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Another option is to mix small amounts of apple cider vinegar with the animal's drinking water; consuming this solution is known to change the blood pH of the horse, thus keeping horseflies away. Cider vinegar, coupled with essential oils, is also used in home sprays as the sharp odor may repeal horseflies.
To make a dish soap spray, take an empty spray bottle and add 4 tablespoons of dish soap, 2 cups of white vinegar to it, then add 1 cup of warm water, shake the mixture and your dish soap spray is ready. Spray it on horse flies and watch them dying instantly.