Heather flavonoids and their impacts on bees

Flowering plants synthesize specialized metabolites, mainly to face biotic and abiotic stressors. These specialized metabolites also occur in floral resources, exposing bees to their effects and potentially impacting pollination interactions and bee fitness. We aimed to determine whether chemical defences are modulated through the plant tissues to maintain pollination interactions while reducing herbivore pressure. We first studied the flavonoid profile of vegetative parts and floral resources of heather (Calluna vulgaris) using liquid chromatography and tandem mass spectrometry. Then, we assessed their effects on bumble bees (Bombus terrestris) using microcolony bioassays. We identified 22 different flavonoids throughout the plant, which appeared highly tissue-specific. Pollen displayed a lower concentration in flavonoids than leaves, and no flavonoids were found in nectar, which may favour pollinator interactions while deterring other herbivores. To test this hypothesis, we fed bumble bee microcolonies with either willow pollen (control), willow pollen supplemented with leaf (herbivory reduction) or pollen flavonoid extract (pollination maintenance). Both flavonoid extracts limited syrup and pollen intakes, and lowered offspring production. Although mortality was not affected, flavonoids also increased fat body mass (detoxification organ), indicating potential deterrent and detrimental effect of flavonoids. These results provide insight into the role of specialized metabolites in plant-pollinator interactions.