Mike, if you do a search on here you'll find I posted quite some time back about having a mite infestation. Brought 'em home from a show. Had no idea what was wrong as I'd never had mites or lice on my place prior to that. They spread like wildfire and I fought 'em for 3+yrs.
Long story short, during that time I tried Ivermectin Eprinex, Pyrethrins, Permethrin, Spidercides, and literally bathing them in DE myself. Sometimes in dosages way over the recommended. Many, many times I'd knock 'em back to pert near nothing but they'd always come back. And I swear the mites developed a resistance to the Pyrethins and Permethrins.
Finally I was Googling and came across an article somewhere about Frontline. Some scientist was giving a chemical breakdown with percentages of how much Fipronil a human would ingest if they ate the eggs from a bird treated with Frontline. (Not enough to come close to even worrying about) So I Googled Frontline and chickens and found a few posts that said to put 3-4 drops down by the vent and a 1-2 drops at the neck.
I went down to Wal-Mart and they had ProNyl about $20 cheaper than Frontline and it had the exact same ingredients and %% of Fipronil as Frontline. So I got the dosage for like the 135 lb. dog. Treated my whole flock (about 150 birds at that time) one Saturday night. The next day I hardly saw any mites. By the 3rd day I saw no mites. I think they recommend treating again after 30 days but I never saw another mite so I didn't. In fact, I've not seen one since.
God Bless,