I just found these same mites in a month-old culture I also purchased from a well-known vendor. The vendor assured me it was normal for this to happen to cultures more than 4 weeks old and that they are harmless to the frogs, but because mites freak me out and I'd like to have cultures that produce flies for more than a month, here is what I did.
1. Poured as many of the flies from this culture as possible into a newly-made culture cup. (Should have done the powder method, but didn't)
2. Threw out old mite-infested culture.
3. Keeping this culture and one I made a week ago using some flies from the old culture in a segregated place and plan to throw them away when they finish producing and won't be making any new cultures using these flies.
4. Just ordered 2 producing cultures and 3 fresh cultures, and 20 Mite-free lids from SE Fruit Flies, who guarantees mite free cultures and has a coupon for free shipping right now as a New Years special. Just enter FREEME in the promotional code box when checking out.
5. Purchased mite spray from petsmart and 2 sterilite containers with locking lids. I will be keeping the new fly cultures in these containers lined with paper towels sprayed with the mite spray, and with a cup filled with wet paper towels to help with humidity. One container will be for cultures less than 3 weeks and the other for those older than 3 weeks.
6. Once these cultures prove to be mite-free for more than a month I will only use these flies in future cultures and will be making my own media to avoid any possibility of "importing" mites from other sources and essentially keeping a "closed herd" of fruit flies in my apartment.
Now because this plan hasn't been proven successful yet, you'll have to take it with a grain of salt, but I thought I'd at least share the ideas I came up with to deal with the same problem you're having. Good luck to us both!
