Nectar and pollen are not the only things collected from flowers. Females of some bees collect fatty oils secreted by flowers, using them as larval food or material for lining brood cells. What odors help these bees find oil-producing flowers?
A study published by Schäffler and colleagues in 2015 addressed this question by combining floral chemical analyses, electrical responses of antennae, and choice behavior in flight cages. A volatile compound called diacetin emerged as central.
A shared compound found even in far-apart flowers
The researchers examined 58 plant species from different regions and evolutionary lineages. Fifty secreted floral oils; eight non-oil-producing species served as comparisons. They analyzed 106 samples obtained by briefly immersing flowers in solvent and also used leaf extracts as controls. Detection here therefore mainly concerns floral extracts, not direct measurements of the amounts each plant releases into the air.
First, they separated the floral compounds of four oil-producing species and looked for compounds that elicited antennal responses in the European oil-collecting bee Macropis fulvipes. Diacetin was present in all four and elicited antennal responses.
Across the wider plant comparison, diacetin was detected in 41 of the 50 oil-producing species, or 82%, but in only one of the eight non-oil-producing species. It is a cue shared by many oil-producing flowers, not a perfect marker applying to every flower.
Different bees show different antennal responses to the same odor
Next, the researchers compared antennal responses to different diacetin concentrations in four bee species: five oil-collecting Macropis fulvipes and six South African Rediviva neliana, alongside five closely related, non-oil-collecting Melitta haemorrhoidalis and nine western honeybees.
In the two oil-collecting species, antennal responses increased with concentration. At the highest concentration, responses were clearly stronger than those to solvent-only controls. In the two non-oil-collecting species, differences from controls were not statistically clear under those conditions.
The authors interpret this difference as evidence that the olfactory systems of oil-collecting bees are adapted to detect diacetin. However, the experiments did not directly identify the molecular functions of receptors or other components. Nor does an antennal response necessarily mean a bee will fly toward the odor. The researchers therefore also examined behavior.
Diacetin attracts bees on its own, but the blend matters
Behavioral experiments used female Macropis fulvipes without experience of the target Lysimachia flowers. Without prior training or rewards after a choice, the researchers recorded which of two odors the bees approached. The aim was to identify flower-finding cues that could not be explained solely by learned preferences.
Diacetin presented alone attracted more bees than solvent alone. However, it was less attractive than a floral extract of Lysimachia punctata.
The researchers therefore made a synthetic blend of diacetin and four other compounds found in the flowers. Approach choices did not differ clearly between this five-compound blend and the natural floral extract. Removing heptanoic acid or (E)-2-dodecenal individually made the blend less likely to be chosen than the complete blend. The combination, not just one leading compound, therefore contributed to attraction.
When diacetin was omitted, bees approached only the complete blend. However, this comparison also omitted geranic acid because that ingredient contained traces of diacetin. The authors judged diacetin to be important in light of this result and the absence of a clear effect when geranic acid alone was omitted.
What was measured here was approach to an odor, not pollen transport or plant reproductive success. The quantities presented were also adjusted to floral extracts under experimental conditions. How far the same effect extends in outdoor air is a separate question.
Why can the odor indicate where oil is available?
Diacetin resembles some constituents of floral oils in chemical structure. The authors proposed that some metabolic pathways or enzymes producing them might be shared. If oil production and odor generation are linked, the odor could provide bees with a reliable cue associated with an actual resource.
However, the study did not directly demonstrate shared biosynthetic pathways. The evolutionary scenario in which a by-product of oil production later became a marker used by bees also remains a hypothesis.
The researchers interpreted this relationship as a private communication channel between particular flowers and pollinators: specialists’ antennae responded strongly to a generally uncommon compound. Many untested bees might nevertheless also detect diacetin. It would be too strong to call it a code absolutely inaccessible to any bee that does not collect oil.
A common marker and differences between flowers
In the authors’ interpretation, diacetin could help bees find oil-producing flowers broadly, while combinations of other compounds could help distinguish plants flowering at the same time. If bees repeatedly visit flowers of one species, pollen is more likely to reach another flower of that species. This division of roles is an ecological interpretation derived from the chemical analyses and behavioral results.
Finding diacetin in plants from distant regions and different lineages also suggests that oil-associated odors may have evolved repeatedly. This connects with the idea of coevolution, in which plant chemistry and bee senses become matched through their interactions, although the study did not directly reconstruct that history.
Summary
Diacetin occurred in many oil-producing flowers, stimulated the antennae of the oil-collecting bees examined, and attracted one of those species toward its odor. The study identified a concrete cue connecting floral chemistry with bee behavior.
The important point is not simply that strong odors attract insects. Bees needing floral oil detect a compound potentially associated with that resource, while differences between blends may help them distinguish flowers. Scent emerges as information linking resources, senses, and evolution.
References
Schäffler, I., Steiner, K. E., Haid, M., van Berkel, S. S., Gerlach, G., Johnson, S. D., Wessjohann, L., & Dötterl, S. (2015). Diacetin, a reliable cue and private communication channel in a specialized pollination system. Scientific Reports, 5, 12779. DOI: 10.1038/srep12779 / Full text. Original article license: CC BY 4.0.
