There are many ways mites may have found their way into the fly cultures, and completely eliminating them is virtually impossible. We can at least keep their numbers manageable. Before we make a new culture, we have to consider each ingredient as a possible source of contamination. Always store your fly culture supplies completely sealed and away from your frog tanks and fly cultures. Even if your fly media is brand new in a sealed container, it doesn't guarantee there aren't mites or eggs in the media. Heating the media to 160°F should kill any mite or egg. If you just make one culture at a time, it is easy to microwave the culture for 30 - 45 seconds. Stir the media and check the temperature.
If you use excelsior, you have to consider it as a possible source of mites or their eggs as well. You might want to buy some new excelsior, new paper plates or coffee filters will work as well.
Your biggest source for mites is going to be your flies from the previous culture. Mite life cycles can be as short as 4 days, each female can lay up to 600 eggs and live for 30 days or more. It's easy to see why a 30 day old culture can be teaming with tens of thousands of mites, even if your original culture only start out with a couple mites or eggs. Before adding any flies to your new culture, you must clean the flies. Once a mite infestation is heavy, some of the mites will become a different form, called a hypopi. The hypopus stage does not eat, but attaches itself to the flies; hoping to be transported to a new source of food. It will then drop off and start the cycle all over again.
Dusting your flies and giving them a few minutes to groom themselves of the dust and mites, can virtually eliminate the mites. Flies can be dusted with food grade diatomaceous earth, will kill the mites but surprisingly does not kill the flies. You can also dust the flies with calcium powder for the same effect. Be careful not to allow any of the dust to enter the new culture. We dust our flies, then transfer most of them to a new cup, carefully tapping the cup to only allow most of the flies to fall into the new cup. We then allow those flies to clean themselves for a few minutes, before adding them to our new cultures, again being sure to not let any of the dust enter the culture.
All new cultures should be stored away from your frog tanks and any older cultures and placed either on mite paper ( which is impregnated with insecticide), or a light dusting of diatomaceous earth. Mites can crawl thru the fabric on insect lids, so we must place a barrier that will kill mites between cups. We don't want the cups to touch one another. this means, no stacking one culture on top of another. We store all of our fresh cultures in a cabinet, in a different room away from our frogs and older cultures.
Always date and dispose of your fly cultures within 30 days of making them. If your cultures are still producing flies after 30 days, start using less media. No sense in using more food than the flies can consume in 30 days.