Hey up there in Peterborough.
The website below has a method for ridding your culture of mites that I have tried and has worked well. I used the Non Chemical Method at the bottom. Good info on this as well.
I have found that shelf paper to work good also. Black Jungle has it in blue, and you can see the wonderful little DEAD mites on it.
I have finally brought myself to through cultures out after 30 days and I keep the new cultures and my starter cultures seperate from the ones that I use to feed the frogs
http://www.fruitflies.net/mites.htm