On facebook, in the Beediverse bee group I was asked why only 50% of their cocoons hatched. This looks like a complicated puzzle, but I have seen it once and think I know why this happened.
I think the problem is that the bees that did not emerge did not have any fat body reserves left by the time spring came around.
It is truly amazing that the female lays the egg with enough pollen and nectar reserves to grow a bee that has enough fat stores for the bee to eat her way through to the outside world, and never mind the overwintering metabolism. It is all about whether the bee uses all her fat bodies while she is going through the autumn and overwintering period. If temperatures are unusually high in the fall, the bees’ metabolism is also unusually high, and the worse case scenario is that the bees run out of fat stores and perhaps die. What I have seen is when bees are kept in a warm basement over part of the fall and winter, there is the little energy or fat stores left to eat through the cocoon to the outside world. Sometimes bees are still moving, but after manually removing the bee from the cocoon, she ends up dead. More usually, you find a perfectly normal dead bee inside a cocoon. I don’t expect that bees can look ‘wasted’ because their exoskeleton keeps them in one shape only, no matter how wasted they are on the inside. Hope this helps.
During the summer months, remove nests from hot sites that for example, face the hot afternoon sun. Store them under cover, away from direct sun.