Bee lice

[Neonach]

Previous fears about acarine mite seem to have been mistaken after all, so as yet I've not unsealed my pack of mentol crystals. However I do definitely have bee lice: in fact I first spotted these when I obtained the bees 2 years ago, but I was told that they were not a significant problem, and indeed all references I've checked have said the same. That said, I thought they'd actually died out last year, but at last inspection I didn't need to look too hard to find a few bees with the little brown dots on their thoraxes. Does anyone know if they transfer from other species of bee - eg Bumbles or whatever? And since it has been such a good year - no reason to doubt good bee nutrition levels and health - I'm wondering why they've made a resurgence, and if there's anything I need to do about it.