Summary

Wild bees form a highly diverse group in Switzerland, distinct from the honey bee in their lifestyles, ecological requirements and conservation needs. Most species are solitary, annual and closely dependent on a combination of floral resources, nest sites and suitable landscape structures.

Switzerland harbours a remarkable richness with 632 inventoried species, of which 575 are currently recorded. This diversity is of great ecological importance, particularly for the pollination of wild plants and crops, but it is also accompanied by considerable vulnerability: a significant proportion of species are listed on the Swiss Red List.

The decline of wild bees is primarily linked to the loss and fragmentation of habitats, the scarcity of host plants and nesting substrates, and the simplification of landscapes. Their conservation therefore requires a functional habitat approach, based on botanical diversity, structural heterogeneity of habitats, proximity between nests and flowers, and adapted long-term management.

1. Introduction: Why wild bees matter

In public debate, the word "bee" most often refers to the honey bee (Apis mellifera). Yet this focus obscures a far broader biological reality. Worldwide, approximately 20,000 bee species have been described, and Switzerland alone accounts for 632 inventoried species, of which 575 are currently recorded.

Wild bees contribute to ecosystem functioning by pollinating numerous wild plants and certain crops. Their functional diversity — body size, flight period, foraging behaviour, floral specialisation — makes their contribution complementary to that of the honey bee.

The situation in Switzerland is concerning: the new Red List indicates that 45.4% of evaluated species are threatened and that 59 are considered nationally extinct. This finding makes it essential to clearly distinguish wild bees from the honey bee: an increase in the number of hives does not automatically improve the situation for wild bee species.

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2. What are wild bees?

In the Swiss and European context, the term "wild bees" refers to all bees other than the honey bee kept in beekeeping, including bumblebees. This is a functional category useful for ecology and conservation, not a distinct taxonomic rank.

Biologically, bees belong to the group Anthophila and derive from ancestors related to predatory apoid wasps. Their major evolutionary innovation lies in the use of pollen to feed larvae. Pollen is not merely consumed by adults: it is actively collected, transported and stored in the nest to provision the brood.

The vast majority of Swiss species are solitary: a mated female builds her nest alone, collects provisions and lays eggs without the help of workers. Bumblebees represent a special case, with social but annual colonies, very different from the perennial colonies of the honey bee.

Understanding this diversity of lifestyles is essential to avoid projecting the familiar honey bee hive model onto all bee species.

3. Diversity of wild bees

The diversity of wild bees is not reducible to species count. It is also taxonomic, morphological, behavioural, phenological and biogeographical. In Switzerland, the 632 inventoried species are distributed across six families, including Andrenidae, Halictidae, Megachilidae and Apidae.

This diversity is expressed in body size, hairiness, pollen-collecting structures, flight periods, nesting behaviours and degrees of specialisation. Some species are generalists and relatively tolerant, while others depend on very precise ecological conditions.

Switzerland's richness is partly explained by the superimposition, within a small territory, of steep altitudinal, climatic and biogeographical gradients. The dry intra-alpine valleys, particularly in Valais and parts of Graubünden, are among the major centres of diversity.

Conversely, heavily intensified or urbanised landscapes are often species-poor and dominated by common species. Documenting this diversity remains difficult, as many species are inconspicuous and require specialised taxonomic expertise.

4. Lifestyles and life cycles

Photo: Brood cell of Andrena vaga, containing the pollen provision and an egg.  © Albert Krebs & Andreas Müller.

Under the temperate conditions of Switzerland, the annual cycle is the general rule. After emergence, adults mate; females then build their nest, provision the cells with pollen and nectar, lay eggs and die before the emergence of the next generation.

Most species are univoltine, meaning they produce only one generation per year. The adult phase visible on flowers is often brief, while most of the cycle takes place in the nest over several months.

As in Hymenoptera generally, sex determination follows a haplodiploid system: females develop from fertilised eggs, males from unfertilised ones. In many solitary species, males emerge before females, a well-documented protandry phenomenon.

Larval provisioning is based on a central principle: each cell is fully provisioned before oviposition. This organisation makes wild bees particularly sensitive to disruptions in floral availability or nest sites during the short reproductive period.

Bumblebees and some Halictidae exhibit forms of sociality, but their colonies remain annual. Here again, this represents a biological model very different from that of the honey bee.

