Fly sheets, bug spray. We our in mosquito country here and they have been more and more nasty each year. I have heard adding garlic to their daily grain helps keep mosquitoes away. I have never tried that theory.
Here there is this pot of stuff, whatever it is that is animal safe that you can hang around horse shelters and watering areas. They produce a lot of smoke which keeps flying bugs, specifically mosquitoes away. Again something I have not tried, just heard about.
Bronco is my weapon of choice and find it works fantastickly well on my horses.