Within the bin(s) cultures should be stored allowing freshest batches to be kept separate from oldest batches. We recommend keeping cultures no older than weeks 1 and 2 together and keeping culture weeks 3 and 4 together. This will prevent mites from your oldest cultures from migrating to your new cultures. This reference is based on a month having 4 weeks. When you make a culture write the date that you made that culture and write an expiration date of 30 days from the present day.
Cultures should be set on diatomaceous earth within that bin. The diatomaceous earth will serve as a barrier so that mites cannot climb up cultures from neighboring infected cultures or leaving infected cultures to spread to neighboring cultures or your living space. Mites will not survive once trapped in diatomaceous earth.
Alternatively you can set your cultures in a moat of water, on commercial "mite paper" or even on paper towels sprayed with a mite spray. Note, in our experience it is difficult to keep mite sprayed paper towels as an effective deterrent. Also, it appears that mite paper has a limited shelf life once opened.
We set our bins on diatomaceous earth and place those bins on paper towels soaked with mite spray or mite paper. Further note that mite spray should be allowed to dry before placing cultures back on. We wait 15 minutes. Melanogaster flies are sensitive to mite spray fumes while drying.