Current BO does not allow “chemical” fly sprays. For some reason, EcoVet is in that category to her; otherwise that would be my choice. Fly Bye Plus may meet with her approval. I use Freedom 45 every 10 days year round, but it’s better against ticks than flies. I tried the Equiderma spray but it really only worked in the exact areas on the horse where it was sprayed.
What helped a lot was fly masks, a fly sheet if it was not too hot (I got a Turtleneck that is awesome, but mare has sweating issues so she could not wear it in the daytime if it got really hot), and fly boots. I used Shoo-fly boots but their quality has declined. Somewhere in the forum, there is a suggestion for another brand, but I can’t find it right now. We also used a lot of Zephyr’s Garden solid fly repellent on areas that flies were especially attracted to, like midlines on the belly and the chest.
Horses are out 24/7 but have nice big sheds with solar powered fans in them. That helps. And when flies are really bad, my mare has a Cashel Fly-Ride getup for her walks. I don’t ride her, so use a surcingle and tie it to the rings.