Echium and Pontechium specific floral cues for host–plant recognition by the oligolectic bee Hoplitis adunca

Flowering plants often have specific floral cues, which allow bees and other pollinators to differentiate between them. Many bee species exhibit specialised associations with flowers (oligolecty) and it is important for them to find and recognise their specific host plants. In this study we compared the visual and olfactory floral cues of different Echium and Pontechium (Boraginaceae) species with the closely related Anchusa officinalis (Boraginaceae). We tested whether plant-specific cues occur in Echium and Pontechium which may allow oligolectic Hoplitis adunca (Megachilidae) to recognise its host plants and to distinguish them from Anchusa non-hosts. Our investigations showed that Echium/Pontechium provides a specific scent bouquet. Furthermore, we identified compounds which were not described as floral scent before ((Z)-3-nonenal and 1,4-benzoquinone). These unique volatiles and the specific bouquet could act as a recognition cue for H. adunca. The corolla colours differed between all species, but were grouped together in the bee colour categories blue and UV-blue and can indicate potential host flowers for H. adunca.

Research highlights
► Echium and Pontechium flowers provide unique volatiles and a specific bouquet.
► Floral scent can act as a recognition cue for oligolectic Hoplitis adunca bees.
► Corollas are coloured bee-blue or bee-UV-blue.
► Colour cues indicate potential host flowers for H. adunca.