Permethrin 10 concentrate, both Gordons or Martins, available at TSC, is an inexpensive product that can be mixed with water according to the label instructions, and used with a spray bottle or garden sprayer to use every month in the coop to keep flies away. An 8 ounce bottle is about $8, makes many gallons of spray, and they have larger quantities. The screw worm spray is also permethrin.