Bumble bees and honey bees on islands harbour reduced viral species richness, yet honey bee populations are dominated by a deformed wing virus recombinant

Pollinators harbour diverse RNA viromes that play a vital role in their
health. Yet, factors that shape viral communities are often unclear. The
European honey bee (Apis mellifera) is experiencing a viral epidemic since
the emergence of the parasitic mite Varroa destructor (varroa) introduced
vector-borne transmission, which has also been linked to increased viral
spillover into wild pollinator communities. Varroa-free island populations
provide natural laboratories to study the effect of varroa, while also
allowing us to ask how islands affect viral communities. Barriers that
restrict the dispersal of wild pollinators and their pathogens to islands
may be overcome by human-mediated transport in managed honey bees. Here we
used islands with and without varroa and matched mainland populations of
honey bees (A. mellifera) and bumble bees (Bombus terrestris) from 2015
and 2021 to explore how varroa presence and island location affect the
virome of managed and wild bees. We find lower viral richness on islands
in both species. Bumble bees harbour a distinct viral community that was
not affected by varroa but geographically structured. In honey bees,
however, varroa-present populations contained more viral reads driven by a
high abundance of deformed wing virus (DWV). Within the six years between
the sampling events, DWV underwent a shift from mostly DWV-B towards a mix
of DWV-B and recombinant strains. Surprisingly, these shifts appeared
independent of varroa. Viewing pollinator virome composition within an
ecological framework provides valuable insights into the barriers to virus
spread and could help to predict drivers of disease emergence.