Landscape structure influences viral infection patterns
Our data show that colonies located in habitats with more floral
resources, (e.g., agricultural or residential areas), generally have
lower viral load and richness. This aligns with previous research that
has demonstrated a connection between higher amounts of floral resources
and reduced pathogen loads in pollinators . Moreover, colonies in areas
with low isolation of agricultural patches had lower total viral load
and colonies in more heterogeneous (i.e., diverse) areas had a lower
number of different viruses as well as a lower probability to be
infected with new viruses. Agricultural patches, mainly consisting of
vegetated vineyards, orchards and grasslands, along with their field
edges, provide suitable floral resources . This suggests that high
habitat diversity might result in a higher chance of finding suitable
and diverse floral resources, providing resource complementation .
Consequently, habitat heterogeneity might reduce virus transmission via
three non-mutually exclusive mechanisms shaping fitness and foraging
patterns of bees: (i) High floral resource availability and resource
complementarity due to landscape heterogeneity increase general colony
performance and health and thereby decrease their susceptibility to
pathogens (Roger et al. 2017), (ii) high floral abundance within a
landscape may decrease the contact between pollinators simply because
pollinators are not concentrated on the few available flower patches
(dilution/amplification effect) and (iii) high habitat/floral diversity
can modify a species diet breath with consequences on virus transmission
.
We further show that colonies located in areas with a higher cover of
forests were infected with more viruses (higher virus richness) and
showed increased virus turnover and appearance of new viruses. B.
terrestris generally prefer open habitats. Consequently, forests do not
provide suitable foraging grounds and may even constitute landscape
barriers to foraging bumble bees . Thus, a higher forest proportion in
the landscape may reduce foraging resources and lead to lower colony
fitness through poorer nutrition and/or higher viral spillover rates as
foraging bees are concentrated in the remaining suitable patches .
Overall, the landscape structure effects operated at rather fine scales,
ranging from 100 up to a 400 meter radius around the bumble bee
colonies, indicating that the habitat structure of the immediate
surroundings is more important than meso- or large-scale conditions .
Even though B. terrestris have been shown to forage over long distances
their main foraging activity occurs within 70-600 m around the colonies
supporting the findings in this study.