The pollen host selection by 19 bee species, which have their main Central European distribution in the Alps, was assessed by microscopical analysis of the scopal contents of about 900 females from museum and private collections. The results of the pollen analyses were complemented by a literature survey as well as by field observations. The examined species widely vary in pollen host spectrum and specialization, revealing a fascinating diversity in bee host plant use. Observed patterns of host plant choice range from narrow specialists, which exclusively collect pollen from the flowers of a single plant genus, to pronounced generalists, which harvest pollen from the flowers of up to 17 different plant families. A quantitative character is given to separate the morphologically very similar females of Panurginus herzi and P. montanus.

Die Pollenwirtswahl von 19 Bienenarten, welche ihren zentraleuropäischen Verbreitungsschwerpunkt in den Alpen haben, wurde mittels mikroskopischer Analyse der Pollenladungen von rund 900 Weibchen aus Museums- und Privatsammlungen ermittelt. Die Ergebnisse der Pollenanalysen wurden durch eine Literaturauswertung sowie durch Feldbeobachtungen ergänzt. Die untersuchten Arten unterscheiden sich stark hinsichtlich Wirtspflanzenspektrum und Spezialisierungsgrad und umfassen sowohl enge Spezialisten, welche den Pollen ausschliesslich auf einer einzigen Pflanzengattung sammeln, als auch ausgeprägte Generalisten, welche bis zu 17 verschiedene Pflanzenfamilien als Pollenquellen nutzen. Ein quantitatives Merkmal zur Unterscheidung der äusserlich sehr ähnlichen Weibchen von Panurginus herzi und P. montanus wird gegeben.

Andrenidae , Apiformes , Halictidae , Megachilidae , mesolecty, oligolecty, pollen analysis, polylecty

Bees are herbivores using nectar and pollen as the predominant food source for their larvae. While no floral specificity is known with respect to the collection of nectar, many bee species restrict pollen harvesting to closely related plant taxa (Robertson 1925, Westrich 1989). The degree of host plant specialization among bees widely varies: “monolectic” bee species are entirely dependent on the pollen of a single plant species even in the presence of sympatric species of the same genus, “oligolectic” species consistently collect pollen from flowers of a single plant genus, tribe or family, “mesolectic” species harvest pollen from flowers of two or three plant families and “polylectic” species exploit flowers of more than three plant families (Cane and Sipes 2006, Müller and Kuhlmann 2008). Even in polylectic species, host plant choice is usually limited to some degree and - as in more specialized taxa - governed by constraints with respect to pollen digestion, flower recognition or flower handling (Sedivy et al. 2008, Haider et al. 2014). Detailed knowledge of pollen host use is important for species conservation, the reconstruction of bee host range evolution and our understanding of the fascinating interrelationships between flowers and their pollinators.

The pollen host preferences of Western, Central and Northern European bee species are fairly well known thanks to several studies, which analyzed pollen loads of collected females by light microscopy (Chambers 1968, Raw 1974, Westrich 1989, Müller 1996, Michez et al. 2008, Müller and Kuhlmann 2008, Sedivy et al. 2008, 2013, Haider et al. 2014, Wood et al. 2016, Wood and Roberts 2017). Gaps of knowledge, however, exist for numerous species, which exclusively or predominantly occur in the Alps. Among the bee species, which have their main Central European distribution in the Alpine arc, pollen host preferences have been thoroughly analysed so far only for Colletes floralis Eversmann, Colletes impunctatus Nylander, Anthidium montanum Morawitz, Chelostoma grande (Nylander), Hoplitis lepeletieri (Pérez), Hoplitis loti (Morawitz) and Hoplitis tuberculata (Nylander) (Müller and Kuhlmann 2008, Müller 1996, 2015, Sedivy et al. 2008, 2013, Westrich 1989, 1993). For many other predominantly alpine species, information on host plant use either completely lacks, is based on field observations, which do not differentiate between nectar uptake and pollen collection, or relies on pollen analysis of female scopal loads without quantification, resulting in a list of pollen sources not considering their relative significance in the species’ larval diet.

The present publication aims to fill the knowledge gap still existing on the host plant preferences of 19 predominantly alpine bee species, which are either endemic to the Alps or have a boreoalpine or boreomontane distribution. For that purpose, pollen stored in the scopae of females from museum and private collections originating from across the Alpine arc was removed and microscopically analysed.

