Four weeks ago, I changed out of the deep litter in my run. I scoped out mostly the manure powder into my drum compost bin. Overnight, the nitrogen and carbon balance was off, the compost smelled horrifically while the flies swarmed in from nowhere. I hung up fly tapes near the compost bin, did not work. Even inside of the adjacent shed was full of flies after a few days. Then I hung up these vanilla car tree fresheners as suggested here, instantly (well, the next day), the flies were gone! Meanwhile, I added more carbon in the compost and got that process under control, no more smell. It has been 4 weeks and there is still no flies! Each week, I spray the concentrated vanilla scent to freshen the "trees", so far, so good. Thanks for this simple and brilliant idea for fly repellent!