My horses are about to experience their first Deep South summer, complete with horse flies. Now, don't get me wrong, we have them up north, but nowhere this early (there's still snow on the ground back home for Pete's sake!) and nowhere near this many! Today was the first day my horses were literally pounding down the barn gates to get back into their stalls after just an hour outside.
Do I just wage the ultimate war with fly masks, spray, and sheets? I'd rather not have them wear sheets when it's 100+ degrees out...
I'd like to get to the point where I can leave them out all day, but right now they want nothing to do with grazing longer than 15 minutes.
I use fly predators as well, and they do work well for flies. Unfortunately, where I am in the swampy south, we are also inundated w/ no-see-ums (biting gnats) and mosquitoes. Just make sure that you apply any fly spray away from where you release fly predators - or you'll kill them. Talk to the people at Spalding Labs, and they'll get you set up.
Fly predators aren't an option for me as I'm at a large boarding barn. I use Pyranha, Mosquito Halt and a fly mask.
The flies barely touch my horse if they get sprayed right before turnout. I use the Mosquito Halt since my horse is turned out at night and they can get pretty unpleasant. I also always worry about West Nile, even though I get them vaccinated. The only people I see that use fly sheets are for horses that are very sensitive to bug bites (ie are covered in welts or are constantly upset) and only during times when the flies are really bad.