Horse Flies Causing a BIG Problem While Riding!?

As it seems you have a very sensitive horse, I would suggest a stronger type of fly spray. Pyranha is an oil based spray that has a longer efficacy. Also, consider giving apple cider vinegar with his regular rations. Start with a small amount, about 2 tablespoons, increasing over a few weeks to 1/4 to 1/2 cup. This helps the horse's basic chemistry, offending flies from biting by changing the ph balance.