Here is an article that EVERY beekeeper should read. We have all seen this exact scenario.
EDITOR'S COMMENT: I put a sticky board under several hives a week ago. One hive that had overwintered well and had a good spring buildup had a 2 day mite drop of 27 varroa! Surprising a 5 week old package hive had only 3 mites. I immediately treated all overwintered colonies. I will check the treated colony again in a few days.