Those things are so painful when they bite! They like sandy areas to breed. IMO, try to attract barn swallows. They like empty run ins with ledges, and pray for dragonflys. I went from being infested with them in Maryland to practically none here in the great state of Arkansas. We have barn swallows and big clouds of dragonflies that eat all types of flys.
Spray the top of your helmet with 40% Deet and your saddle and blanket.
Also, your horses tail and the croup. If one lands there and you don't see it you will most likely get bucked off. You shouldn't really put Deet on your horse in any amount, their skin will peel off.
They come in patches. try to ride the same trails consistently and kill as many as you can, you have to hear the crunch to know you killed them. I used to tell the kids I killed all the dumb ones, now we are down to the smart ones, and we'd make a game of it. I won't ride with anyone that refuses to kill them (swishing them away). They are worse when you leave woods and go into the sun. They sit on branches in the shade and swoop when you are in the sun. Drop Mosquito dunks in any open puddles, that will help get rid of them for next year.