since your horses are already dressed in fly gear...
I've had excellent luck with fly predators. They're a small bee that eats fly larvae. You "sprinkle" them all around your property in high waste areas like a manure pile, run in shed etc. all natural and they really work if you follow the protocol.
https://www.spalding-labs.com/products/fly_control_products/fly_control_for_horses/p/what_are_fly_predators.aspx
Here's another tip I've found that works - put @ 10 pennies in a zip lock Baggie. Add about 2 inches of water. Zip it closed and hang it in doorways with a thumbtack.
The shiny reflective water and the copper pennies deter the flies from crossing the threshold. I've got one in every barn doorway, feed room doorway, tack room doorway, viewing room etc etc.
Other than fly spray which I have dubious feelings about, I got nothing. The flies here in N IL are tolerable - it's the Mosquitos that are making the horses (and humans) nuts since we had so much rain in May, June and July.
Take care and remember at least it's not snow!!!
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