Both my hives also have SBB.
I closed them up in the fall as well. On the few warm days we've had, I pulled them out and open up the front entrance.
Then I can check the death rate by pulling out the dead ones. I leave it open for half the day then close them back up.
Last warm day was almost 70 I think?
They came out bigtime! like a swarm in front of the hive......they pooped everywhere!
I was out there observing them.....or ya' know just kinda' hanging out with them, and i had my bee jacket on....... yeah, it's gonna need a good washin'
Yikes! I had no Idea what a mess they could make!
Great to hear from fellow Michiganders!....oh and everyone else as well....of course!