The bees are my (now estranged) husband's project, so I probably shouldn't be saying 'we.' And I am speaking as a hobbiest, not a knowledgeable expert. I don't know what commercial honey producers are doing now regarding mite control. But as I understand it, there is, or was, virtually no immunity to the mites among the US bee population and integrated pest management methods would not have worked before the entire population was gone. I know we did not have one bee survive. The mites inhabit 'gills' on the bees. the old world strains that are not effected by the mites either don't have the gills, or the gills are conformed differently so that the mites can't inhabit them as readily.