There are two commonly occuring types, the first lives on the fly, and generally is not too much of a problem. The second predates on fly eggs , and can wipe out cultures. Avoiding mites completely can be difficult, it is essential to have a tight fitting bung, and as much as possible avoid contact with wild flies. Eventually they should dissappear. Mites can be seen on the vial sides as tiny light pale brown dots, and should be discarded if found.
Water is preferable as it keeps cultures at the required humidity, and stops the media from drying.
I have always found wild type drosophila melanogaster to be far more resistant to mites than the larger flightless flies, probably due to the much shorter development time of the former. A typical drosophila vial, for flighted flies. A home-made drosophila vial, for flightless flies. The 'walkway' is essential. No graphics or content from this site may be used elsewhere without prior consent of the author, however you may print out pages for your own use.