Horse Fly

Horse Fly

Horse flies are often called forest flies and are often mistaken for deer flies. These flies are considered pests but they are one of worlds largest true species of fly. There are 3,000 species of Horse Flies in the world and 300 of them are found in North America.

Horse flies range in size from 3/4 to 1-1/4 inches long and usually have clear or solidly colored wings and brightly colored eyes. Attack by a few of these persistent flies can make outdoor work and recreation miserable. The numbers of flies and the intensity of their attack vary from year to year.

Numerous painful bites from large populations of these flies can reduce milk production from dairy and beef cattle and interfere with grazing of cattle and horses because animals under attack will bunch together. Animals may even injure themselves as they run to escape these flies. Blood loss can be significant. In a USDA Bulletin 1218, Webb and Wells estimated that horse flies would consume 1 cc of blood for their meal, and they calculated that 20 to 30 flies feeding for 6 hours would take 20 teaspoons. This would amount to one quart of blood in 10 days.




These flies apparently are attracted to such things as movement, shiny surfaces, carbon dioxide, and warmth. Once on a host, they use their knife-like mouthparts ( like Black Flies) to slice the skin and feed on the blood pool that is created. Bites can be very painful and there may be an allergic reaction to the salivary secretions released by the insects as they feed. The irritation and swelling from bites usually disappears in a day or so. However, secondary infections may occur when bites are scratched. General first aid-type skin creams may help to relieve the pain from bites. In rare instances, there may be allergic reactions involving hives and wheezing. Male flies feed on nectar and are of no consequence as animal pests.

A hunting female fly often waits patiently in shrubs at the edge of a forest, marsh, field, or roadside until a potential food source passes by. In the summer these flies can be seen in swarms on livestock, often around the eyes of the animal.




After eating, she lays her eggs on plants sticking up out of wetlands. When the larvae hatch, they fall into the water. Larvae live as predacious worms in the water where they live through winter. Pupae develop along the water’s edge in early summer. The adult stage lasts through much of June and July. Only one generation develops each year.

Horse flies carry tularemia and other diseases that can harm humans and other animals. (Tularemia, also known as “rabbit fever,” is a disease caused by the bacterium Francisella tularensis. Tularemia is typically found in animals, especially rodents, rabbits, and hares. Tularemia is usually a rural disease and has been reported in all U.S. states except Hawaii.)