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This large, bluish-black horse fly with black wings can exceed 1 inch in length and inflict a painful bite.

Horse flies

Large populations of horse flies (Tabanidae family) are severe pests of Missouri cattle-annoying animals and causing economic losses for farmers. Tabanus atratus is one of the most easily recognized and notorious horse flies in eastern North America.

Like many insects, the female horse fly is the bloodsucker of the family. She uses her sharp mandibles to slash a wound in the host's skin and laps up the blood. This hurts and can lead to economically significant blood loss when horse fly populations are large and their feeding pressure is severe. Horse flies can transmit cattle anaplasmosis and other blood-borne diseases in livestock.

Horse flies are adept at locating warm-blooded animals, including people near swimming pools, streams and in the woods. Apparently the flies are sensitive to parts of the electromagnetic spectrum--their sight may be like "thermal vision" cameras used to detect heat leaks in houses.

Most Missouri horse flies have aquatic larval stages. The mother affixes her eggs to vegetation near ponds and streams. The hatching larvae drop into the mud or water where they live as predators for up to several years, depending on the species. When they are ready to pupate, they move from the water to terrestrial sites and generally emerge synchronously; that is, adults of a particular species all emerge at once, which facilitates mate-finding. Horse flies seem to be present all summer long. In fact, synchronous emergences result in one species being present for a time, then being replaced by another species, and so on. To cattle, however, it means being bitten the entire summer.

Field trials have demonstrated that daily treatment of cattle with synergized pyrethrins (a natural insecticide extracted from plants of the genus Chrysanthemum) will reduce biting rates by horse flies without adversely affecting cattle. Generally, this has been feasible only for dairy cattle (and for horses) because of their high management level. Few producers are capable of, or willing to, round up beef herds every day, and even if this could be accomplished, the disruption of grazing might actually result in a greater economic loss than that caused by horse flies.

Because most horse flies are day-feeders, it is theoretically possible to protect animals and people from attack by staying inside during daylight hours.  

The economic equation changes, however, if cattle can be made to treat themselves. Almost 50 years ago, MU entomologists Curtis Wingo and Stirling Kyd described an economical cattle self-spraying approach that produced effective horse fly control. It will still do so today.


Dear flies

Deer flies are slightly larger than house flies, and mostly yellow or black with darker stripes on the abdomen and dark markings or patterns on the wings. They have brilliant green or golden eyes with zigzag stripes. Deer flies frequently attack humans, whereas horse flies usually attack livestock.

No satisfactory methods have been developed for control of horse and deer flies. It is impractical in most regions to eliminate the breeding areas. Draining marshes and wet meadows where flies develop is of the greatest value, but should be done in such a way as to preserve the desirable wildlife of such areas, if possible. Fortunately, the season for deer flies is rather short, usually four to five weeks in June or July, and three to four weeks in August for horse flies. If the problem lasts six to eight weeks, there may be several species present or that species lasts longer than most. Humans are better able to protect themselves than wild or domestic animals by swatting flies away and by using repellents.

The greatest deer and horse fly activity occurs on warm, sunny days when there is little or no wind. A slight drop in temperature or a sudden breeze reduces biting attacks. Deer flies seem to be attracted to moving objects and dark shapes. They attack humans especially around the face and neck areas (four to five deer flies attack at one time).

A few years ago, several Ohio State University Extension agents field tested TRED-NOT DEER FLY PATCHES as a non-chemical control method. Some reported good results of these odorless, non-chemical, adhesive patches. The patches are three inches wide by six inches long, and are worn on the back of a cap to trap and hold biting deer flies. Patches worked best when moving. For more information, contact DETEX, 6910 W. Ten Mile Road, Leroy, Michigan 49655. Prices are 12 patches for $12.00, 40 patches for $25.00 and 80 patches for $40.00. Shipping is free and within 24 hours.

Persons hiking, picnicking, camping or involved in outdoor activity should protect themselves with repellents such as N-diethyl-meta-toluamide (Deet, Off) or permethrin. Deet-based repellents are effective for a few hours, but the user should not apply indiscriminately since human allergies can develop. Permethrin-based repellents applied only on clothing usually last longer. (Follow label directions.)

Horse flies are usually attracted to shiny surfaces, motion, carbon dioxide, and warmth. Some people report attacks while in swimming pools. Flies light on the skin and bite, causing much pain (one to two horse flies attack at one time). Others fly erratically near the head, often "banging" recklessly into windows and other objects in a crazy fashion. Some rest on back porches, etc., especially in mid to late August.

There are also nylon head nets that keep insects away from the neck and face yet are suitable for clear vision and reinforced for pipe or cigarette smoking. And, Bug-Off Jackets make it unnecessary to use aerosol cans or messy, oily bottles of rub-on repellents. A handy mesh jacket slips on easily over regular outdoor clothing and is very effective when a strong repellent is applied. Store the jacket in a sealed plastic bag.

Gloves are useful, as are tightly woven, long-sleeved shirts and long pants. Be sure that the shirt cuffs button tightly. Light colors, such as army suntans, seem less attractive than dark blue jeans.

Area repellents with citronella or naphthalene are a good way to repel deer flies and mosquitoes in or near a patio, yard, tent, or cabin. Providing daytime shelter for humans and animals is important as horse and deer flies do not appear to bite greatly at night.


Department of Medical Entomology (Photographs)


Horse Flies & Deer Flies of Kentucky

University of Kentucky Department of Entomology




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