I have 3 free range, happy hens, have used fly predators, natural spray, commercial spray, made my own, our manure is hauled away weekly, and the flies are horrible right now. It seems they get stronger & more resistant to traps, sprays, etc as the summer goes on. My hens do eat the bugs, but, like you say, the flies travel & none of my neighbors have any sort of fly program in place, so yes–it is gross right now. I think the hens do help, and they sure are fun to watch & interact with. Easy keepers, too-and of course, eggs!