I am puzzled about this phenonmena too.I lost three our of four hives Last winter. The carry over colony is going gangbuster style. Great brood, great brood chamber honey and pollen stores and three honey supers filling up.. lower two are 95%capped. And this despite the fact that two strong wind storms blew the outer cover off with a brick on it in January and on February.I do not know how long it was off becasue I was in hospital both times.oulld be as much as three or four days. Other colonies that died stayed closed up.Perhaps not enough ventilation has some effect on the mites. One of my new colonies is doing well, but the other is dragging and looks queenless. The weak colony started off OK with some brood and suddenly stopped,no eggs, no more larvae but a small number of earlyu capped cells hatched. Then two weeks nothing and I could not find the queen. Put in a new queen about May 20 . Had some progress but now again no new eggs or larvae. I used apistan at both periods There are a good many worker bees there. But without some new eggs a ll will be lost.