Summer brings warmth, long rides, and biting flies. Of all the insects that make summer difficult for horse owners, the horse fly is among the most persistent and potentially harmful. This guide covers what horse flies are, how they behave, the risks they pose to horses, and the most effective ways to protect against them.

What Is a Horse Fly?

Close-up of a horse fly on a mossy surface

Close-Up of a Biting Horse Fly

Horse flies are large, persistent biters most active on hot, windless days.

According to PestWorld, horse flies are large, stout-bodied insects capable of flying more than 30 miles and often found near bodies of water, wooded paths, and areas where mammals graze. They are most active on hot, windless days in full daylight and inactive at night.

Only female horse flies bite. Males feed on pollen and nectar. Females require a blood meal to reproduce and use scissor-like mouthparts to cut and tear skin before lapping up the blood, which is why horse fly bites are significantly more painful than mosquito bites. They actively target moving, dark objects and will chase and pursue a host repeatedly until they succeed or are killed.

Horse flies are known by several other names depending on region:

  • Gadfly
  • Cleg or Clegg
  • Breeze Fly
  • Bull Dog Fly (Canada)
  • March Fly (Australia)
  • Green-headed monster, a nod to their large, iridescent compound eyes

The Health Risk Horse Flies Pose to Horses

Horse in a pasture with flies covering its head and face

Horse Head Covered with Flies in the Pasture

Fly pressure during summer months creates real health risks for horses.

Horse flies do not transmit disease to humans, but they pose a specific and serious risk to horses. Because females interrupt their blood meals when a horse moves defensively, they frequently move on to a second horse before finishing feeding, mechanically transferring blood between animals in the process.

This is the primary natural transmission route for Equine Infectious Anemia (EIA), a viral disease caused by the EIA virus. Per USDA APHIS, there is no vaccine and no treatment for EIA. Horses that survive infection become lifelong carriers and must be permanently isolated or euthanized to prevent transmission to other horses. Fly control is one of the key prevention measures USDA recommends for all horse owners.

Protecting Your Horse from Horse Flies

Horse wearing a protective fly mask in the pasture

Horse Wearing Fly Mask for Fly Protection

Fly masks and ear nets provide a physical barrier against biting insects.

A combination of physical protection and environmental management offers the best results. For direct horse protection:

  • Fly masks, ear nets, fly boots, and fly mesh sheets provide a physical barrier against biting insects
  • Oil-based fly sprays last longer than water-based products; apply to a cloth first before using near eyes, ears, and muzzle
  • A powerful fan in barns discourages horse flies since they avoid wind
  • Disposable fly traps with attractants activated by sunlight and water can capture significant numbers of flies over several weeks

For a simple DIY trap: cut the top third from a plastic bottle, drill a small hole in the cap, place rotten meat and water in the bottom section, invert the top section into the bottom, and seal the edges with tape. The smell draws flies in through the cap hole but they cannot escape.

Other Biting Flies to Know

Beyond horse flies, horses contend with several other biting insects during summer months:

  • Stable flies: bite legs and lower body, known for painful repeated biting

  • Deer flies: similar behavior to horse flies, also potential EIA vectors

  • Black flies: small but aggressive, often attack the ears and belly

  • Biting midges (sandflies): extremely small, capable of biting through thin areas of skin

Each fly type has similar preferences for warm, humid environments near water and manure sources, making sanitation the common thread in preventing all of them.

Fly Prevention: Barn and Pasture Best Practices

Barn interior showing manure and damp straw that attracts flies

Removing Damp Straw and Manure for Fly Prevention

Removing damp manure and bedding cuts off the breeding sites flies depend on.

A single fly can produce close to 1,000 offspring in under a month, which means prevention is always more effective than reactive control. These six practices form the foundation of a solid fly management program:

  • Remove manure from stalls, yards, and fields regularly, since manure is both a breeding site and a food source

  • Eliminate damp areas: wet straw, rotting hay, pooled rainwater, and moist soil under hay bales are all fly breeding grounds; consider rubber matting as an alternative to straw bedding
  • Spread manure and bedding so it dries quickly, cutting off the moisture flies need to lay eggs
  • Treat barn surfaces, stall sides, fences, and manure piles regularly with a reliable fly control product
  • Replace sticky fly traps and flypaper regularly
  • Turn off barn lights at night to avoid attracting insects

Natural Fly Repellent Options

For horse owners who prefer to avoid chemical sprays, several natural options are effective:

  • Apple cider vinegar (ACV): Fill a spray bottle with unpasteurized, organic ACV and apply directly, or dilute 50/50 with water if the scent is too strong for the horse
  • Essential oil sprays: A blend of citronella, eucalyptus, lemongrass, peppermint, cedarwood, or lavender in a witch hazel base makes an effective DIY repellent; combine half a cup of witch hazel, one teaspoon of vanilla extract, and 100 to 150 drops of essential oils in a 16-ounce amber spray bottle, fill to the top with water, and shake before each use; store in a cool, dark place and avoid applying near eyes and nose

Grooming After Fly Season

EquiGroomer WaterWisk sweat scraper being used on a horse

WaterWisk Horse Sweat Scraper by EquiGroomer

After fly season, regular bathing and grooming helps keep a horse's coat healthy.

Regular grooming during summer months does more than keep a horse’s coat in good condition. It also provides the opportunity to check for fly bites, skin irritation, and signs of infection before they become larger problems.

After washing down a horse, the 7-inch WaterWisk removes excess water and sweat from every part of the body, including bony areas like hips, hocks, and legs where standard plastic scrapers are difficult to use. Keeping a horse clean and dry reduces the moisture that attracts flies.