Good point about "not needing treatment" being influenced by individual
beekeeper perceptions and expectations. But I still think the regional
contrasts exist. There are good beekeepers (which requires good
observation skills) in all those countries. I doubt they would all be
following some guideline without some reality checks. People who have
reported these contrasts to me are in the know of what goes on in the
field.
I suspect Africanized bees in Brazil went through serious natural
selection to acquire resistance to varroa as it spread through the
country after the introduction to Paraguay. The advancing front of
Africanized bees was far away from there and moved through varroa free
environments pretty much until they hit Mexico. So there could be
Africanized bees with different levels of resistance.
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