But if a broodless period of 2 weeks significantly reduces the varroa load, why does the winter break not wipe them out?
The "theory" is that in warm weather the phoretic mites are looking for a place to breed. The brood break lets them all out of capped cells and looking for a cell to enter. As soon as the queen start laying those first cells are host to more than one mite. Multiple mites per cell means the larva dies, and then by default those mites die also. So the sudden spike in mites are groomed out in the first few eggs the new queen lays.
This is the theory presented, I make no claims about it other than knowing it. I will say that my "treatment free" hives that have survived have all practiced/had brood breaks. Learned this a while back when I went to graft from my best TF hive. Queen was marked and present, but stopped laying in july.
Charles
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