The viral landscape of honey bees (Apis mellifera) has changed dramatically since the emergence of the ectoparasitic mite, Varroa destructor. The global spread of Varroa caused decreased honey bee health and increased colony losses throughout the beekeeping world. It is now clear that the pathology caused by mites is largely due to the spread of virulent viruses that Varroa harbours and transmits to bees upon feeding. In particular, the spread of one RNA virus, Deformed Wing virus (DWV) has been described as a global pandemic: DWV titres increase dramatically upon infestation with mites. However, multiple DWV strains exist, and specific strains are apparently associated with increased virulence. Furthermore, recombination between DWV strains frequently occurs, generating novel viral haplotypes.
In most cases, chemical miticides are necessary to prevent colony loss. However, multiple honey bee populations have naturally evolved or been selected for Varroa-resistance. While these populations can withstand Varroa infestation, it is unclear whether the bees also differ in their response to viruses. Therefore, we examined the viral landscape of Varroa-resistant honey bee populations from Europe, Africa and the Pacific. We find that DWV titres can be extraordinarily high, similar to levels found in collapsing, Varroa-sensitive colonies. However, we see multiple instances of similar DWV recombinant strains that have occurred in different populations of Varroa-resistant bees. These results indicate that the relationship between honeybees, Varroa mites and DWV is constantly evolving, and subtle differences in viral genomes could alter the balance between colony survival and mortality.