Warding off flies and the ailments they transmit is the obvious key to protecting your horse's health. Although you may take satisfaction from direct combat with the "divells" in your horse's habitat, there are considerably more effective ways to reduce fly population than smashing by swatter and crushing by hand. You may not be able to control the "wild" flies that breeze in from adjoining forests and fields, but you can eliminate opportunities for "homegrown" pests to reproduce in your own backyard.
Preventive stable management includes daily collection and disposal of manure, soiled bedding and spoiled feed--all excellent breeding grounds for house- and stable flies. Manure that has to be stockpiled for weeks or months can be treated with topical larvicides or feed-through larvicides, which your horse ingests along with his feed and passes out with his manure. Fly predators, small, nonstinging members of the wasp family, can also be introduced to manure piles, where their larvae feast on fly larvae.
You can eradicate fly breeding grounds by disposing of water-catching refuse and junk and filling those perpetual puddles on your property. These days, you may run afoul of wetlands-protection laws by draining muddy or swampy pasture areas in the interest of fly control, but certainly no one would object to stocking the farm pond with fish that feed on insect larvae or welcoming insect-eating birds by allowing barn swallows to next or erecting a purple martin house.
But since this is a fly's world, some adults will always survive to bite, tickle and just generally torment your horses no matter how discouraging you've made the environment. A shady shelter, be it a stall or a run-in shed, is among the best fly protection you can provide for horses. Many daytime feeders, such as deerflies, black flies and horseflies, are reluctant to enter dimly lit buildings. Organophosphate sprays, either surface or residual, provide effective control of other flies inside the stable, as do automatic misters that release fly repellents or insecticides. Window screens and baited fly strips and traps will also reduce the interior fly population.
When your horse must be turned out in a shelterless pasture or you're working him outdoors during the fly season, you might try any number of pyrethroid repellents on his coat. Unfortunately, sunlight and sweat tend to render these repellents ineffective more rapidly than most horses and their riders would wish. Face masks, ear nets and fringed veils will protect the most sensitive areas of the head from face flies and gnats, and even a petroleum jelly coating on the inside of their ears will minimize bites.
Chances are neither you nor your horses will ever enjoy the bliss of a fly-free environment. But with effort, you can greatly reduce the opportunity for flies to infect your mount with a contagious disease, parasite or fungus, especially if wounds are carefully tended and if ivermectin is used periodically. Good horse care and good stable management can prevail against the age-old evil that flies do.
Excerpted from an article that first appeared in the June 1991 issue of EQUUS Magazine.