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.