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.
KeywordsChelostoma Campanula Floral scents Host location/recognition Oligolectic bees Spiroacetals
We thank Hannah Burger and Irmgard Schäffler for methodological support and constructive discussions, Hans-Joachim Flügel (Lebendiges Bienenmuseum Knüllwald) and Jochen Fründ (University of Göttingen, Germany) for providing nests of Chelostoma rapunculi, Gregor Aas for use of a greenhouse in the Ecological Botanical Garden of the University of Bayreuth for flight cage experiments, and the team from the Botanical Garden of the University of Ulm for providing Campanula fields to rear Chelostoma rapunculi bees used in our experiments. We also are thankful to Heather Carew, Jana Seaman, Michael Peterson, Debrakaye Nelson, Jennifer Telfer, Jamie Williamson, and Robert Davey for conducting multiple-choice behavioral experiments, the Station Linné in Sweden for its research facilities, and Whitman College for faculty-student Research funding, including two Perry Awards. Grants were provided to PM-P (Deutscher Akademischer Austauschdienst - A/08/71732) and to CS (Conselho Nacional de Desenvolvimento Científico e Tecnológico - 305692/2009-7 and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - BEX/4236/08-4).
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