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Horse & Deer Flies

- Large, biting flies that attack people and livestock -

Summary: These large flies bite people and livestock. Horse flies are serious pests that can disrupt normal animal behavior and even animal health. Both horse and deer flies can be annoying and persistent pests of people, too. Traps are usually the best control option.

Jack DeAngelis, PhD

OSU Ext. Entomologist (ret.)

Horse and deer flies bite

Horse and deer flies are large flies that bite people, livestock and wildlife. Bites can be very painful but these flies do not transmit disease in humans. Horse flies are more likely to attack livestock, including horses, where their persistent biting can adversely affect animal health. People are generally more bothered by deer flies but horse flies will also attack us, the difference may be related to body size. Both flies are active during daylight hours.

Horse fly - about 3/4". Note large eyes. Deer flies tend to be smaller and more colorful.

Horse flies are dark in color often appearing almost black (above right) while deer flies [picture of deer fly] are somewhat smaller and more colorful. Both have very large, prominent eyes which they use in hunting for animal hosts. Both flies can be very persistent and annoying when encountered and they are relatively strong fliers so can follow a host animal, or us, a long distance.

Horse & deer flies need damp soil

Larvae of these flies develop in the moist soil around bodies of freshwater so adults tend to occur near water as well. Adults, however, are stong fliers so they may occur some distance from their breeding grounds.

Avoiding bites

It is almost impossible to treat the soil around freshwater sources where horse and deer fly larvae live so the best approach is to trap adult flies and use insect repellent. Repellents containing DEET, and permethrin-based fabric protectants work well for these annoying flies (see Insect Repellents for more information). Also see our article about Sweet Itch In Horses for a list of effective fly repellents for horses.

Horse fly traps

Horse flies are attracted to dark objects that mimic the silhouette of an animal and there are a number of non-insecticidal traps that incorporate dark shapes that attract and capture these flies. One such commercial trap, the Horse Pal Trap, is effective and relatively easy to set up and move around in pastures.

Flies are attracted to the dark ball suspended below the trap (right). Flies tend to fly upward when they leave an object and so are trapped in the pyramid netting and jar trap (jar not shown in photo at right). One advantage of the this particular design is the ease of moving it from place to place in the field. Other trap designs are larger and more difficult to move around. Horse and deer fly traps use no pheromones or insecticides.

Horse Pal trap. Image used with permission.

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