I just had a bunch of flies in the last couple of days. I am using DE, I clean the roost area out daily. We did have a big rain last week so it got wet but I moved the coop and let the area dry, dusted it with DE (a lot) dusted the roost with DE, but they free range and they drop poop all over and that is where the flies are. I have a cat litter scoop that I keep with me and I scoop as I see it and put it in the compost bin. I also bought a fly trap that you put the smelly bait in will with water and hang, I used to have horses near a cattle farm and they worked well, I also put up 3 sticky strips, I hope all this works the flies were bad yesterday. I walked around with a scoop and fly swatter! ( I am unemployed right now so I have the time) I have a book with "home" remedies, it says put 1 egg yolk, 1 T molasses (I used honey) and a pinch of pepper in a shallow dish like a tuna can, and put it in the yard/house, I did catch a few that way, just have to put it where the hens can't get to it and eat it...
I use my grass clippings for the coop, I let them dry in the sun and then put them in, that way I can change them every week when I mow, I will usually add more during the week too. I guess that is ok...I use pine in the roost and nests..the girls love the grass they will leave the yard and go in the coop and pick at the grass...
We have a neighbor with dogs that stay chained up outside beside us so I think some of the flies are from all the poop, but I put pine shavings in the coop then it rained to help soak up some of the wet and I dumped those in the big compost pile, not in a bin, maybe I will dust that with DE, although that will kill all the good bugs working...darn, next time I will put it in a plastic bag...