Sometime in the 20^th century, the Varroa mite switched hosts from the Eastern to the Western honeybee, with disastrous consequences. Varroa's spread into American and European bee populations was accompanied by outbreaks of viral disease, which often lead to colony collapse. Intriguingly, there are no viruses unique to Varroa-infested colonies. Instead, the mite's presence appears to drive the evolution of higher levels of virulence in ordinarily benign pathogens, which Varroa carries between bees as it feeds on their haemolymph.  These observations match the theoretical prediction that the emergence of an arthropod vector should select for higher levels of virulence in a parasite, by reducing the parasite's dependence on the host for transmission.

Varroa has not yet arrived in Australia, but the majority of its associated viruses are present in benign form. Whether the arrival of the mite will foment outbreaks of virulent diseases remains an open question of considerable interest. Moreover, the theoretical prediction that vector-based transmission selects for higher levels of virulence lacks solid empirical support. Australia's status as the last remaining large Varroa­-free landmass affords a unique opportunity to answer these questions. We are currently using a serial injection procedure to manually transfer benign Australian virus strains between bees. We hope to imitate the selection pressures exerted by vector-based transmission and track the resulting evolution of virulence, to better understand how the introduction of a vector can change the viral landscape of Australian honey bees.