Hi,
 
I can't say what CO2 does for honeybees, but I know that it is
extensively used on Queen Bumblebees raised for pollination because
it's part of the process that it used to change the life cycle so that
there are plenty of workers very early in the year for tomatos etc.
under glass. If I remember correctly (maybe a breeder will confirm/deny)
a combination of CO2 and chilling is used rather as one would stratify
seeds before planting. I think the effect is to fool the Queens into
believing winter is over. Of course, Bumblebee Queens naturally
hibernate so maybe the combination is 'safe' for them?
 
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I have also been assured several times that varroa only live a few days
on dead bees ('definitely not more than 8', but I can't remember who
said it) I occurs to me that if the bees died out due to Tracheal Mite
infestation, then maybe some of the 'dead' bees could have been in that
strange 'comotose' state that they get into. This may be worth
investigating because if they really are living that long then we may
need to rethink some of our views on the mites.
 
A further possibility could be robbing bees bringing them in, but it
seems odd that the mites would alight in a dead colony.
 
Regards, Gordon.