5. Nesting modes and nesting ecology

In wild bees, nesting constitutes one of the cores of reproductive ecology. The nest is not simply a shelter: it is the site of egg-laying, larval development and often overwintering, and therefore a decisive element for the maintenance of populations.

The majority of species nest in the soil. They use a range of substrates: sandy or loamy soils, embankments, well-exposed slopes, path edges or pioneer surfaces. Preferences are, however, precise and depend on texture, moisture, compaction, aspect and degree of vegetation cover.

Other species use pre-existing cavities in dead wood, hollow stems, pithy stems, old walls or other interstices. Here too, the diameter, depth, orientation and microclimate of cavities play an important role.

Some specialisations are very pronounced, such as in osmia bees nesting in snail shells. The internal construction of cells also involves specific materials: earth, mud, resin, leaf fragments, petals or plant hairs.

Proximity between the nest and floral resources represents a central ecological constraint: the longer the foraging distances, the higher the reproductive costs become. Insect hotels can help certain cavity-nesting species, but they address only the needs of a minority of wild bee species; the majority depend primarily on natural substrates.

6. Bee–flower relationships and trophic specialisation

The relationships between wild bees and flowers cannot be reduced to simple flower visitation. The decisive point is the pollen actually collected to feed the larvae. A species may appear to be a generalist when its floral visits are observed, while remaining biologically specialised for brood provisioning.

The literature classically distinguishes polylectic species, which collect pollen from many plant groups, from oligolectic species, specialised on a narrow range of host plants. In Switzerland, this specialisation plays an important role in species vulnerability.

Among non-parasitic species evaluated from this perspective, 33.2% are oligolectic. The proportion of threatened species is markedly higher among specialists than among generalists. This relationship does not mean that all specialisation is inherently a handicap, but it strongly reduces the margin for substitution when host plants become scarce.

Experimental work shows, moreover, that certain specialised bees do not develop correctly on non-host pollen, even when it is abundant. Trophic specialisation is therefore not only a matter of behaviour: it may also rest on subtle physiological, morphological and sensory constraints.

It follows that a flower-rich landscape is not necessarily good habitat. The actual botanical composition, the presence of relevant host plants and their proximity to nest sites are determinative.

7. Importance of wild bees as pollinators

The importance of wild bees as pollinators can only be understood by avoiding two simplifications: reducing them to a marginal role, or assuming that the honey bee could replace them without functional loss.

In natural habitats, they contribute to maintaining plant–pollinator interactions and support the reproduction of numerous flowering plants. For crops, the findings are particularly robust: wild pollinators increase fruit set (fruit set is the initial stage of fruit development. It is the point at which the flower's ovary develops into a fruit following fertilization) independently of honey bee abundance (Garibaldi et al., 2013).

This complementarity is explained by the diversity of biological traits in wild bees: different flight periods, activity under varied weather conditions, access to complex flowers and varying pollination efficiency depending on the system.

Species diversity also has intrinsic value, as it reinforces the stability of pollination services over time. In Switzerland, insect pollination represents a real ecological and economic issue, even if precise attribution of each group's contribution remains unevenly quantified across crops.

8. Threats to wild bees

The situation of wild bees in Switzerland is concerning. The most recent Red List evaluates 615 species: 279 of them, or 45.4%, are red-listed, and 59 are considered nationally extinct.

This situation does not affect all species equally. Oligolectic species, ground-nesting species, late-season fliers and those tied to narrow microhabitats appear particularly vulnerable.

Habitat loss and degradation constitute the best-supported factor. Agricultural intensification, landscape simplification, and the disappearance of hedgerows, fallow land, embankments, extensive meadows and other semi-natural structures simultaneously reduce floral resources and nest sites.

Additional pressures compound these effects: urbanisation and soil sealing, pesticide exposure, possible pathogen transmission from managed colonies, and the effects of climate change. These factors do not act in isolation; they interact, and their precise weight varies according to habitat, region and species guild.

9. Habitat requirements: What wild bees need

A functional habitat for wild bees is not simply a flower-filled backdrop. It must bring together, within an ecologically coherent space, three types of resources: a diverse and continuous floral supply, suitable nest sites, and a spatial organisation that connects these resources at distances compatible with the biology of the species.

