We can differ, but we've used it as a preventative for 7 years now and know that's why we don't have lice/mites as our chickens hang out where squirrels, chipmunks, and wild birds do daily...under the wild bird feeders.
I'd get permethrin/garden powder. I assume DME is diatomaceous earth, which is a good preventative, but it will take too long to kill all those now that they've got them, if they in fact do.