Role of floral secondary compounds for pathogen transmission and establishment in pollinators

Background/Question/Methods

Although plant-herbivore and plant-pollinator interactions have often been studied separately, they are linked by a common theme, consumption of plants. Nectar and pollen are the primary food items of the majority of bees, and frequently contain plant secondary compounds as well as sugars and amino acids. Secondary compounds can directly reduce herbivore performance while providing indirect benefits by reducing predation or parasitism; however, the impacts of secondary compounds on bee pollinators are almost entirely unknown. Parasites are strongly implicated in the decline of both managed and wild bees, but the impacts of secondary compounds in mediating plant-bee-parasite interactions remain an almost unexplored frontier. We infected Bombus impatiens bumble bees with the common trypanosome pathogen Crithidia bombi and then assigned bees to treatments consuming one of eight secondary compounds at naturally-occurring nectar concentrations, compared to a sucrose control solution without secondary compounds. Additional research is examining the effect of exposing Crithidia to four different secondary compounds pre-infection, which occurs when Crithidia are deposited in flowers and then consumed by uninfected bees. 

Results/Conclusions

Consumption of four of the eight secondary compounds post-infection, including alkaloids, terpenoids, and iridoid glycosides, significantly reduced Crithidia loads, with a reduction of up to 81% compared to the control treatment. By comparison, exposing Crithidia cells to secondary compounds prior to infection had relatively little effect on subsequent pathogen loads. This suggests that the mechanism by which secondary compounds reduce pathogen load is not through direct toxicity to the pathogen. Current and future work is examining other mechanisms underlying this result, and addressing how floral traits mediate transmission dynamics in the field. Considering the role of floral defenses as mediators of a broad range of interactions will provide a more in-depth understanding of how such traits evolve, and their consequences for population dynamics of both plants and insects.