The pollen host spectra of 19 bee species belonging to the six genera Andrena and Panurginus (Andrenidae), Dufourea (Halictidae) and Megachile, Hoplitis and Osmia (Megachilidae) were assessed by microscopical analysis of the scopal pollen contents of 877 female specimens deposited in museum and private collections and captured between the beginning of the 20^th century and 2018. For each species, the aim was to analyze a minimum of 40 and a maximum of 50 pollen loads all originating from the Alpine arc. This goal was not attained for Osmia alticola Benoist and Osmia steinmanni Müller due to their rarity and poor representation in collections. For Panurginus herzi Morawitz, the pollen samples analysed by Vögeli (2001) were included in the present study and supplemented with numerous new samples. To account for potentially deviating pollen host use by specimens from different populations, pollen samples from females collected at as many different localities as possible were analysed. Specimens with identical information on the collection label (i.e. locality, date and collector) were regarded as originating from the same locality. Before removing pollen from the female scopae, the degree to which they were filled was estimated. The amount of pollen in the scopae was assigned to five classes, ranging from 1/5 (filled to one-fifth) to 5/5 (full load). The pollen grains were stripped off the scopae with a fine needle and embedded in glycerol gelatine on a microscopic slide. When a pollen load was composed of different pollen types, their percentages were estimated by counting the grains along two transects chosen randomly across the cover slip at a magnification of 400×. Pollen types represented by less than 5% of the counted grains were excluded to prevent a potential bias caused by contamination. For pollen loads consisting of two or more different pollen types, the proportion of the different types was corrected by their volume (Buchmann and O’Rourke 1991, Silveira 1991). For that purpose, the relative volumina of all pollen types within the sample were estimated by eye and the counted numbers of each type multiplied by a factor that corresponded to its volume. After assigning different weights to scopae according to their degree of filling (full loads were weighted five times more strongly than scopae filled to only one-fifth), the estimated percentages were summed up over all pollen samples for each species. To characterize the degree of host plant association, such as oligolecty, mesolecty or polylecty, definitions and methods proposed by Müller and Kuhlmann (2008) were applied. The pollen grains were identified down to family or, if possible, to subfamily, tribal or genus level at a magnification of 400× or 1000× with the aid of the literature cited in Westrich and Schmidt (1986), Beug (2004) and an extensive reference collection. Flower records written on the collection labels often facilitated pollen identification to a taxonomic level lower than the plant family. Difficult pollen types were identified by the palynologist Katharina Bieri (Biological Institute for Pollen Analysis, Kehrsatz). Certain closely related plant genera could not be properly identified with the method applied in the present study, such as Sedum and Sempervivum (Crassulaceae), Fragaria and Potentilla (Rosaceae), Euphrasia and Rhinanthus (Orobanchaceae) and Campanula and Phyteuma (Campanulaceae). Pollen grains of these taxa, however, were assigned to genus level if altitude, phenology or habitat excluded the alternative genus or if own field observations, literature data or label records all invariably suggest only one of the two genera. Such non-morphologically based pollen identifications are marked with “cf.” in Table 1. The distinction of pollen from Rhododendron and Vaccinium (Ericaceae) and from Euphrasia and Veronica (Plantaginaceae) is difficult. For the former pair the thickness of the exine and for the latter pair the shape of the swollen grains was used as distinctive characters. However, these characters could only be recognised in grains that were specifically oriented within the gelatine layer and/or completely swollen. If only typical Rhododendron (or Vaccinium) grains or typical Euphrasia (or Veronica) grains were present in a pollen sample, all grains for which the distinctive characters were not discernible were assigned to Rhododendron (or Vaccinium) and Euphrasia (or Veronica), respectively. Data based on a comprehensive literature survey on confirmed pollen hosts and observed flower visits as well as field observations made by the author during the last three decades were used to complement the results obtained by the microscopical analysis of scopal pollen loads.

The taxonomy of Panurginus in Central Europe is still under discussion. While the validity of P. herzi Morawitz as a biological species is undisputed, there is no consensus yet whether P. sericatus (Warncke) is a species of its own (e.g. Schwarz and Gusenleitner 1997, Amiet et al. 2010) or should rather be treated as a western subspecies of P. montanus Giraud (e.g. Warncke 1972, Ebmer 2001). No morphological characters are known to discriminate between the females of these latter two taxa. In contrast, the males slightly differ in the shape of the gonostylus (Amiet et al. 2010). However, there are populations in eastern Switzerland, which show an intermediate gonostylus shape (Amiet et al. 2010). Due to these morphological transitions, the two taxa are regarded as conspecific in the present study. The characters given by Amiet et al. (2010) to separate the females of P. herzi from those of P. montanus proved to be partly unreliable. The shape of the head, however, was recently found to be a reliable character allowing for the unambiguous identification of P. herzi (Hopfenmüller 2017). To quantify this character, the ratio of head length to head width (sensu Michener 2007) was measured under a stereomicroscope for 150 Panurginus females originating from 78 different localities in Switzerland, Austria and Germany. The measurements revealed an almost non-overlapping bimodal distribution in the head length to head width ratio (Fig. 1), corroborating the reliability of this character to separate the females of P. herzi and P. montanus. For the present study, Panurginus females with a head length to head width ratio of 0.76–0.82 were identified as P. herzi and those with a ratio of 0.84–0.90 as P. montanus. Pollen loads of females with a ratio of 0.83 were not considered for pollen analysis.

