Horse flies and deer flies belong to the fly family Tabanidae. Horse flies (genus Tabanus) are considerably larger than deer flies (genus Chrysops). Both horse flies and deer flies have large heads and large eyes. Horse flies can range from 20 mm (3/4 inch) to more than 25 mm (1 inch) in length. A common species typically found near beaches and saltmarshes is Tabanus nigrovittatus, or "greenhead fly." This species has brilliant green eyes which are sometimes crossed with red or purplish bands. Larger species of horse flies are brown to black and have varying stripes or triangles on their abdomens. The larger species of horse flies are less common, but inflict a painful bite similar to other species of horse flies. The antennae of horse flies are thick and lengthen into 5 thinner segments; the antennae of deer flies are long and thin. The wings of horse flies are usually clear or completely dark, whereas deer fly wings have varying patterns. Deer flies are comparable in size to house flies and are mostly yellow or black with varying stripes and shapes on their abdomens. The wings of deer flies are usually marked with dark patterns. Deer flies also have brilliantly-colored eyes, ranging from gold to green, with large brightly-colored stripes.

Deer fly	Horse fly (photos courtesy of the Florida Cooperative Extension)

Damage:

Only female horse flies and deer flies feed on blood. Both flies are vicious, painful biters which feed on the blood of cattle, horses, mules, hogs, dogs, deer and other mammals, including humans. These flies cut through the skin using razor-sharp mouthparts that are shaped like a knife or razor. The flies will then suck the blood up from the wound for several minutes. This process makes these flies potential mechanical vectors of such diseases as anthrax, tularemia, anaplasmosis, hog cholera, equine infectious anemia and filariasis. Deer flies and horse flies are also suspected of transmitting Lyme disease (New England Journal of Medicine 322:1752, 1990). The open wound left by the fly bite also permits secondary feeding sites for other nuisance insects. Biting deer flies frequently attack humans along beaches, streams, ponds, lakes and around woods and dirt roads near large bodies of water. Some people suffer severe lesions, high fever and even general disability when bitten. Allergic reactions may occur from the saliva, which is poured into the wound to prevent clotting while the fly is feeding. A person can become increasingly sensitive to repeated bites. However, horse flies and deer flies are generally thought of as primarily nuisance pests.

Life Cycle:

The lifecycles of both horse and deer flies are similar. The first stage of development is the egg stage. The eggs are dark, shiny and spindle shaped. They are layered in masses ("tiers") which contain a few to several hundred eggs. These masses are laid on vegetation which hangs over water. Eggs hatch within five to twelve days, and small larvae drop down and burrow into moist soil. Suitable habitats include saltmarshes, swamps, bogs and areas along the edges of ponds, lakes and streams. Deer fly larvae feed on organic debris and other small organisms. Horse fly larva will feed on organic debris, insects, small crustaceans, snails, earthworms and other organisms. Horse fly larvae are also cannibalistic and will eat other larvae.

Larvae overwinter in muddy soils, maturing in late spring. In some cases, larvae take one to three years to complete development. In late spring, the larvae migrate towards dryer soils and develop into pupae. The pupal period varies between species and may range from six to twelve days, depending on temperature. Adult flies emerge from pupae and immediately begin mating and blood feeding. Adults are strong fliers, searching visually for hosts and mates. The females require a blood meal for their eggs, but also feed on nectar and plant juices for flight energy. Males also require nectar and plant juices for flight.

Control:

Unfortunately, there are no satisfactory methods for control of horse flies and deer flies. It is impractical in most regions to eliminate their breeding areas, especially along endangered wetlands, where these flies are commonly found. Larval control is equally impractical, especially in recreation areas or reservoirs. Adulticiding is not practical because it requires the use of broad-spectrum insecticides, which can be toxic to fish, birds and mammals. Traditional repellants are not effective in keeping away horse and deer flies. Repellants containing DEET (N-diethyl-meta-toluamide) do not prevent flies from landing, although they may inhibit the flies from biting.

The greatest horse and deer 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. Horse and deer flies are visual insects, locating hosts by movement. Dark, moving objects and shapes are most attractive to the flies. They are also attracted to carbon dioxide that is released from their hosts. To reduce exposure to bites, it is best to wear light-colored clothing, including a light baseball cap, especially on warm, sunny days when flies are most active.

Many devices have been designed to catch flies attracted to moving objects. One device is the trolling deer fly trap. This device consists of a blue cup coated with sticky material; the cup is placed outside of a moving vehicle, attracting horse and deer flies with its movement and color. Another device is the Tred-not Deer Fly Patch, which is a new, non-chemical sticky patch for controlling horse and deer flies. Some testers have reported good results from these odorless, non-chemical, adhesive patches. The patches are 7.5 cm (3 inches) wide by 15 cm (6 inches) long, and are worn on the back of a baseball cap to trap and hold biting deer flies.

A number of fly traps have been developed which attract these flies using dark, moving objects, as well as carbon dioxide and other attractants. Although these traps will not completely eliminate all the flies, they will reduce the populations to a more tolerable level. Examples of trap designs include canopy traps, box traps, malaise traps and light traps.

Commercially available horse and deer fly repellants are available for use on animals only. Most of these repellants contain permethrin, which may be harmful to humans. One must take precautions to use these repellants only as instructed. Repellants safe for horses are not necessarily safe for other animals. Always follow label directions.

By Kristen Bartlett, 1999