The dynamics of virus epidemics in Varroa-infested honey bee colonies

1. When the parasitic mite Varroa jumped species from the eastern to the western honey bee, millions of infested bee colonies died. Recent work has revealed that the probable cause of this collapse is that mites provide a new route of transmission, by acting as a vector, for certain bee viruses. 2. Using a mathematical model parameterized by recently collected data on bee viruses, we investigate the relationship between the mite load in a colony and the possibility of a virus epidemic occurring within a bee colony. 3. The model suggests that the balance of coexistence between mite, virus and bee in the eastern honey bee, has been lost in the western bee host, not simply because of the new transmission route, but also because mite populations in western honey bee colonies has exceeded a critical epidemic threshold. We quantify the critical epidemic mite load for two well-studied bee viruses, acute paralysis virus and deformed wing virus, through the colony's yearly life cycle. 4. As well as providing practical insights into mite control strategies, the model allows us to disentangle the relative importance of different bee and mite behaviours in virus spread. We consider the evolutionary aspects of the new route of virus transmission, looking in particular at how changes to social organization might bring about collective resistance.