There are mites out there that are becoming resistant to Permethrin which is the main ingredient in Provent-a-Mite. I believe they're out west, though it hasn't been completely identified and I'm not putting any names out in the air. But if you do get new animals, please make sure you're QT appropriately. Also it doesn't hurt to be proactive and treat any new snakes regardless if you see mites or not, this is what I do. This way it's like an added layer of protection.