The good news is that our chickens don't have Northern Fowl Mites after all. The bad news is they have lice. Thankyou to AZ for pointing this out to me. I guess I should have continued my research before jumping on the mite bandwagon. Not sure which pest is more difficult to eradicate, they both seem, well: pesty.
Using what I had on hand already, I put some diatamaceous earth in their nesting boxes. I put some oregon grape root and licorice powder in their dust bath area, but they began eating it rather than bathing in it. Unfortunately I grew several herbs this summer that were recommended for treating lice and mites. But I failed to harvest them. I started the seeds indoors, transplanted them and watched as the mullein, yarrow and feverfew flowered. By then I had forgotten why I'd grown them and failed to harvest them. Feverfew is the source of pyrethrin and mullein is a natural source for rotenone. I'd feel more comfortable using my own fresh or dried flower mixtures rather than something out of a bottle. I'll be using what I have around the place already, but if all else fails I'll be making a trip to the feed store next week. Keep ya posted.