In the early days of varroa mites, at least in the southern U.S.,
immigration of mites into treated colonies did not appear to be a
problem. Treatments were effective and lasted for a year or more. A
summer treatment got colonies through the next winter. A winter
treatment usually lasted through the summer and into the next winter.
Presumably many feral colonies and unaware beekeeper colonies were
collapsing in big numbers. Where did those mites go? What has changed
to make mite bombs so devastating, or have they always been like that
and we were not aware of them?
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