Dirt from the garden. I used to be a big advocate of DE - swore by the stuff - talked about it's benefits ad nauseum! I put it on the floor of the coop, put it in the nest boxes, put it in the dust bath.....then one day I realized that I was out there replenishing the dust bath and I was wearing a bandanna over my mouth and nose. The bag said that it was a respiratory irritant and that I should wear protection to use it. What the heck was I doing??? I was putting something down where my chickens would be fluffing and rolling, scratching and laying, and they weren't wearing any breathing protection from the dust they kicked up, even though their respiratory systems are more sensitive than mine and they were exposed to it 24/7. Yep, it kills mites. But there are plenty of other things that help do that! Besides, I was using the deep litter method in my coop and run and the DE was also killing the very critters that I was depending on to help break down the litter.
Eureka moment! I put that stuff away and my chickens have survived just fine without it.