For ground-nesting species, this means permeable surfaces, areas of bare or sparsely vegetated soil, well-exposed embankments and pioneer zones. For cavity-nesting species, dead wood, hollow or pithy stems, old walls and other favourable microstructures are needed.

Proximity between nests and flowers is decisive. Experimental work has shown that longer foraging distances significantly reduce offspring production in several solitary bee species (Zurbuchen et al., 2010). The general ecological logic is therefore clear, even if no single threshold can be applied to all species.

Extensive meadows, wildflower strips, hedgerows, woodland edges, herbaceous fallows and transitional zones are particularly important when embedded in a sufficiently dense and connected landscape network. Habitat quality is thus built at the scale of the network and the mosaic, not from a single isolated measure.

10. The honey bee as a specific topic: utility, limitations, competition

The honey bee occupies a particular place in an article devoted to wild bees. It is important for beekeeping, contributes to the pollination of certain crops, and has a real economic, social and cultural value.

This utility does not mean, however, that it can replace the functional diversity of wild pollinators. Available studies show on the contrary that wild insects enhance pollination independently of honey bee abundance.

The overlap in floral resources between Apis mellifera and wild bees is well documented, but it is not sufficient to demonstrate demographically significant competition. Available findings remain heterogeneous and depend strongly on context: colony density, resource availability, habitat type and species concerned. The review by Mallinger et al. (2017) illustrates precisely this variability in effects.

In certain resource-poor natural or protected habitats, high densities of managed colonies can affect certain components of the wild bee community. In other contexts, particularly urban ones where resources are abundant, coexistence appears more straightforward.

Pathogen transmission represents a further dimension of the problem, distinct from food competition. The risk is biologically plausible and partially documented, but its population-level consequences remain unevenly quantified. The most robust conclusion is therefore contextual: beekeeping should neither be presented as a general solution for wild bees, nor condemned without nuance.

11. What can be done? Some principles for supporting wild bees

Available knowledge suggests that no single measure is capable, on its own, of sustainably benefiting all wild bee species. Several relatively robust action principles can nevertheless be identified.

Think about habitat in functional terms. A favourable habitat is not defined solely by abundant flowering, but by the combination of floral resources, nest sites and a spatial organisation compatible with the biology of the species.

Ensure a diverse and continuous floral supply. Botanical composition is decisive. For specialised species, the actual presence of host plants at the right time and close to nest sites matters more than general floral abundance.

Preserve genuine nesting resources. Open soils, well-exposed embankments, dry stems, dead wood, old walls and other natural microstructures are of central importance. Insect hotels can supplement these resources, but cannot replace them.

Manage habitats extensively and in a differentiated manner. Over-intensive management depletes resources, while complete abandonment can lead to habitat closure. Staggered interventions, refuge zones and the maintenance of structural heterogeneity appear to be the most coherent approaches.

Think at the landscape scale. Measures are more effective when extensive meadows, wildflower strips, hedgerows, woodland edges and ruderal areas complement one another within a coherent network. In the dry intra-alpine valleys, the priority often lies in conserving already highly rich habitats.

Support measures with monitoring. Even simple, repeated monitoring over time, combined with observations on vegetation and habitat structure, makes it possible to distinguish genuinely effective measures from those that produce only a transient effect.

In summary, supporting wild bees requires an integrated approach based on the complementarity between flowers, nests, landscape structure, adapted management and long-term evaluation, rather than a single recipe presented as universal.

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See also:

• Solitary bees
• Protecting pollinators
• Wildflower strips benefit bees
• Apis mellifera & other Apis
• How do bees choose pollen?
• Pollen consumption and colony development

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Selected bibliography
 

Albrecht, M., & Ganser, D. (2023). Grassland extensification enhances nest densities of ground-nesting wild bees. Journal of Applied Ecology, 61(3), 521–531. https://doi.org/10.1111/1365-2664.14527

Albrecht, M., Knecht, A., Riesen, M., Rutz, T., & Ganser, D. (2021). Time since establishment drives bee and hoverfly diversity, abundance of crop-pollinating bees and aphidophagous hoverflies in perennial wildflower strips. Basic and Applied Ecology, 57, 102–114. https://doi.org/10.1016/j.baae.2021.10.003

Antoine, C. M., & Forrest, J. R. K. (2021). Nesting habitat of ground-nesting bees: A review. Ecological Entomology, 46(2), 143–159. https://doi.org/10.1111/een.12986

