Q&A: Controlling biting flies

A professional advises on how to identify biting flies and protect your horse from them.
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A professional advises on how to identify biting flies and protect your horse from them.

Q: I have a 10-year-old mare who came to me about three years ago. My problem is that I cannot ride her in the hot months of summer because she freaks out over horseflies. I’ve tried sprays, wipes, collars and even the spot-on product that you apply every two weeks. Nothing I’ve tried so far helps. Her barn area and stall are kept clean. I really like this mare, although at times I know why the previous owner sold her---it had to be the flies. She’s fine at everything else. I don’t want to sell her because the problem will go with her. Please help me enjoy and keep my mare.


A: I truly sympathize with your horsefly problem. My last horse was imported from Europe, and he also had terrible trouble with American horseflies and deer flies---he wanted to go home! Several species of flies bite horses: 

• Horseflies, or greenheads, can grow to more than an inch long and bite on the rump, shoulders, head and neck. Smaller ones may be more apt to bite on the legs or belly.

• Deer flies range in size up to a half inch long and tend to bite on the neck and head.

• Black flies, or buffalo gnats, are around a quarter inch long. They tend to bite on the ears, genitals and other sparsely haired areas. 

• Stable flies look very similar to houseflies and tend to bite on the legs as well as the underbelly, face and neck.

Mosquitoes and biting gnats (“no-see-ums”) will also bother horses but these tend not to cause the type of behavior you describe.

If your problem were limited to stable flies, the solution would be a little easier. Because this species lays its eggs in soiled bedding, manure and other decaying organic matter, taking steps to remove or compost these materials can greatly reduce stable fly populations around your farm. Predatory wasps added to your manure pile would also help, as would baited traps with scented attractants. Finally, repellent products, including sprays, wipes and spot-ons, tend to deter them, and mesh fly wraps applied to your horse’s lower legs help to keep them away.

However, from your description, it sounds like you are dealing with horse-, deer and/or black flies. All of these species, but especially horseflies, inflict painful bites with their sharp, knife-like mouthparts, and a horse’s attempts to evade them can sometimes get frenzied. Only the females feed on blood. They hunt by sight---attracted to large, dark, moving objects that emit warmth and carbon dioxide---and they are fast fliers who may be persistent at pursuing their victims until they have finished their meal. They prefer horses but will also bite people, dogs and other animals.

These species all lay their eggs near natural waterways and wetlands, including streams, ponds and marshes, especially in wooded areas. This means that keeping your premises clean has no impact on their numbers, and there is little you can do to control their breeding, especially if you live near woods or wetlands. All of these larger biting flies are most active on warm, sunny days with little wind. A slight drop in temperature or a breeze reduces biting activity, making riding more pleasant.

Unfortunately, horse-, deer and black flies are extremely difficult to control. Because they hunt by sight, not smell, scent-based deterrents will have no effect. Fly sprays and other products that contain pyrethrins, permethrin, DEET or resmethrin are most effective, but even these will not prevent all bites. These substances can also be harmful, so it’s essential to follow the instructions on the products. Aerial spraying of a local area is only a temporary measure, because these flies can fly for miles, and new ones will appear in a few days.

Traps designed specifically for sight hunters can be helpful in pastures and stable yards. A number of specific types are available. Most exploit the flies’ attraction to dark objects by using a black lure of some sort, then trap them in a container or on a sticky surface. But these traps won’t help you while you’re riding, and if you live near a prolific breeding area, they may barely make a dent in the local population.

Your best defense may simply be avoidance: Keep your mare in a shady barn during the hot, sunny hours when horsefly feeding is at its peak, and turn her out at night. Ride early or late in the day when it is cooler or when there’s a breeze, and avoid riding near horsefly breeding sites such as wetlands, ponds, lakes, beaches and woods. When she’s turned out, make sure she has a deep, dark run-in shed she can escape into. The flies won’t follow her inside.

Protective clothing may also be helpful, for both you and your horse. Light-colored fly masks, nose nets, ear covers, full hoods, belly covers, leg wraps and full body sheets will cover the skin and physically prevent many bites. You can find versions of most of these covers that can be worn with a saddle and bridle while you’re riding. You might also carry a fly whisk---a short crop with a “tail” made of long horsehair, to swish flies off your mare as you ride. Remember, though, that biting flies are attracted to movement, so don’t wave it too wildly or more flies will come.

To protect yourself, wear light-colored, long-sleeved clothing and a light-colored helmet. Hikers and fishermen sometimes wear Bug Off shirts and hoods made of fine mesh permeated with DEET or other fly repellents. You can also get sticky cloth tape patches that can be placed on the back of a hat to catch deer flies that hover around your head.

I hope that you’re able to manage your mare’s sensitivity to biting flies with some of these management practices.

Susan E. Harris is an international clinician and equestrian author from Cortland, New York. She teaches Centered Riding Clinics and gives Anatomy in Motion/Visible Horse presentations around the world. Harris, who has been honored as a Master Instructor by the American Riding Instructor Association, is the author of Horse Gaits, Balance and Movement, Grooming to Win and the U.S. Pony Club Manuals of Horsemanship.

This article first appeared in EQUUS issue #443.