Female horse flies and deer flies slash skin with blade-like mouthparts to create small pools of blood from which they feed. Both groups of these closely related flies can be serious pests of cattle, horses, and humans. Horse flies range in size from 3/4 to 1 1/4 inches long. They usually have clear or solidly colored wings and brightly colored eyes. Deer flies, which are more likely to bite humans, are smaller with dark bands across the wings and colored eyes similar to those of horse flies.
Attack by these persistent flies can make outdoor work and recreation miserable. Seldom thwarted, they attack persistently to get enough blood to make a batch of eggs. The numbers of flies and the intensity of their attack vary from year to year. Numerous painful bites from large populations can interrupt grazing and result in violent efforts by the animals to try to stop or escape from the attacks. In addition to blood loss, these biting flies can transmit anaplasmosis and equine infectious anemia from infected to healthy animals on their blood-contaminated mouthparts.
Vision is a key factor in finding hosts. Female horse flies and deer flies feed during the day; from their resting spots, they respond by sight to large, dark moving objects within their range. While body temperature, carbondioxide, and other chemicals released by host animals may play a role in the final decision to bite, the long range attraction means that insecticides or repellents play no role in protection until the fly lands.
- Pyrethrins and pyrethroids have an irritating effect on insects but may not be at full strength or effective enough to interrupt feeding until after the painful skin cut has been made. In addition, only the flies’ feet come into contact with a treated surface, so the insects will bite and may get their meal before becoming irritated enough to move.
- Several types of traps are sold for horse fly control. They may have some effect in protecting animals in a relatively small space but the impact may be reduced in large pastures.
By Lee Townsend, Extension Entomologist