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Deer Flies

Horse and Deer Flies

William F. Lyon

Common Name Scientific Name
Horse Fly Tabanus spp.
Deer Fly Chrysops spp.

Female horse and deer flies are vicious, painful biters. They feed on the blood of cattle, horses, mules, hogs, dogs, deer, other warmblooded animals, and even humans. These flies cut through the skin with their knife-like mouthparts and suck the blood for several minutes. When they fly away, a drop or two of blood usually exudes from the wound, permitting secondary feeding sites for other nuisance insects. The flies are potential vectors of such diseases as anthrax, tularemia, anaplasmosis, hog cholera, equine infectious anemia, and filariasis. Also, deer and horse flies are suspected of transmitting Lyme disease (New England Journal of Medicine 322:1752, 1990). Biting deer flies frequently attack humans along summer beaches, near streams, and at the edges of moist, wooded areas. Some people, when bitten, suffer severe lesions, high fever, and even general disability. Symptoms are allergic reactions to hemorrhagic saliva poured into the wound to prevent clotting while the fly is feeding. A person can become increasingly sensitive to repeated bites. Nuisance Autumn horse flies "buzz" people and rest on porches during late summer.


Horse flies are considerably larger than deer flies, heavy bodied, with huge heads (large eyes in males) and from 3/4 inch to over an inch long. Smaller species are brown, black or gray, and often have brilliant green eyes. The eyes are sometimes crossed with reddish-gold bands that disappear when the fly dies. Larger species are brown to black and may be slightly striped. Horse flies have their antennae divided into three parts, the third being long and composed of five to eight rings. Horse flies usually have clear wings whereas deer flies have wings with dark markings.

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.

Eggs are dark, shiny, spindle shaped and in layered masses (tiers) of a few to several hundred on vegetation in or hanging over the water. Fully grown larvae are cylindrical, tapering toward both ends, whitish or yellowish gray, banded with black or brown and a fleshy elevated ring on each body segment. They are tough skinned (leathery), up to two inches long and are often used as fish bait.

Life Cycle and Habits

The life cycles of both flies are similar. Eggs are deposited in masses usually on vegetation or other objects over water near the larval habitat. After eggs hatch in 5 to 12 days, small larvae drop down and burrow into moist, wet soil found in marshes, stream banks, and bottoms of lakes and ponds. They may drop into rapidly flowing streams or burrow into dry soil. Larvae feed on organic debris, other insects, tiny crustaceans, snails, earthworms, and aquatic or semiaquatic organisms. Larvae overwinter in muddy soils, maturing in late spring. Pupation occurs in dry soil. The larval stage is about one year or up to two to three years for some species. The pupal period may range from 6 to 12 days depending on temperature and species. Adults are strong fliers, and appear in early summer with females feeding on blood while males feed on flower nectar, honeydew, plant juices, and other liquids. The life cycle may require from two months to two or three years, depending on the species and geographical region. Some species must obtain a blood meal before the development of each batch of eggs. Adults of most species are seen for only about one month, but often there is an overlapping succession of species during the season. Humans are seriously annoyed by deer flies around woodland areas with streams.

Control Measures

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 organic 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).

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. 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

Question: My husband and I are looking for a safe way to get rid of the biting deer flies and horse flies that love to hang around our pool and patio. C.D., Philadelphia

Answer: A nontoxic solution that has worked well for me is a fly trap from AgraCo. It attracts the flies into a container of water and kills them. You can learn more about it online at or call the company at 1-800-337-4169.

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