The chemical basis of host-plant recognition in a specialized bee pollinator

Many pollinators specialize on a few plants as food sources and rely on flower scents to recognize their hosts. However, the specific compounds mediating this recognition are mostly unknown. We investigated the chemical basis of host location/recognition in the Campanula-specialist bee Chelostoma rapunculi using chemical, electrophysiological, and behavioral approaches. Our findings show that Ca. trachelium flowers emit a weak scent consisting of both widespread and rare (i.e., spiroacetals) volatiles. In electroantennographic analyses, the antennae of bees responded to aliphatics, terpenes, aromatics, and spiroacetals; however, the bioassays revealed a more complex response picture. Spiroacetals attracted host-naive bees, whereas spiroacetals together with aliphatics and terpenes were used for host finding by host-experienced bees. On the intrafloral level, different flower parts of Ca. trachelium showed differences in the absolute and relative amounts of scent, including spiroacetals. Scent from pollen-presenting flower parts elicited more feeding responses in host-naive bees as compared to a scentless control, whereas host-experienced bees responded more to the nectar-presenting parts. Our study demonstrates the occurrence of learning (i.e., change in the bee's innate chemical search-image) after bees gain foraging experience on host flowers. We conclude that highly specific floral volatiles play a key role in host-flower recognition by this pollen-specialist bee, and discuss our findings into the broader context of host-recognition in oligolectic bees.