Tom,
 
If the disease report indicates that greater than 30 per
cent of bees are infested then in my experience the bees
will eventually dwindle and die out.  Even if you try Folbex
(which should NOT be used anymore )at this late stage I
doubt if it would help much as all the young winter bees
have been reared up and are probably infested.  They will
probalbly survive the winter but come a nice sunny warmish
day at the end of Feb you will witness a massive amount of
crawling and the colony will eventually die out leaving a
small cluster of bees about the size of a tennis ball.
 
Wintergreen does work but only if you apply it at the end of
july or as soon as the supers are off, it needs the heat to
evaporate and diffuse through out the box.
 
Frow mixture is rather a nasty stuff which you want to keep
out of the food chain and basically should NOT be used
anymore.  It can be made up but your recipe, which I assume
is for original and not the modified version, is incorrect -
you need nitrobenzene instead of benzene and the chances of
getting your hands on that in this country are rather slim
!! I doubt it would work for reasons given above.
 
 
The only thing you can do at this time of the year is wait
and hope then requeen from clean/resistant stocks
 
Phil