Well....I'm getting into this thread late....but ....I'll give my 2cents anyway....
I'm a big fan of Food Grade DE...Sevin Dust has perythrins and other stuff in it. Perythrins are toxic to cats and should not be inhaled.....so that's why I chose not to use it myself.
I treat every bird that comes onto my property. Mites can be transferred by hay...but many times your birds can get it just because the wild birds have been infested. This is why I'm ademate about putting DE everywhere my chickens hang out as a preventative.
IN the coop shavings....
IN the nesting boxes...
Weekly add DE to favorite dusting areas....
THis makes it so that the chickens are effectively dusting themselves with DE regularly.
SOmeone I know had a huge infestation come into his flock of >100birds. I told him about DE and he just ended up putting piles of it in each coop (he had big ones)...the birds began to dust bathe in those piles on their own right away---he said it was almost as if they knew that's what they needed!!! Anyway- he's been spot checking chickens, and he hasn't seen any more sign of mites....
My treatment for scaley mites is a mixture of mineral oil and tea tree oil. Really- anything greasy or oily works, but my thinking is the more liquidy it is means it can get into the nooks and crannys under the scales better. The tea tree oil has natural antibiotic and antifungal properties which helps with secondary infections from advanced scaley mites (I had a rescue that lost the majority of her scales due to mites). It's important to oil the roosts as well. For this I spray orange oil (sold as orange guard at ace hardware). I spray ALL the wood in the coop, and reapply on the roosts as necessary (when they look really dry....it means it needs more oil). The oily roosts also helps control mites, as it helps kill them when they drop off. If you have a lot of chickens...keeping the oily mixture in a coffee can with a lid makes it easy to dip the legs in and then have it available for future use. I do treatments once a week for 3 weeks...
Additionally- if you have a wood stove...WOOD ash is great for ridding mites....PUt them in piles in your run...and they will dust bathe in it. DOn't do this if you burn trash in your stove.
Good Luck
Sandra