Horse flies are a common pest that bite people, horses and livestock. Insect repellent is often an ineffective option, due to the large area in need of protection and the possibility that harsh chemicals might cause unintended harm to plants or livestock. Homemade horse fly traps are a simple and inexpensive alternative that can produce effective results. These traps use natural bait to attract and catch flies; the traps can then be emptied and reused.
Choose the best location for the trap, such as the doorway of your horse stable, a wooden balcony beam, or wherever you see flies congregate. Hammer the nail into place at eye level; the trap will hang from this nail.
Use a straightedge knife to cut six holes all around the bottle about 1/4 inch from the bottom. The holes should be about 1/8 inch in diameter.
Cut two more holes on opposite sides of the neck of the bottle. These should be wide enough to feed the twine or string through.
Mix together vinegar and sugar until the sugar is dissolved and a paste is formed. Add water a few tablespoons at a time until the mixture reaches a consistency that can be poured.
Remove the bottle cap and pour the sugar mixture into the bottle. Be sure to pour carefully in order to not splash or have it spill out of the punched holes. Replace the cap and rotate the bottle for about 30 seconds to coat the bottle's interior with the sugar mixture. Don't worry if the coating is thin; it will be enough for the flies to get stuck.
Feed twine or string through the top two holes and tie it. Hang the trap from the nail.
If flies congregate in a location that isn't conducive to hanging trap from a nail, place the trap on any flat surface near the animals they attack. Empty the trap at least once a week. The bottle can be rinsed and reused, or you can make a new trap.