Assessing chemical mechanisms underlying the effects of sunflower pollen on a gut pathogen in bumble bees

Many pollinator species are declining due to a variety of interacting
stressors including pathogens, sparking interest in understanding factors
that could mitigate these outcomes. Diet can affect host-pathogen
interactions by changing nutritional reserves or providing bioactive
secondary chemicals. Recent work found that sunflower pollen (Helianthus
annuus) dramatically reduced cell counts of the gut pathogen Crithidia
bombi in bumble bee workers (Bombus impatiens), but the mechanism
underlying this effect is unknown. Here we analyzed methanolic extracts of
sunflower pollen by LC-MS and identified triscoumaroyl spermidines as the
major secondary metabolite components, along with a flavonoid
quercetin-3-O-hexoside and a quercetin-3-O-(6-O-malonyl)-hexoside. We then
tested the effect of triscoumaroyl spermidine and rutin (as a proxy for
quercetin glycosides) on Crithidia infection in B. impatiens, compared to
buckwheat pollen (Fagopyrum esculentum) as a negative control and
sunflower pollen as a positive control. In addition, we tested the effect
of nine fatty acids from sunflower pollen individually and in combination
using similar methods. Although sunflower pollen consistently reduced
Crithidia relative to control pollen, none of the compounds we tested had
significant effects. In addition, diet treatments did not affect
mortality, or sucrose or pollen consumption. Thus, the mechanisms
underlying the medicinal effect of sunflower are still unknown; future
work could use bioactivity-guided fractionation to more efficiently target
compounds of interest, and explore non-chemical mechanisms. Ultimately,
identifying the mechanism underlying the effect of sunflower pollen on
pathogens will open up new avenues for managing bee health.