University of Maryland Extension

Horse and Deer Flies

adult horse fly
Horse fly

deer fly
Deer fly
Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Horse and Deer Flies 

  • Horse flies are about 3/8 to 1 1/8 inches long, usually gray or blackish in color. The wings usually lack dark areas, but some species have wings that are entirely dark. The eyes often have horizontal stripes.
  • Deer flies are smaller, about 1/4 to 1/2 inch long, black or brownish, usually with dark areas on the wings. Their eyes usually have spots.

General Information

  • Females are blood-sucking and inflict a painful bite.
  • Males feed primarily on pollen and nectar and are often found on flowers.
  • Females deposit eggs on plants, rocks, and other objects usually in damp locations.
  • The larvae live in wet areas and eat immature insects and other small animals. Larvae pupate in the mud near the edge of these areas.
  • The life cycle may take from 3 months to 2 years to complete.
  • Most are strong fliers and have a range of up to several miles. They are most active during the warmest parts of the day and often follow moving objects.
  • The use of pesticides for control is not feasible due to the habits of these flies.
  • Insect repellents and wearing protective clothing may help to prevent bites. 
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