Horse Fly

Actual Size: 1/2″–1¼”

Characteristics: Stout flies with large, iridescent eyes and clear or solid colored wings.


  • They are bothersome in that they pursue their target incessantly.
  • Adult horse flies drink nectar and plant juices, and females consume blood for protein.
  • They are active during the day and do not like darkness.
  • Adults are fast and strong fliers that flock to streams, marshes, and wooded areas.

Horse Fly Identification

Unlike other types of flies, horse flies are not hairy and have characteristically large eyes. In females, these often-colorful eyes are separated whereas male eyes are always touching. They are a bit larger than house flies and slightly rounder as well. Horseflies have transparent wings with one black dot on each, as well as short antennae that are orange-red in color.

Horse Fly Behavior

Female horse flies lay batches of eggs on areas over wetlands or water sources. Once hatched, the larvae feed on decaying matter or small organisms in the surrounding soil and water. They emerge as adults in anywhere between one and three years.

While male horse flies feed on nectar and sap from plants, female horse flies are active during the day as they search for cattle, horses, and even humans to feed on in the late summer months. Once they’ve found a host, they use their knife-like mouth to slice open skin and suck on the blood created. When female horse flies are abundant, they can suck three ounces or more of blood from their victim.

Are Horse Flies Dangerous?

When livestock or humans are bitten by a horsefly, the bite can be felt right away and usually causes a red bump. Because female horse flies are a nuisance and typically relentless in their pursuit, they can make outdoor work or recreation miserable in areas infested by them. Besides the painful bite, some humans may have an allergic reaction to the secretions released by the flies as they feed, and secondary infections are possible when bites are scratched. Over-the-counter first-aid skin creams or ointments could help relieve any pain from a bite.

When female horse flies attack livestock, the consequences can be more dire. Numerous bites from a large population of female horse flies can effectively reduce milk production from dairy and beef cattle, and blood loss can be significant in serious attacks. Grazing cattle and horses can also be affected, as animals under attack will group together for protection.

Protection from Horse Flies

To protect animals from horse flies, there are several permethrin-based insecticides on the market that could provide temporary help. However, this is not a permanent solution and the horse flies will continue to swarm around areas of the animal that have not been sprayed. One tip is to keep animals in barns or deep shade during infestations in the daytime; they can graze at night when the flies aren’t active.

Humans can also make use of repellents during the summer to protect themselves from horse fly attacks. These repellents can be bought anywhere and can prevent flies from landing, but they do not ward off a swarm. Wearing light-colored clothing and outdoor mesh garments could provide relief from female horse flies as well.