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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