Does a Nectarless Orchid Smell Like Its Neighbors?

Some orchids attract insects and have them transport pollen without providing food such as nectar. Does their scent resemble nearby rewarding flowers, or do they emit compounds that broadly suggest flowers?

In 2025, Corinna Etl and colleagues compared the scent of the lady’s-slipper orchid Cypripedium calceolus, with its yellow pouch-shaped flowers, with plants flowering at the same place and time. The aim was to investigate how a rewardless orchid attracts insects by examining shared scent compounds and their proportions.

Imitating one flower or signaling flowers more generally?

The lady’s-slipper flower has a yellow, shoe-shaped part. Small insects entering it touch the reproductive structures as they pass through an exit at the back, transporting pollen. Yet the flower supplies none of the rewards, such as nectar, that they expect.

Several ideas explain how such flowers attract insects. One is that they display colors and scents widespread among rewarding flowers. Another is close resemblance to one particular rewarding plant. An intermediate possibility is partial resemblance to several nearby flowering plants.

To consider where this orchid fits, the researchers examined scent compounds shared with surrounding flowers, rather than flower shape alone. This is not a claim that plants consciously imitate: it is an evolutionary question about which traits may be useful in their relationships with insects.

Comparison with neighboring plants in one German locality

The study site was on the shore of Lake Königssee in the Bavarian Alps, Germany. The researchers selected eight rewarding plant species flowering within ten meters of orchids, known to be visited by insects visiting the orchid or their close relatives.

For the orchid, they used existing scent data collected from 14 individuals in 2014. Neighboring plants were sampled mainly in 2023, with some samples from 2015. The same collection and analytical methods were used, but this was not a comparison in which everything was measured side by side in the same year.

Flowers or inflorescences were enclosed in bags, and volatile compounds captured from the airflow were analyzed by gas chromatography–mass spectrometry. Empty bags and leaves were also sampled as controls to identify flower-specific compounds.

No flower-specific compounds were detected in two of the eight neighboring species. One sample from another species also yielded none, and these were excluded. The final statistical comparison comprised 14 orchids and 31 individuals across six rewarding species. Failure to detect compounds does not establish that those plants have no scent at all.

Twenty of the orchid’s 67 compounds were shared

Across samples from the 14 orchids, 67 compounds were detected. Twenty, about 30%, were shared with at least one of the six neighboring species. This does not mean that every individual flower necessarily emitted all 67.

The orchid shared between one and seven compounds with each of the six species. Comparing pairs, the median number shared between the orchid and its neighbors was 3.5, versus zero for pairs of neighboring species. Pairs involving the orchid shared significantly more compounds.

In other words, the orchid’s scent contained compounds connecting it with several surrounding plants. However, the orchid also had a large detected compound repertoire overall. The number of shared compounds alone does not establish that insects perceive the scents as the same or that mimicry evolved.

Shared compounds do not imply identical proportions

For example, linalool was a major orchid component, averaging about 30%, but constituted about 1% in a neighboring legume sharing it. Conversely, some major constituents of surrounding flowers occurred only in small amounts in the orchid.

Taking these differences into account, the researchers compared not only compound presence or absence but also each compound’s proportion of the total. The orchid’s scent composition differed statistically from that of every one of the six comparison species.

The results therefore do not show that this orchid reproduces one particular flower’s scent exactly. Chemical analysis instead revealed a distinctive blend with components shared with several flowers.

Widespread compounds do not tell the whole story

The 20 shared compounds included substances such as benzaldehyde and benzyl alcohol that occur in flowers of many plants. This is consistent with using scents broadly associated with rewarding flowers rather than mimicking one particular species.

However, 14 shared compounds were not among the very widespread floral compounds used by the authors for comparison. These included compounds such as p-cresol and indole that invite consideration of relationships with local plants.

The authors discuss the possibility that partial resemblance to several neighboring floral scents may help attract insects alongside more general floral signals. Earlier findings of regional differences in compounds also prompted consideration of local plant–insect relationships. However, local adaptation was not tested experimentally.

Another hypothesis: resemblance to insect odors

The orchid contained 20 aliphatic compounds not found in the six surrounding species. These included octyl acetate and decyl acetate, which made substantial contributions to its scent.

Some of these compounds are also known from secretions of Andrena bees that visit the orchid. The authors therefore suggest investigating resemblance not only to rewarding flowers but also to odors used in communication between insects.

However, Andrena bees are not the main pollinators at this site. The same compounds are not known to function as pheromones in the halictid bees that predominate there. It is important that this study did not demonstrate sexual attraction of insects.

Antennal detection does not necessarily mean flower visitation

Earlier research showed that eight of the 20 shared compounds elicited antennal responses in bees or hoverflies carrying this orchid’s pollen. This suggests that differences in compounds are not necessarily irrelevant to insects.

But detecting an odor with the antennae is different from being attracted to it. This study did not test whether changing compounds altered visitation or whether insects mistook the orchid for a rewarding flower. How shared scent components translate into behavior remains a question for future work.

The comparison was also limited to one locality, with different sampling years. Neighboring plants were selected using information about their visitors, not by surveying every local plant. Even for the same orchid, the significance of shared compounds could change with habitat or visiting insects.

The research was funded by the Austrian Science Fund. The authors declared no commercial or financial relationships that could represent a potential conflict of interest.

Summary

The rewardless lady’s-slipper orchid shared scent compounds with all six surrounding species examined. Taking blend proportions into account, however, it retained a distinctive scent composition different from every one of them.

The authors propose a mechanism extending beyond an exact copy of one flower. Widespread floral odors, similarities to several nearby flowers, and possibly similarities to insect-derived odors may overlap. Which elements matter could vary with local flowers and insects. Comparing compounds has laid out a path for testing these possibilities in behavioral experiments.

References

Etl, C., Etl, F., Guilhot, R., Braunschmid, H., Gross, K., & Dötterl, S. (2025). Deceptive Cypripedium calceolus shares more floral scent compounds with co-flowering rewarding species than those species share among each other. Frontiers in Plant Science, 16, 1627890. DOI: 10.3389/fpls.2025.1627890. Full text. © 2025 Etl, Etl, Guilhot, Braunschmid, Gross and Dötterl. The original article is published under CC BY.