DE won't get rid of them. You need to dust with a garden or poultry dust such as Sevin - the birds and the coop. Then you need to clean the coop out and dust that down completely too. You need to repeat the process in a week because this only kills the mites, not their eggs, so you need to dust again to kill the ones that will have hatched from the eggs.
And because someone says this every time I post, you can also use permethrin - the spray is great to get into nooks and crannies in the coop - but it is extremely toxic to cats and WILL kill them so don't use it if you have cats. It's also highly toxic to fish if it gets into streams etc so I really would steer clear but someone always posts about it when I post to use Sevin so I'm just going to throw it out there with all the information.
