Just pulled this off their website (fly predators):
Fly Predators parasitize Stable Flies and are an effective control tool. Proper sanitation and management of breeding areas will help control this pest. Research at Texas A&M University has shown that over one million Stable Flies may develop from the residue of one round bale feeding site. By cleaning any straw and hay and spilled feed that accumulates in pens and spreading it out to dry or disposing of it in a compost pile, their numbers can be significantly reduced. Stable Flies are not attracted to “odor” traps, but to specially designed Stable Fly traps. Since after a blood meal, the adults fly to a vertical surface to digest their food residual pesticide applied to those surfaces are often effective in decreasing Stable Fly populations.
I have been using them again this year (2nd year) and they seem to be working even better than last year (this year I did start earlier too). I really don't even think about it until I have someone over who comments on my Lack of flies even though I have horses/cows. I do still use fly spray on the horses, but I've cut done alot on how much and how often (only a couple times a week and I just buy the stuff that is 5-6 dollars a bottle). And they are not all over me either when Im spending time with the horses. Makes for a much more enjoyable time spent with the horses.