Floral shape predicts bee–parasite transmission potential

Abstract

The spread of parasites is one of the primary drivers of population decline of both managed and wild bees. Several bee parasites are transmitted by the shared use of flowers, turning floral resources into potential disease hotspots. However, we know little about how floral morphology and floral species identity affect different steps of the transmission process. Here, we used the gut parasite Crithidia bombi and its primary host, bumble bees (Bombus spp.), to examine whether floral traits or species identity better predict three basic steps of parasite transmission on flowers: feces deposition on flowers, survival of the parasite on flowers, and acquisition by a new host. We also identified which traits and/or species were most strongly associated with each step in the transmission process. We found that both trait‐ and species‐based models fit the data on deposition of feces and survival of C. bombi on flowers, but that species‐based models provided a better fit compared with trait‐based ones. However, trait‐based models were better at predicting the acquisition of C. bombi on flowers. Although different species tended to support higher fecal deposition or parasite survival, we found that floral shape provided explanatory power for each of the transmission steps. When we assessed overall transmission potential, floral shape had the largest explanatory effect, with wider, shorter flowers promoting higher transmission. Taken together, our results highlight the importance of flower species identity and floral traits in disease transmission dynamics of bee parasites, and floral shape as an important predictor of overall transmission potential. Identifying traits associated with transmission potential may help us create seed mix that presents lower parasite transmission risk for bees for use in pollinator habitat.

Urban habitat fragmentation and floral resources shape the occurrence of gut parasites in two bumblebee species

“…This result confirms the well-known role of flowers as hubs for parasites spread among individuals (Pinilla-Gallego et al, 2022). Indeed, about 10% of flowers were found to host one or more parasites of bees (Graystock et al, 2020) and shorter and wider flowers promote higher transmillability (Pinilla-Gallego et al, 2022). This could be a peculiarity of "poor-quality", fragmented landscapes and of disconnected urban green areas, where flowers occur in aggregated ways or cover specific areas, thus aggregating pollinators too and promoting infection, as higher bee pathogens are found in flower strips near isolated semi-natural patches (Piot et al, 2019).…”

“…In this context, both the probability of infection and the parasite richness shown by B. pascuorum were higher where more flowers were available for foraging. This result confirms the well-known role of flowers as hubs for parasites spread among individuals (Pinilla-Gallego et al, 2022). Indeed, about 10% of flowers were found to host one or more parasites of bees (Graystock et al, 2020) and shorter and wider flowers promote higher transmillability (Pinilla-Gallego et al, 2022).…”

Manipulation of multiple floral traits demonstrates role in pollinator disease transmission

“…Our manipulative results complement those of a new observational study which found that floral shape variation across 16 plant species affected several aspects of Crithidia transmission in B. impatiens . Short, wide flowers were more likely to have feces deposited and acquired, but pathogen survival was lower on these more exposed flowers; the net effect was that plant species with short and wide corollas were more likely to transmit pathogens (Pinilla‐Gallego et al, 2022). Both observational and manipulative approaches are important to demonstrate the potential for floral traits to influence pathogen dynamics at landscape levels.…”

“…Pathogen transmission via flowers can differ between plant species, but few specific traits have been identified that mediate transmission. A large observational study quantified fecal deposition, C. bombi survival, and pathogen acquisition by Bombus impatiens across 16 plant species, and found that plants with short, wide corollas were more likely to transmit pathogens (Pinilla‐Gallego et al, 2022). By contrast, a previous study that examined C. bombi acquisition rates from flowers of 14 plant species found no such relationship; the only trait that consistently correlated with acquisition was the number of reproductive structures per inflorescence (Adler et al, 2018).…”

“…For pathogens like Crithidia with fecal‐oral transmission, traits could alter transmission by affecting the location of contaminated feces. Plant species with longer or narrower corollas may be less likely to have fecal deposits in the flower than species with short, wide corollas (Bodden et al, 2019; Figueroa et al, 2019), although pathogens may desiccate more quickly in short, wide flowers (Pinilla‐Gallego et al, 2022). The arrangement of flowers in an inflorescence could affect transmission by changing how bees move (Durrer & Schmid‐Hempel, 1994), altering the likelihood of encountering feces.…”