Range and pastured cattle are affected by many ectoparasites, which can cause significant economic losses (Table 1). The most widely recognized ectoparasites include flies in the order Diptera. Horn flies, face flies, heel flies, horse flies, deer flies, and stable flies are most bothersome to cattle. The horn fly, Haematobia irritans (L.), is considered the most important blood-sucking pest of pastured cattle and can have a significant economic impact on cow/calf operations.
 

 Table 1  Estimated losses in cattle production due to ectoparasites.*
 

Ectoparasite	$ Loss (millions)
Horn fly Stable fly Ticks Lice Face fly Mosquitoes Mites  Total	$ 800.0 $ 607.8 $ 398.9 $ 126.3 $ 53.2 $ 38.7 $ 29.7  $ 2054.6

*Adapted from Drummond et al., 1981 and Foil et al., 2000.

Biology and Life Cycle The horn fly is an obligate blood-sucking external parasite of cattle and is widely distributed throughout the United States. Adult horn flies are 3/16-inch long and are about half the size of a housefly. They are dark gray in color, with two dark stripes on the thorax and a few faint spots on the top of the abdomen. Horn flies can be differentiated from houseflies and stable flies by their slender appearance. At rest, the wings are held partially open, forming a V-shape. Horn flies are typically seen on the backs, side, and poll areas of cattle. During hot or rainy days, horn flies can be observed on the belly of cattle. When disturbed, horn flies will ascend up in a swarm, but return to the animal almost immediately.

In general, horn flies appear in the early spring and the population increases in numbers until it peaks during mid- to late- summer and early fall, depending on the region and weather. Horn flies overwinter as diapausing pupae in or beneath cattle manure. Adults emerge the following spring and begin a new life cycle. Adult horn flies will spend most of their lives on cattle, but can infest horses, sheep, goats, deer, and elk. The average life span of an adult horn fly is 10-21 days. Both male and female horn flies feed on cattle by piercing the animal’s skin with their bayonet-like mouthparts and ingesting a small amount of blood. Horn flies feed intermittently approximately 20-30 times daily for 10-25 minutes at a time. A single female can ingest 11-21 mg of blood daily.

The horn fly life cycle is relatively short. After mating, the female leaves the animal only long enough to deposit eggs in fresh cow manure or in vegetation under the manure (Figure 1). The female deposits as many as 400-500 eggs during her lifetime. Eggs hatch within one day, and larvae feed and mature in the manure, and then migrate below the manure into the soil to pupate. Upon emergence from the pupal stage, adult flies seek a host and begin feeding (Figure 1). If no immediate host is found, newly emerged horn flies may travel several miles searching for a host. The entire life cycle can be completed in 10 to 20 days depending on the weather.

   Economic Impact

Horn flies have a significant impact on pastured cattle. Economic losses associated with horn flies are estimated at more than $800 million annually in the United States. Feeding habits of horn flies are irritating and painful to cattle, as well as causing blood loss. Heavy infestations of horn flies can result in decreased grazing efficiency, reduced weight gains, and a decline in milk production. In addition, horn flies have been implicated in the spread of mastitis.

The number of horn flies on the heads, backs, and shoulders of at least 15 animals are used to obtain visual counts of horn flies, which in turn are used to quantify horn fly populations. Approximately 100 horn flies per animal is considered the “treatment threshold” when control measures may be required. The “economic injury level” for horn flies on cattle is considered to be 200 flies per animal.

Numerous studies from the 1970s to the early 2000s have been conducted to assess the economic affects of horn fly infestations on cows and calf weaning weights. Most agree that when horn fly populations exceed 200 per cow, cattle group together and expend considerable energy combating flies. Cattle often stand in water or search for shade to get relief from flies, resulting in a decrease in grazing efficiency. Data from these studies indicate a general reduction in weaning weight of 10-20 lb per calf when horn flies exceed 200 flies per cow. Horn flies generally infest larger, more mature animals such as yearlings, stockers, replacement heifers, and cows. They tend to avoid infesting young calves weighing less than 300-400 lb. Horn fly infestations on yearlings can result in up to 18% reduction in weight gain. Horn fly feeding on mature cows results in reduced milk production, which indirectly reduces weight gains in calves. Every 100 flies per cow can reduce calf-weaning weights by 3-16 %. Table 2 summarizes studies examining the effects of horn fly control on cattle production.

Table 2  Relationship between horn fly infestation of cows and weaning weights
or average daily gain (ADG) of the their calves.^a

Author(s)	Average no. of flies on cows	Weaning Weight (lb)	ADG (lb)
Campbell, 1976 Huston et al., 1979 Kunz et al., 1984 Quisenberry and Stohnbehn, 1984 Schreiber et al., 1987	30 938  10 270 0 700+ 2 220 2 197	387.9 374.9 - -  - - - - - -	- - 1.96* 1.83 2.03* 1.94 1.83* 1.72 2.60 2.60

^aAdapted from Drummond, 1987.
^*P < .05
 

Horn Fly Control
There are many effective methods available to control horn flies on cattle – sprays, dusts, backrubbers (self-oilers), insecticidal ear tags, pour-ons, boluses, and oral larvicides (feed-throughs). Efficacy, cost, convenience, and herd management practices should be considered when designing a horn fly control program.

Ear tags are one of the most convenient methods of horn fly control. They provide good control of flies throughout the fly season. However, ear tag use has resulted in widespread resistance to some of these insecticides (pyrethroids). Sprays and pour-ons are easy to apply, but their duration of control is often limited (28 days). Backrubbers and dust bags are effective ways to apply insecticides; however, they are less convenient. They can provide good control of horn flies if cattle are forced to use them. Insect growth regulators that prevent larvae from developing into harmful adults have been used to reduce horn fly production related losses. The key to success is often related to proper consumption of the oral larvacide. Uncontrolled horn fly populations in cattle herds located in close proximity to treated herds may hamper control methods regardless of treatment methods.

Conclusion
An effective horn fly population reduction program can help curb economic losses related to irritating, biting horn flies. Lower horn fly numbers allow cattle to graze peacefully and uninterrupted, enabling them to gain more weight and produce more milk (better calf weaning weights). Calf weaning weights may be reduced 3-16% when the horn fly population reaches 100 flies per cow. The choice of control method should be dictated by product efficacy, cost, associated labor requirements, facilities, and management capabilities and preferences.