregnated cattle ear tags (eg, pyrethroids) release small amounts of insecticides that are distributed over the animal during grooming or rubbing. Animals should be tagged at or near the beginning of fly season, the tags removed at or near the end of fly season, and alternative methods with nonpyrethroid insecticides used near the end of fly season. Pour-on insecticide formulations are also effective against horn flies. These compounds are applied to cattle in measured doses based on body weight. Most of these pour-ons function as contact insecticides.

Last full review/revision August 2013 by Charles M. Hendrix, DVM, PhD

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