Several years ago, someone told me to try sprinkling dried basil all around in my coops, and runs. I thought they were nuts, but I decided to give it a try. Sprinkle it like you would on meat. Not overly heavy, but broadcast around giving nice coverage. I was shocked to find out it really works. The next day, there will be few, if any flies. You may want to sprinkle a bit more the next day, if your chickens eat some of it. Mine do. Don't worry, it won't hurt them any at all. Within a couple days, your fly problem will be gone.