Figure 1. 

Head length to head width ratio in females of Panurginus herzi (light blue) and P. montanus (dark blue) (n = 150). Definitions for head length and head width according to Michener (2007).

The 19 bee species investigated in the present study widely vary in their pollen host spectra and degree of host plant specialization, revealing a fascinating diversity in bee pollen host use (Tab. 1). The examined set of species encompasses i) narrowly oligolectic species, which exclusively collect pollen on a single plant genus, ii) broadly oligolectic species, which harvest pollen on a single plant family, iii) mesolectic species, which are dependent on two plant families, iv) polylectic species, which exhibit a strong but not exclusive preference for a single plant taxon, and v) polylectic species, which do not prefer any single plant taxon and exploit up to 17 different plant families.

Comparison of pollen host use among closely related species of the same subgenus or the same monotypic genus reveals different patterns (see species accounts above and Tab. 1). The six Osmia species of the subgenus Melanosmia all exhibit a pronounced affinity for the pollen of Fabaceae, supporting other studies which demonstrated that host plant preferences are often conserved in clades of closely related bee species (Müller 1996, Wcislo and Cane 1996, Michez et al. 2004, Sipes and Tepedino 2005, Patiny et al. 2007, Larkin et al. 2008, Michez et al. 2008, Sedivy et al. 2008, 2013). However, the degree of dependence on Fabaceae pollen differs among the O. (Melanosmia) species, ranging from a narrow specialization on Fabaceae as in O. xanthomelana, which collects pollen solely from the flowers of two closely related Fabaceae genera, to a moderately strong dependence on Fabaceae as in O. uncinata, which exploits the flowers of at least 14 additional plant families. In contrast to O. (Melanosmia), the three Andrena species of the subgenus Andrena widely differ in their pollen host use as do the two Dufourea species. While the differences between A. fucata and A. rogenhoferi are potentially due to deviating distribution and habitat selection with A. fucata restricted to forested areas of the montane and subalpine zone and A. rogenhoferi colonizing a wide spectrum of habitats from the submontane to the alpine zone (SwissBeeTeam 2018), the pronounced preference of A. lapponica for Ericaceae pollen might possibly be genetically based as it is the case for other bee species with a specialized diet (Praz et al. 2008). A genetic basis for the differing pollen host choice is also suggested for the two Dufourea species, which often colonize the same habitats in the Alps, where they encounter a similar flower supply but nevertheless collect the pollen of different plant taxa. For both Panurginus species, pollen of Potentilla (Rosaceae) plays an important role in the larval diet. Interestingly, P. herzi is entirely dependent on Potentilla, whereas P. montanus is capable of exploiting several additional hosts. If future studies show that the polylectic habit of P. montanus is evolutionary derived, this would support the view that many generalist bee species that evolved from specialized ancestors had broadened their diet under maintenance of the exclusive host of their ancestors (Sedivy et al. 2008).

I gratefully acknowledge the help of the following colleagues who generously permitted pollen removal from bee specimens of their collections or under their curation: F. Amiet (Solothurn), G. Artmann (Olten), H. Baur (Naturhistorisches Museum Bern), M. Bur (Rechthalten), R. Eastwood (ETH Zürich), K. Hirt (Menziken), A. Freitag (Musée de Zoologie Lausanne), F. Gusenleitner and M. Schwarz (Biologiezentrum Linz), M. Herrmann (Konstanz), S. Liersch (Bündner Naturmuseum Chur), R. Neumeyer (Zurich), C. Praz (Université de Neuchâtel) and C. Schmid-Egger (Berlin). Katharina Bieri (Biological Institute for Pollen Analysis, Kehrsatz) kindly helped with the identification of several difficult pollen types. D. Bénon, S. Falk, A. Jacobs, A. Krebs, W. Kreisch, H.-J. Martin, R. Prosi and P. Westrich provided photos. Comments by J. Litman, M. Kuhlmann, C. Praz and V. Mauss improved the manuscript. The project was generously funded by the Tierschutzverein Glarus.