Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Horses say "SHOO FLY" to annoying pests!

Pet Column for the week of August 6, 2001

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Jennifer Stone
Information Specialist

In the heat of the summer, horses often expend a lot of energy trying to stomp and squish the flies that buzz around them. Not only are these flies extremely annoying to horses, but they can also be a health hazard.

The weather seems to have a big effect on the activity of most flies. Dr. Dean Scoggins, an equine Extension veterinarian at the University of Illinois College of Veterinary Medicine, says, "When the temperature gets high, flies become more active and feed more aggressively."

Many different types of flies bother horses. Some, such as houseflies, feed off manure, while others, called "face flies," feed off secretions from the eyes.

The most damaging flies are blood-sucking flies that feed off animals. These flies will often ravage the skin, leaving it raw and bloody. Parts of the body that stick out, such as the ears, are a favorite target. The blood feeders not only cause skin sores, but can also spread blood-borne disease, such as infectious equine anemia.

One type of blood-sucking fly, called a culicoid, can cause severe allergic reactions in some horses. Large areas of skin become covered with itchy bumps resulting from fly bites, which can become infected if the horse scratches them too much. These flies feed only outdoors at dawn and dusk and can be avoided by putting the horse out to pasture at other times of the day.

There are many methods for controlling flies. Sprays can be applied directly to the skin, but many flies develop a resistance to these over time. One of the most popular remedies is a liquid with a syrupy consistency that is poured over the mane and shoulders. This is more effective than spray, but some owners put formulas made for cattle on their horses. This must be done with caution since the cattle formulas may be toxic to horses.

Because many kinds of fly sprays have kerosene or diesel fuel as a component, chemical burns can occur if the horse stands in the sun after they are applied. Putting a saddle or other equipment over an area that has recently been treated with a topical insecticide can also cause a chemical burn. One form of insecticide that is safe and effective is a "feed through" type put in the horse's food; it kills the fly larvae when the fly eggs hatch in the manure. It is fairly effective, but flies can develop a resistance to it over time.

A mechanical barrier provides an alternative to managing flies chemically. There are fly nets available that cover the head and prevent the flies from reaching the eyes and ears. Dr. Scoggins warns, "These nets can be effective, but you must be sure that they will easily come off if they get tangled in something; otherwise strangulation could occur."

Another non-chemical method for controlling flies is natural predators. A special kind of wasp that does not bite people or horses but feeds on fly larvae can be used. The wasps cannot kill enough larvae to bring the fly population down once it is established, so the wasps should be released early in the spring before there are many flies around. If you use this method, then you cannot use insecticides, which will kill the wasps as well as the flies.
Regardless of what method you use, starting an active campaign against the flies early in the spring is a must, because preventing flies is much more effective than trying to control them after they are present in large numbers in your pasture.

By far the easiest way to help keep the flies away is to pick up the manure regularly. If the pasture is clean, the flies will have to look elsewhere for food and a place to reproduce, and they will not be around to bother your horse.

If you have any questions about how to control flies in your pasture, contact your local equine veterinarian.