Everything You Should Know About Flies

Flies irritate not only horses, but also their owners; especially when it comes to controlling their populations!

Classification of Flies Seen in Horse Barns

Most flies are considered external parasites and are classified as either a biting or non-biting fly. As an external parasite, the fly’s life cycles exist outside of the animal, are not affected by dewormers, and often require focused management to reduce their numbers. An exception to this would be the bot fly. Bot flies have a life cycle with an internal and external phase and can be controlled with a proper dewormer.

Biting flies, often multiple times a day, administer a painful bite to a horse to get a blood meal. Not only is the pain associated with multiple bites a concern, but these flies can also spread illness with diseases such as Equine Infectious Anemia (EIA). Furthermore, these flies can irritate horses not only with their bites but also with their perpetual buzzing. This can lead to abnormal behaviors, poor body condition, or even infection and irritation around the bite site.

Types of Biting Flies Associated with Horses:

The types of biting flies associated with horses include:

  • Horse fly
    • Large fly, body lengths ¾ to 1 ¼ inch long, yellowish-grey to black bodies with clear or solid colored wings, and brightly colored eyes
    • Most active during mid to late summer
  • Deer fly
    • Larger than a house fly but smaller than a horse fly, body length up to ½ inch, can be black, yellow and black, or grey, wings are cross banded, eyes are brightly colored
    • Most active during the summer months
  • Stable fly 
    • Size of a house fly, less than ½ inch long, grey in color with dark spots on top of its abdomen, brown eyes
    • Most active during mid-summer and early autumn
  • Horn Fly
    • Smaller than a house fly, less than ¼ inch long, brownish-gray or black bodies, with brightly colored wings and brownish eyes
    • Most active during late spring to mid-summer

Non-biting flies are less of a concern but can be a major source of annoyance to any horse. While these flies do not bite horses to get a blood meal, they typically will feed off secretions around the eyes or muzzle of a horse. The associated buzzing, tactile sensations, and constant flying movement, all leads to irritation to horses. When bothered by large populations of non-biting flies, this too can lead to abnormal behaviors, weight loss or even poor body condition.

Types of Non-Biting Flies Associated with Horses:

The types of non-biting flies associated with horses usually include:

  • House fly
    • Most recognizable fly, can be black to grey in color, with red eyes, and usually not longer than 1/3 inch
    • Most active during the summer months
  • Face fly
    • Similar in appearance and size to house flies, can be distinguished by four dark stripes along their thorax
    • Most active mid- to late-summer

Overall, flies can be bothersome when not managed. However, with a careful and well thought out strategy, these pests do not have to serve as an irritation either to you or your horse. For a comprehensive fly control program, there are many considerations. For example, feed additives such as Essential K® with Fly Control is a good start. Other strategies could include:

  • Composting manure and maintaining clean stalls and barn
  • Removal of stagnant water; excellent for mosquito control as well
  • Application of fly spray, usually effective for 1 to 2 days
  • Fly masks, sheets or boots to reduce irritation to your horse
  • Insect traps or fly predators

When in doubt with any parasitic control program, it is advisable to always consult with your local veterinarian. They can tell you more about which flies are endemic to your area.

Article By: Chris Mortensen, Ph.D.
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