Battling flies in the summertime is a given. Some years, though, are worse than others... and I have a theory about that.
We've used every known means for battling flies at the barn. We've used fly spray on the horses... they hate it, and I have never found one that truly works. I swear the flies have little tiny gas masks!
The horses spend their summers with fly masks on during the day to keep them out of their eyes and face. We also keep multiple fans blowing in and around the barn.
We have about 15 of these solar fly traps around the farm - that start catching flies in Spring and continue until autumn. We catch billions of flies this way. Eventually, the sun bakes them and they end up in a pile inside the solar catcher. The traps are eventually dumped into the compost pile, where those flies provide necessary protein.
We keep fly tapes on hand and change them liberally. Lately, we catch this amount of flies on a fly tape in a single day. We hang them around the barn, in the henhouses, in the duck house, in the donkey house, and in the greenhouse. They are effective, but need to be changed on a daily basis... and, even through they are fairly inexpensive, we could...