Sunflower Pollen and Bumble Bee Health: Mechanisms, Modifiers and Trade-Offs

Bumble bees face increasing pressure from interacting stressors, including pathogens, nutritional limitations, and agricultural intensification. Among natural dietary factors that modulate disease, Asteraceae pollen-particularly sunflower (Helianthus annuus)-has repeatedly been shown to reduce infection by the trypanosomatid Crithidia bombi in bumble bees under laboratory conditions. Yet the mechanisms, generality, and ecological relevance of these effects remain incompletely resolved, and field-based evidence from European systems, particularly for Bombus terrestris, is scarce. Here, I synthesise current knowledge on how Asteraceae pollen traits influence bumble bee health, focusing on the interplay between pollen morphology, phenolamide chemistry, nutrient composition, gut microbiota, and host physiology. I evaluate evidence for three non-exclusive mechanistic pathways-mechanical abrasion, chemical activity, and microbiome-associated effects-and review emerging evidence for nutritional, immunological, and colony-level trade-offs associated with medicinal pollen. To place these mechanisms in a field-relevant context, I integrate pollen-trap data from B. terrestris and Apis mellifera colonies foraging in Central European agricultural landscapes, indicating strong seasonal reliance on Solanaceae pollen, no uptake of sunflower pollen by B. terrestris, and moderate use of Silphium perfoliatum, a perennial Asteraceae crop of growing agroecological interest. Together, these patterns highlight a mismatch between laboratory efficacy and field-level pollen use, indicating that sunflower pollen is unlikely to function as a standalone medicinal resource under realistic foraging conditions. Instead, potential health effects of Asteraceae pollen appear context dependent and embedded within diverse nutritional landscapes. I identify key knowledge gaps-including cultivar-level chemical variation, species-specific responses, and interactions with co-occurring stressors-and outline research priorities for evaluating when and how medicinal pollen may contribute to pollinator-supportive cropping systems.