Effect of peak bloom timing on the transmission of the bumble bee pathogen Crithidia bombi (Trypanosomatida: Trypanosomatidae).

Pathogen spread is a major driver of wild and managed bee decline, with shared floral resources often serving as transmission hubs. However, the abundance of floral resources changes throughout the season, potentially altering bee visitation rate per flower. This could affect pathogen transmission by influencing both the likelihood of pathogen deposition on flowers and of new hosts encountering pathogens. We assessed the effect of peak bloom timing on transmission of a bumble bee (Bombus impatiens Cresson) (Hymenoptera: Apidae) pathogen (Crithidia bombi Lipa & Triggiani) in a 2-wk tent experiment. We manipulated floral abundance using Monarda fistulosa L to provide more flowers either in the first week (early peak bloom) or second week (late peak bloom). Each tent received 1 infected (donor) and 1 uninfected (recipient) microcolony. We recorded floral visitation, and after 2 wk, assessed infection prevalence and intensity in both microcolonies. We found that peak bloom timing affected donor bee infection, but not transmission to recipient bees. Donor infection intensity was lower when bees had more flowers available early than late in the experiment. However, peak bloom timing did not affect the probability of infection in donor or recipient microcolonies, or the intensity of infection of recipient microcolonies. Furthermore, donor probability of infection and recipient infection intensity were positively related to inflorescence visitation rates in the second week. These results suggest that peak bloom timing could influence bee-pathogen dynamics by altering visitation rates per flower, highlighting the importance of phenological changes in plant-pollinator interactions for pollinator health.