Casanelles-Abella, J., Fontana, S., Fournier, B., Frey, D., & Moretti, M. (2023). Low resource availability drives feeding niche partitioning between wild bees and honeybees in a European city. Ecological Applications, 33(1), e2727. https://doi.org/10.1002/eap.2727

Fürst, M. A., McMahon, D. P., Osborne, J. L., Paxton, R. J., & Brown, M. J. F. (2014). Disease associations between honeybees and bumblebees as a threat to wild pollinators. Nature, 506(7488), 364–366. https://doi.org/10.1038/nature12977

Garibaldi, L. A., et al. (2013). Wild pollinators enhance fruit set of crops regardless of honey bee abundance. Science, 339(6127), 1608–1611. https://doi.org/10.1126/science.1230200

Henry, M., & Rodet, G. (2018). Controlling the impact of the managed honeybee on wild bees in protected areas. Scientific Reports, 8, 9308. https://doi.org/10.1038/s41598-018-27591-y

Mallinger, R. E., Gaines-Day, H. R., & Gratton, C. (2017). Do managed bees have negative effects on wild bees? A systematic review of the literature. PLOS ONE, 12(12), e0189268. https://doi.org/10.1371/journal.pone.0189268

Maurer, C., Sutter, L., Martínez-Núñez, C., Pellissier, L., & Albrecht, M. (2022). Different types of semi-natural habitat are required to sustain diverse wild bee communities across agricultural landscapes. Journal of Applied Ecology, 59(10), 2604–2615. https://doi.org/10.1111/1365-2664.14260

Milet-Pinheiro, P., et al. (2013). The chemical basis of host-plant recognition in a specialized bee pollinator. Journal of Chemical Ecology, 39(11–12), 1347–1360. https://doi.org/10.1007/s10886-013-0363-3

Müller, A., & Praz, C. (2024). Liste rouge des abeilles : espèces menacées en Suisse. État 2022. Office fédéral de l'environnement & info fauna.

Oertli, S., Müller, A., & Dorn, S. (2005). Ecological and seasonal patterns in the diversity of a species-rich bee assemblage. European Journal of Entomology, 102(1), 53–63. https://doi.org/10.14411/eje.2005.008

Pfiffner, L., Ostermaier, M., Stoeckli, S., & Müller, A. (2018). Wild bees respond complementarily to "high-quality" perennial and annual habitats of organic farms in a complex landscape. Journal of Insect Conservation, 22(3–4), 551–562. https://doi.org/10.1007/s10841-018-0084-6

Praz, C. J., Müller, A., & Dorn, S. (2008). Specialized bees fail to develop on non-host pollen: Do plants chemically protect their pollen? Ecology, 89(3), 795–804. https://doi.org/10.1890/07-0751.1

Praz, C. J., Müller, A., Bénon, D., Herrmann, M., & Neumeyer, R. (2023). Annotated checklist of the Swiss bees: Hotspots of diversity in the xeric inner Alpine valleys. Alpine Entomology, 7, 219–267. https://doi.org/10.3897/alpento.7.112514

Rogers, S. R., Tarpy, D. R., & Burrack, H. J. (2014). Bee species diversity enhances productivity and stability in a perennial crop. PLOS ONE, 9(5), e97307. https://doi.org/10.1371/journal.pone.0097307

Senapathi, D., et al. (2021). Wild insect diversity increases inter-annual stability in global crop pollinator communities. Proceedings of the Royal Society B, 288(1947), 20210212. https://doi.org/10.1098/rspb.2021.0212

Sutter, L., Herzog, F., Dietemann, V., Charrière, J.-D., & Albrecht, M. (2017). Demande, offre et valeur de la pollinisation par les insectes dans l'agriculture suisse. Recherche Agronomique Suisse, 8(9), 332–339.

Zurbuchen, A., Cheesman, S., Klaiber, J., Müller, A., Hein, S., & Dorn, S. (2010). Long foraging distances impose high costs on offspring production in solitary bees. Journal of Animal Ecology, 79(3), 674–681. https://doi.org/10.1111/j.1365-2656.2010.01675.x

Zurbuchen, A., & Müller, A. (2012). Wildbienenschutz – von der Wissenschaft zur Praxis. Haupt.