Fly control is probably the most annoying, but necessary, component of summer care. With so many brands on the market, including those water-based botanical alternatives to chemical sprays, you will find a product that is just right. Most contain pyrethin, which may come mixed with lanolin, aloe or glycerine. Citronella oil is a popular base for fly sprays. Look for products containing sunscreens.
Some horses are skittish about sprays.
Approach slowly, and if possible, hold your horse's lead rope yourself, or have someone hold it, in contrast to tying the horse the first time you spritz. Spray a dozen or so times on each side of your horse's body, and do not forget to spray under the belly and around the tail, and between and down the legs. For the horse's face, spray a sponge or towel that you only use for this purpose, and rub it on the face, being careful to avoid the immediate eye area. Unfortunately, fly spray only works when you apply it, so some diligence is required in terms of repetition.