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When you mean these red-colored mites... yes, they must have come with the flies. If it was in only one of the boxes, the contamination most assumalbly came from inside the box.

When you want to freeze at -18 degrees C, you may have to keep them in the fridge much longer to be sure possible eggs or larvae are all killed. I wouldn't know when you are safe then. Maybe just try?

Boxes stored in a dark place seem to be easier contaminated than boxes stored in the light. Also humidity may play a role. As well as some luck :)

Anyway, I am not a specialist in this field. Maybe Kahis can add some info on this, since he is permanently living in a freezer....? :D
(I must say the summer of 2003 was very nice in Finland, though!)

Andr