Essential Oil Lavender diluted with olive oil, rubbed onto the bridle path or ears (stay away from eyes) and along the mane will deter pesky deer flies. Pour a little on yourself, too.
Learn More about essential oils https://holistichorse.com/equine-therapy/essential-oils-first-aid-kit-for-animals/
Thanks to Susan Schultz www.basichsacres.com for this tip