Protecting Cattle from Horn Flies

BY Robert Fears | | Comments (0)

Horn flies are the number one pest that affects cattle in the United States. Economic losses were estimated in 1992 to be about $800 million annually. Horn fly presence or absence is temperature dependent, while abundance is influenced by humidity and precipitation. As a result, horn fly populations peak in early summer and decrease when the weather turns hot and dry. Their numbers increase again in the fall with occurrence of rainfall and lower temperatures. These flies are no longer a major problem after September or October.

Both sexes of flies live on a cow and feed 20 to 30 times per day on the animal’s blood by cutting through the hide with their piercing mouthparts. One fly will consume approximately 10 micro liters of blood per day and it is not uncommon for 3000 flies to reside on one cow. This number of flies will consume a total daily blood amount of 30 milliliters, which is slightly over one fluid ounce. Bites from the horn fly hurt and they leave welts on the skin.

“Horn flies adversely affect cattle production through annoyance in addition to causing blood loss,” says Dr. Kelly Loftin, University of Arkansas. “Annoyance results in energy losses associated with combating the flies, changes and/or reductions in routine grazing patterns and bunching of animals. Significant reduction in calf weaning weights, due to decline in milk production, is well documented. Studies have shown that effective horn fly management can result in a 15 to 30 pound increase in stocker calf weights during the growing season. University of Arkansas researchers noted a 17 pound reduction in calf weaning weights for every 100 flies feeding on the cow.”

Identifying Characteristics and Life Cycle

“Horn flies look like house flies and stable flies, but are slightly smaller,” says Dr. Sonja Swiger, Texas A&M AgriLife Extension Service Entomologist. “Both horn and stable flies have piercing mouth parts that resemble a beak. The horn fly was accidentally introduced into the United States from southern France prior to 1886.”

“To distinguish horn flies from stable flies, observe their feeding behavior,” continues Swiger. “Horn flies rest on an animal between feedings, whereas stable flies only remain on the animal while feeding. Stable flies feed principally on the animal’s legs, but horn flies feed most often on the back, shoulders and sides. During very hot or rainy weather, horn flies may move to the belly.”

“Horn flies leave cattle to lay eggs in fresh manure that is less than 10 minutes old,” states Loftin. “Development from egg to adult occurs in as little as 9 to 12 days. Eggs hatch and larvae develop in the manure. Mature larvae migrate to the lower portion of the manure pat or into soil to pupate. Adults emerge after about five or six days, if weather conditions are suitable. During winter, they will survive in the soil as pupae. Approximately two days after emergence, horn flies mate and seek a host to begin blood feeding. An adult female may begin laying eggs three days after emergence and may lay up to 400 eggs during her lifetime. With such a short life cycle, many generations per year are possible.”


“Monitoring horn fly abundance on cattle is important in making appropriate management decisions,” explains Loftin. “Routine monitoring will help producers determine when best to initiate control methods and evaluate efficacy of the current control program. Monitoring will also provide early warning to potential insecticide tolerance or other issues that negatively impact control.

“Horn fly abundance should be monitored weekly throughout fly season. Monitoring is best achieved by counting flies on the head, shoulders and back of 10 to 15 cattle. Whole body counts are the most accurate, but in practice, are difficult to make. Counting flies on one side of an animal may be the only available option. Horn fly counts from at least 10 animals should be used to obtain an average. When average counts approach the economic threshold of 150 to 200 per head on beef cattle, control should be considered. “

“To suppress horn fly populations efficiently, use an integrated pest management (IPM) approach,” recommends Swiger. “IPM relies on multiple tactics including cultural, biological and chemical methods to suppress insect pests.” She suggests the following control methods:

Biological control – Parasitic wasps suppress horn fly populations naturally and fly pupae parasitized with the wasps can be obtained from insectaries across the United States. The parasitized pupae are best used around barns where manure accumulations allow for fly pest development. Research has not proven that releasing the wasps suppresses horn flies or that use of parasitized pupae reduces fly populations in pasture situations.

Fire ants and dung beetles also suppress horn fly numbers. Dung beetles compete for manure use and shrink the manure pats where horn fly larvae grow. Moxidectin and, to a greater extent, avermectin kill dung beetles; so be careful when applying these two pesticides. Fire ants feed on horn fly larvae and pupae, thus reducing the number of horn fly adults.

The walk-through Bruce trap helps control horn flies by dislodging them as the animal passes through. Dislodged flies are trapped in three elements located on each side of the trap. The elements catch flies by the “inverted cone” principle, similar to a minnow trap, upon which many insect traps depend. The trap forces insects to crawl from a large opening through a small one. The triangular shape formed by the bending of the screen makes alternate large ends that face the interior. Holes punched through the small end of the triangle permit the flies to crawl toward the outside of the trap. There, the screening on the exterior of the trapping element interrupts the flies’ progress. Flies are trapped between the exterior screen on one side and the zigzag screen on the other. Because the cone now faces the wrong way for the insects — that is, the small end faces them — it is unlikely that many will find the holes and escape. Trapped flies die from starvation or dehydration. For more information and design instructions, see the University of Missouri’s link.

Cultural methods – Remove and properly dispose of fresh manure from barns and stalls to interrupt the horn fly’s life cycle and help prevent new populations from developing.

Chemical Control – Several chemical control methods such as ear tags, sprayers, dusters, feed additives, and boluses can help reduce the number of horn flies on cattle. Contact your county or state extension office to obtain their recommendations for chemical control. To prevent resistance, rotate chemical classes of insecticide products each year and even within a year if a mid-to-late season horn fly increase warrants further insecticide applications.

Regardless of the chemical method you choose, follow these guidelines:

Do not treat infestations of less than 200 flies per cow. Treating when horn fly populations are below this level is not cost-effective and the unnecessary use of insecticides can speed development of resistant fly populations.

Read the product label to ensure that it is suitable for use on beef cattle.

Be careful when applying insecticides and wear protective clothing as recommended on the product label.

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