I know it's been mentioned before, but with all the current talk about Varroa,
I haven't heard anyone talk much about inter-relationships between Varroa and
bee viruses.  As I was looking at Andy Nachbaur's web site, he has a link to
a UK site where research on these relationships is presented.  It is at
http://www.res.bbsrc.ac.uk/entnem/research/chdpage1.htm
The overview information I got from reading it is that inapparent viral
infections can be activated by chemicals injected into the bees' blood when
the mites feed on them and can result in a fatal infection within 5 days.  The
virus can also be transmitted between bees by the mites feeding on them.  It
appears that if your bees are a "carrier" of particular viruses that may not
have any apparent affect on them, when you add Varroa it becomes a fatal mix.
Of course, the higher the population of mites, the more rapidly the epidemic
would spread.  So it appears that you have a large combination of factors at
work, including a fatal viral epidemic as well as weakened bees with a shorter
life expectancy, so that at least in some cases, the demise of a hive can be
quite dramatic and due to multiple cases, each exacerbating the others.
 
I've heard bee viruses mentioned in conjuction with Varroa, but not recently.
I thought this might be a good time to mention it again.  I don't know any of
this from first hand experience, just from what I've read and thought about.
I have Varroa, I have lost hives, I'm still trying to figure it out through
studying and experience, but my "visit" to the above Web site helped several
things "fall into place" in my mind--like when to treat and why it's important
to treat at those times.
 
Layne Westover
College Station, Texas