Heat to keep brood warm is indeed the limiting factor in spring buildup. My bees are still tightly covered with an insulated wrap and the I" bored holes in the upper brood box are more and more resembling a fire hose. Last two nights have been below freezing and snow on the ground two days ago. I think I will let others adapt their tropical insects to a gaping hole in the bottom of their hive in such conditions. In a week or so I expect swarm preparations in the hives and I can make a few splits and wrap them up tightly too. It will be mid July before nights are even in the fifties here.