Pretty interesting paper on how mites vector DWV and collapse colonies. A little technical but the takeaway is that you need to control varroa.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3375299/

Through an integrated analysis of the population and molecular changes associated with the collapse of honeybee colonies infested by the parasitic mite Varroa destructor, we show that this parasite can de-stabilise the within-host dynamics of Deformed wing virus (DWV), transforming a cryptic and vertically transmitted virus into a rapidly replicating killer, which attains lethal levels late in the season. The de-stabilisation of DWV infection is associated with an immunosuppression syndrome