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.