An arthropod deterrent attracts specialised bees to their host plants

Many bee species are adapted to just a few specific plants in order to collect pollen (oligolecty). To reproduce successfully, it is important for oligolectic bees to find and recognise the specific host flowers. In this study, we investigated the role of floral volatiles used by an oligolectic bee to recognise its host plants. We compared the attractiveness of natural and synthetic scent samples of host flowers to foraging-naive and -experienced Hoplitis adunca (Megachilidae) bees that are specialised on Echium and Pontechium (Boraginaceae) plants. The investigations showed that naive H. adunca females are attracted to 1,4-benzoquinone. During their lifetime, bees learn additional floral cues while foraging on host flowers. In contrast to naive ones, experienced H. adunca females use, in addition to 1,4-benzoquinone, other compounds to recognise their host plants. 1,4-Benzoquinone is an uncommon floral compound only known from the host plants of H. adunca, and is therefore ideally suited to be used as a plant-specific recognition cue. Several arthropods use this compound to deter insect predators. Therefore, 1,4-benzoquinone as an attractant in Echium flowers may have evolved from a primary function as a defensive compound against insect herbivores.