Due to mounting concerns about pollinator declines, there has been a recent surge of interest in assessing factors that affect bee populations, including pathogens. Although pathogen transmission between bees from different species or colonies is thought to occur during flower foraging, we know remarkably little about the role or consequences of floral traits for disease transmission. We assessed the role of plant species identity and floral traits for transmission of Crithidia, a common gut trypanosomatid, to Bombus impatiens, the common eastern bumble bee, and compared the ability of plant species identity and floral traits to explain variation in disease transmission. We then manipulated key floral traits, such as floral number and nectar chemistry, and assessed their causal role in disease transmission. Finally, we assessed the consequences of plant species variation in disease transmission for colony health and disease load using tent microcosms in which we manipulated the presence and composition of co-flowering ‘hedgerow’ species with canola, Brassica rapa, as a focal crop.
There was a fourfold difference in Crithidia transmission to bumble bee hosts. Transmission was low in species including Digitalis purpurea, Linaria vulgaris, and Thymus vulgaris, and highest in Asclepias incarnata. Although we assessed a wide range of floral morphological and chemical traits, the only floral trait that consistently predicted transmission was the number of reproductive structures per inflorescence. Furthermore, traits alone had nearly as much predictive power for transmission rate as did plant species identity, and so a trait-based model had a lower AIC due to greater simplicity. Manipulative experiments showed that the number of open flowers per inflorescence increased pathogen transfer in one plant species but not two others. Finally, choosing high-transmission plant species doubled colony pathogen loads relative to low-transmission plant species, with intermediate pathogen levels without a hedgerow, but colony performance was improved with either hedgerow compared to no hedgerow. Taken together, this work demonstrates the importance of floral traits and species variation in shaping bee-pathogen transmission dynamics.