I've lived in the forest for the best part of 15 years and have found no spay or magic method to deter these little blighters
, a friend rides and drives her cobs in barrier method style as
@domane shows. This works for her, my big lad hated crab flies with a passion but we worked out way through it by never expecting him to keep still when one landed on him, if I went into anything faster than a trot it would normally move, and I would swipe away at it with my schooling whip. Horrible nasty things.
Pony at the moment hates them but bot flys drive her to another level
so I purchased a ride on fly rug for her which seems to help, not helpful having a pony getting worked up and distressed with a nervous child on her ...