I've read that the broodless period in the fall is brought about by a lack of pollen. So far I have not been able to find out when broodless period in the spring is or what causes it.
The reason I ask is I want to treat for Varroa using oxalic acid vaporization or the trickle method. As stated many times in this forum, these treatments are most effective when delivered during a broodless state.
I would appreciate any information you may have.