Flower fragrances not only act through the air, but also as flavours in nectar. This was discovered by a German-US research team led by Ulm University. The scientists were able to show that impatiens flowers lead bumblebees to their nectar using a sophisticated chemical road map. This strategy also prevents the insects from simply stealing the nectar without doing their job as pollinators. The results of the study were published in the prestigious journal Current Biology.
When bees, including bumblebees, and other insects buzz through gardens and parks in early spring, they follow a sophisticated chemical guidance system: to attract pollinators and show them the way to pollen and nectar, flowers release volatile organic compounds as scents, not only into the air, but also into the nectar: bees and bumblebees can not only smell, but also taste them thanks to numerous sensory cells on their mouthparts. This dual function has hardly been scientifically investigated to date. An international team led by Ulm researchers Kim Heuel, Dr Hannah Burger and Professor Manfred Ayasse has now been able to show that odorants are spatially distributed in plant species with complex flowers and that some volatile organic compounds are only found in nectar.
"We were able to show for the first time that plants create a kind of chemosensory map in their flowers," explains first author Kim Heuel. "Certain odorants - some for smelling, others for tasting - are found exclusively in nectar and guide pollinators to their destination like a navigation system." The scientists studied North American impatiens species with a curved nectar spur and their typical pollinators, the bumblebee species Bombus impatiens. While some bumblebees crawl deep into the flower to reach the nectar and pollinate it at the same time, others take a shortcut: they bite a hole in the nectar spur from the outside. This turns them into "nectar thieves", as they don't pollinate the flowers in the process.
Anti-theft device protects against nectar thieves
However, impatiens have a kind of anti-theft device built in. In elaborate behavioural experiments with inexperienced bumblebees exploring flowers for the first time, the researchers discovered that the insects react differently to the volatile organic compounds - depending on whether they perceive them as scents or flavours. "Typical nectar fragrances such as vanillin tasted good to the bumblebees, while they tended to avoid fragrances from other parts of the flower such as the nectar spur," says Dr Hannah Burger from the Institute of Evolutionary Ecology and Conversation Genomics at Ulm University, who coordinated the study. "This spatial distribution of volatile organic compounds promotes pollination and deters nectar thieves with unpleasant-tasting substances in the flower wall," explains Dr Burger. "The plants have developed a kind of chemical bouncer."
The significance of the discovery goes beyond the species studied. The results show that floral odorants also play an important role as flavour signals in the communication between plants and their pollinators. "Until now, science has underestimated the role of scents as taste signals," emphasises Professor Manfred Ayasse, a bee expert from Ulm and co-coordinator of the study. "The findings open up completely new fields of research in the sensory and behavioural biology of pollinators and non-pollinating counterparts."
The study was conducted as part of Kim Heuel's master's thesis at the Institute of Evolutionary Ecology and Conversation Genomics at Ulm University. She analysed the flower scents together with Professor Robert Raguso at Cornell University, USA. Professor Robert Gegear from the University of Massachusetts contributed his expertise in the behavioural ecology of pollinators. For the interdisciplinary investigations, the team used state-of-the-art analytical methods to identify volatile organic substances.
The project was funded by the mentorship programme of the Zukunftskolleg at the University of Konstanz and by PROMOS scholarships; (Erasmus) grant from the German Academic Exchange Service (DAAD).
Publication reference:
Kim C. Heuel, Robert A. Raguso, Emma Coogan, River Mallick, Kirsten J. Keleher, Manfred Ayasse, Robert J. Gegear, Hannah Burger: Spatial partitioning of floral volatiles provides a "chemosensory roadmap" for bumblebee pollinators. Current Biology, 2025, ISSN 0960-9822
https://doi.org/10.1016/j.cub.2025.02.010
Further information:
Dr Hannah Burger, Institute of Evolutionary Ecology and Conversation Genomics, Mail: info(at)hannah-burger.de
Text and media contact: Christine Liebhardt