I bought a 50# bag of DE and it didn't appear to me to do anything. My coops were also loaded with flies and in one coop I realized I also had a problem with mites. The mites seemed to thrive as well as the flies no matter how much DE I put down. After doing some research I bought some permethrin and after a few weekly treatments no more mites and no more flies. It doesn't kill the mite eggs so weekly treatments were necessary for awhile, but it worked. I think if you use one product too much the pests will become immune to it. I also discovered that some manufactures of ear tags for cattle use permethrin for the flies.