Fly predators work in some situations--but you have to be the consistent type. They don't last forever, and if there are cows around, you will have flies. Deer also bring their own brand of torment. The predators work primarily on the house and stable flies that breed in moist manure and garbage. The blood-suckers--deerflies, horn flies, bull flies, horse flies, etc. don't breed in those areas and the predators are weak fliers. So don't expect miracles from the little guys.
Also, mosquitoes are the bigger disease carriers-not flies. OOPS!
Then there are the blasted pesky gnats down this way that get inside the horses' ears and leave them bloody and raw. Oh, yeah, been down that road.
Feed her? Watch the garlic--it needs to be air-dried granules and the ONLY place I know of that carries them is Springtime, Inc. Everybody else uses the freeze-dried stuff and it does make a difference. You run the risk of making a horse anemic through a complex reaction. The air-dried stuff loses the compound that is guilty of the problem. Other than that, lose any molasses based feeds. She complain and try to convince you that you are the meanest owner on earth, but the pests will leave her ALONE. Sugar isn't good for them either. Ditto the apples, carrots, and other goodies. Just say no. Remember that you can't buy love and that it's for her own good.
As for sprays--Avon's Skin So Soft in the original formula works--1/2 capful in a quart. Just don't spray under the saddle--it's slippery. I mix in Neem oil, too. The bugs hate the stuff.
Equi-spot works for pastured horses quite well. Just remember to check midlines for summer sores that the face flies will start. My mares object to the oil--I think the creeping feeling makes them nuts. We're still in negotiations on that one.
Best of luck--if all else fails-try Bounce dryer sheets-original scent-and I'm not joking! It works!
No moment of life is lost that is spent in the saddle. Young men have come to grief by backing them or owning them, but never by riding them. Unless, of course, you break your neck which is not a bad death to die. --Sir Winston Churchill