How Is Floral Scent Diversity Related to Insects and Bacteria?

Insects visiting flowers carry microbes as well as pollen. Flowers that welcome a variety of insects may receive an equally diverse range of microbes. Could floral scent therefore be involved both in attracting visitors and in regulating the microbes that live on flowers?

A study published by Hanusch and colleagues in 2025 compared the chemical diversity of floral scent with the richness of flower-visiting insects and bacteria across 39 plant species in the Austrian Alps. It found contrasting associations: more chemically diverse scents were linked to a greater variety of insect visitors but a smaller variety of bacteria on flowers.

Scent “Diversity” Is More Than the Number of Compounds

Diversity here is different from scent intensity or a human impression of complexity. The researchers evaluated a combination of the number of compounds, the proportion contributed by each, and the extent to which their molecular structures differ.

For example, even if two flowers contain the same 10 compounds, their mixtures differ if one is strongly dominated by a single component while the other contains several in relatively even proportions. The approach also distinguishes mixtures of structurally similar compounds from mixtures of compounds with different structures.

Behind this approach is the idea that effects on organisms may depend not only on individual compounds but also on their combinations. However, structural differences offer clues to possible differences in action; the biological effects of every compound were not measured in this study.

Investigating Mountain Flowers, Visiting Insects, and Floral Bacteria

The fieldwork took place from May to July 2014 at seven sites around Grossglockner Mountain, at elevations of 1,146–2,750 meters. Floral scent was examined in 92 individuals from 39 species and combined with data on insects, bacteria, and flower morphology recorded in the same region.

Scent was sampled by enclosing wild inflorescences in bags, collecting the volatile compounds released into the surrounding air, and analyzing them instrumentally. Of the 264 compounds detected, structures could be assigned to 158. Rather than excluding all compounds with unknown structures, the researchers included them in diversity calculations by assigning average values for structural dissimilarity.

Insects were collected when touching the reproductive parts of flowers and counted in groups defined by their external appearance. A total of 656 flower visits were recorded. The dataset used to analyze the relationship with scent comprised 53 individuals from 31 plant species. The number of visits and the number of visitor types are different measures; the focus was on the latter.

For bacteria, the researchers analyzed the 16S rRNA gene in samples taken from flowers and assessed richness based on differences between sequences. This association analysis used 39 individuals from 31 plant species. It did not directly count bacterial cells or the quantity of living bacteria, but used the number of detected sequence types as an indicator.

Because sampling used up the flowers, scent, insects, bacteria, and floral morphology were not all measured in the same individuals. The study integrated data from different sets of individuals sampled in comparable environments within the same plant communities.

A Positive Association with Insect Richness and a Negative One with Bacterial Richness

The statistical models showed an association between higher floral scent chemodiversity and more types of flower-visiting insects. In contrast, bacterial sequence richness tended to be lower. Both associations were statistically significant.

Elevation was also considered in the analysis, but its relationships were not clear in this dataset. This does not mean that all environmental influences, such as temperature or humidity, had disappeared.

The researchers also examined flower size, the width and depth of the nectar-bearing tube, pollen position, and flower orientation. In this dataset, those morphological traits did not show significant associations with insect or bacterial richness. The links among scent diversity, insect richness, and bacterial richness stood out more than their links with morphological traits.

However, flower color was not included in the analysis. Nor was this an experiment in which scent was increased or decreased, so it cannot establish that scent alone caused the differences in insects and bacteria.

Microbes Arrive Alongside Pollen

These results led the authors to propose a hypothesis focusing on “pollinators that also carry microbes.” They call it the Filthy Pollinator Hypothesis. It views insects that help plants by transporting pollen as organisms that simultaneously carry a variety of microbes from flower to flower.

Flowers that welcome a wider range of insects might be expected to receive more diverse microbes. Yet in this study, flowers with diverse scents and more visitor types had fewer bacterial types. The authors suggest that combinations of scent compounds might act like a “sieve,” selecting which microbes can readily establish themselves on a flower.

Previous studies provide examples of individual scent compounds acting on microbes, and combinations of compounds might broaden or strengthen such effects. However, the present study did not directly test the antimicrobial action of mixtures or track the subsequent establishment of bacteria delivered by insects.

Microbes that tolerate a flower’s chemical environment and remain there are not necessarily beneficial to the plant. Pathogenic organisms may also be adapted to that environment. Fewer bacterial types cannot simply be reinterpreted as “fewer harmful bacteria and better health,” and the authors distinguish these points too.

Considering Floral Scent in Relation to Both Insects and Microbes

The significance of this hypothesis is that it treats floral scent diversity as more than a feature for attracting insects. If the benefit of welcoming a wide range of visitors is linked to handling the microbes they bring, scent mixtures may have multiple roles.

The authors propose future controlled experiments that independently manipulate the number of compounds, the unevenness of their proportions, and their structural differences, then test changes in insect visitation and microbial establishment. The hypothesis also needs to be tested by examining which microbes remain and how they relate to plant growth and reproduction.

Summary

In Alpine flowers, scent chemodiversity was positively associated with the number of insect visitor types and negatively associated with the number of bacterial types on flowers. This combination led to a hypothesis that considers insect attraction and microbial filtering together.

Floral scent may help shape a place through which many different organisms pass. By considering microbes as well as insects, the study offers a way to explore why flowers combine diverse scent compounds within the broader relationships of an ecosystem.

References

Hanusch, M., Dötterl, S., Larue-Kontić, A.-A. C., Keller, A., & Junker, R. R. (2025). Floral scent chemodiversity is associated with high floral visitor but low bacterial richness on flowers. New Phytologist, 248, 3270–3279. DOI: 10.1111/nph.70600 / Full original article. Original article: © 2025 The Author(s), CC BY 4.0.