Fig 1. Schematic of experimental designs.

In a series of four experiments, we examined virus deposition on flowers by honey bees and/or virus transmission between honey bees and bumble bees. To examine the effect of plant species and/or plant diversity, flowering plant species were provided to foraging bees as either single plant species (A) or in diverse arrays consisting of all three species (B). To test whether chronic exposure to contaminated flowers is necessary for virus transmission, bumble bee microcolonies were exposed three times to honey bee-visited flowers over the course of three days (C). To test if direct contact or comingling is necessary for viral transmission, honey bees and bumble bees were allowed to forage together in tent enclosures (D). Blue boxes in the schematic represent tent enclosures assigned as the honey bee tent (where infected honey bees were allowed to forage on flowers), the exposed bumble bee tent (where plants exposed to honey bees were transported into three hoop houses to be foraged on by bumble bee microcolonies), and the control bumble bee tent (where bumble bee microcolonies foraged on ‘clean’ plants brought directly from the greenhouse). Red semi-circles represent hoop houses within bumble bee tents, each containing a single bumble bee microcolony. Green arrows represent the movement of plants from the honey bee tent to the exposed bumble bee tent after a 15 hour nectar regeneration period. In the chronic experiment, the same three bumble bee colonies were used on each of three days (depicted by red arrows connecting the hoop house through time). Three plant species were used throughout the series of experiments: Trifolium repens, T. pratense, and Lotus corniculatus. Photos of inflorescences and tent enclosures (with hoop houses) are provided for visualization (E).