Might check on Tracheal mites. This sounds very familiar. Massive
kill-off followed by its disappearance for years so it is no longer a
problem.
It is the forgotten mite but has been making a silent resurgence here in
Maine. And we get all those pollinators from Florida.
Problem is, when you have a collapse, Varroa is always the culprit,
while it may not be.
Bill Truesdell
Bath, Maine
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