Floral fragrance in sweet osmanthus majorly repels pollen feeders

Why some flowers are so fragrant remains mysterious. The hypothesis of pollinator-mediated selection suggests that floral scent offers olfactory signals attracting pollen vectors, but it may be susceptible to pollen consumers. To explore the filtering hypothesis of floral scent that lures potential pollinators but repels pollinivores, here we provide the first report that thrips were the only effective pollinators for Osmanthus fragrans (Oleaceae), a famous fragrant tree whose flowers produce no nectar, although co-flowering species were pollinated by diverse insects, including honeybees, in both wild and cultivated populations. A series of experimental tests of ecological functions in floral scent demonstrate that two major aromatic compounds, β-ionone and linalool, both deter honeybee visits, but β-ionone attracts thrips. Honeybees obviously avoided O. fragrans, and ceased re-visiting Hibiscus mutabilis flowers that were artificially added linalool or β-ionone, illustrating a push-pull strategy using floral scent to recruit thrips pollinators but repel pollen feeders to reduce pollen consumption. The finding of β-ionone as an olfactory signal filtering pollen vectors highlights that the diversity of floral volatiles could also be selected by antagonists via pollen protection from pollinivore repellence.