I stand corrected--one species of mites, the Northern Fowl Mite, lives on the host. The other species, Chicken Mites, lives off the host. Yes, check at night. Check while wearing white cotton gloves (so you can see them on your hands)
http://ohioline.osu.edu/vme-fact/0018.html
I bought Elector PSP that can be used on laying hens with no withdrawal time. It is fairly expensive, but very effective. A vet who raises Cochins recommended it. I've never had a mite issue, but the new birds I bought came with a lot of lice eggs on them. I dipped the new birds in a Permectrin II every two weeks and used the Elector spray after the poultry shows, plus bathed them in Adams Flea and Tick shampoo before the shows. I'll dip them again in a week if the weather is good. I'm not sure about withdrawal for Permectrin II, and if there is any, I'll just do a routine spray with Elector PSP which is incredibly effective.
You have to concentrate around the vent since that is the preferred place for them to go.
My understanding is the mites and lice are really easy to get, especially if you don't make sure new birds are quarantined and clear of parasites. Infestations are persistent, too. Most people don't check their birds. A bird can be horribly infested, to the point where it dies and the owner doesn't realize.