I dusted mine a couple times w/poultry dust and still had mites. I have fluffy cochins, and broody ones, so here's what I did. Being that you mentioned organic, you may not go for this, but I was where you are (I just want them gone!, and they were getting on ME...) and this is what worked for me.
I completely cleaned out the coop, emptied nest boxes, and vacuumed the whole place. Got Flea/Tick premise spray (Adams) and sprayed the whole place down. Let that dry, put down DE, fresh bedding in coop, and DE, cedar shavings, then fresh hay in nestboxes. All the chickens got Frontline Spray- one squirt under each wing, one on back of head, one near vent. This was back in November, and no signs of the creepy crawlys since (furiously knocking on wood!).
Good Luck!
~Terry