Nesting site

Nests of Osmia uncinata were found at each of the eight study sites. In total, 80 nests were discovered, 71 of which were abandoned and one to several years old, whereas nine were new containing living or freshly dead progeny (n = 8) or an adult female, which had died within her first unfinished brood cell for unknown reasons (n = 1). All nests were built inside the outermost bark layer of trunks of living and healthy trees of Pinus sylvestris, which grew at sunny places of open pine stands (Figs 5–10). The trunk diameter at breast height of trees selected as nesting sites varied from 16–76 cm (Fig. 15a). All nest entrances, which were circular in shape and had a diameter of 4.25–4.75 mm, were located below 1–3 cm thick prominences of longitudinal bark ribs and were visible only from below (Figs 4, 11–14, 17). The only exception was a single nest that entered the bark on the lateral side of a rib. The nest entrances were at a height of 16–220 cm above the ground (Fig. 15b). As the bark of pine trunks usually becomes considerably thinner at heights between two and three metres offering increasingly less suitable nesting sites, the proportion of overlooked nests higher than two metres above the ground is expected to be low. The bark areas containing nests were exposed in all directions (Fig. 15c). However, most of the nests were located on the east, southeast, south and southwest side of the trunks, whereas a much smaller proportion was on the less sun-exposed trunk areas.

Figures 5–14. 

Nesting site of Osmia uncinata. 5–10) Position of nests in the bark of Pinus sylvestris trunks. 11–14) Nest entrances below prominences of longitudinal bark ribs.

Figure 15. 

Nest parameters of Osmia uncinata. a) Trunk diameter at breast height of Pinus sylvestris trees selected as nesting site. b) Height of nest entrance above the ground. c) Exposure of nest. d) Length of nesting burrow. e) Number of brood cells. Grey = abandoned nests; blue = nests occupied upon discovery.

The nesting site described above corresponds to the behaviour of three females of O. uncinata, which examined the bark of Pinus sylvestris while searching for a suitable nesting place near Trimmis on 8.6.2019. One female checked the bark of a dead pine stump, while the others patrolled the bark of living pine trunks. The latter two females started their search at the base of the tree and flew slowly along the sunny side of the trunk up to a height of 1.5–2.5 m, before they left and flew to the base of a neighbouring pine tree to start a new upwardly search flight. All three females repeatedly interrupted their flight and landed on the bark to carefully inspect fissures, fractures and prominences. The same behaviour was observed in two females of the sapygid wasp Sapyga similis (Fabricius), which examined the pine bark most probably in search for nests of their host O. uncinata (see below). Host-nest searching females of S. similis at the bark of old pine trees were also reported by Tischendorf (2002).

Three nests containing living progeny or freshly dead adults at the time of their discovery were most likely reused as indicated by the presence of at least one year old remains of leaf pulp walls. Whether such nests were enlarged in the year of reuse by prolonging the main burrow or by excavating side burrows is unclear. Other aculeate Hymenoptera also used the abandoned nesting burrows of O. uncinata to place their brood cells, such as Heriades, e.g. H. truncorum L. (Megachilidae, n = 1), Hylaeus, e.g. H. communis Nylander (Colletidae, n = 11), Passaloecus, e.g. P. eremita Kohl (Crabronidae, n = 2), Trypoxylon (Crabronidae, n = 1), Deuteragenia (Pompilidae, n = 2) and eumenine wasps (Vespidae, n = 1); in five other reused nests with remains of earthen cell walls the owner could not be identified.

Nest architecture

The nests were all tunneled out by the females of Osmia uncinata with no indication that pre-existing burrows e.g. of cerambycid beetle larvae were used. The nesting burrows were carefully carved out and measured mostly 4.25–4.75 mm in diameter (range 4.0–5.75 mm). They were neither lined with glandular secretions nor with leaf pulp except sometimes for small areas of a few square millimetres covered with a thin layer of leaf pulp, which was probably applied to smooth out irregularities or to fill small cracks. The burrows, which were completely hidden 2–13 mm below the bark surface, ran more or less vertically upwards and parallel to the trunk surface (Figs 16, 18–25).

Figures 16–25. 

Nest architecture of Osmia uncinata. 16) Several years old nest after uppermost bark layer has flaked off. 17) Entrance to old nest seen from below. 18–25) Dissected nests consisting of a single burrow (18–22) or of one to three side burrows branching off the main burrow (23–25).

The nests usually consisted of a single straight to slightly curved burrow of 1.3–8.0 cm length (n = 63; Figs 15d, 16, 18–22). In eight nests, however, one (n = 5), two (n = 2) or three (n = 1) side burrows branched off the main burrow (Figs 23–25). These side burrows were excavated in the same plane as the main burrow and the longer ones ran parallel to the main burrow. The total length of nests with a main burrow and one to three side burrows varied from 4.7–12.0 cm (Fig. 15d).

The new nests as well as those old nests, for which the number of brood cells could properly be ascertained due to remains of cell partitions or dead bee progeny, contained 1–6 brood cells (Fig. 15e). The brood cells were constructed either singly in short burrows or in a linear series of 2–5 in longer burrows (Figs 28–30). They had a length of 7.5–11 mm including the outer cell partition, a width of 4.5–5.75 mm and were slightly broadened in the middle (n = 49). Given these brood cell dimensions, some old nests with a total burrow length exceeding 7–8 cm might possibly have contained more than six brood cells. In spite of the vertical orientation of the nesting burrows, the larval provisions were located in the basal, i.e. upper half of the brood cell (Figs 29, 30).

Figures 26–30. 

Nests of Osmia uncinata. 26–27) Nest entrances sealed with leaf pulp. 28) Dissected nest with three brood cells each containing a cocoon. 29) Dissected unfinished nest with cocoon in uppermost cell and dead larva on food provision in two cells. 30) X-rayed nest with six cells containing four overwintering females, one overwintering male (outermost cell) and a dead larva on food provision.

Each brood cell was sealed towards the nest entrance with a one-layered wall built from leaf pulp without addition of other material (Figs 28–30). These cell walls had a maximum marginal width of 1–2 mm and a minimum central width of 0.25–0.75 mm (n = 13). As revealed by a threshold of leaf pulp at the entrance of a still empty brood cell, the females mark the position of the later cell wall before they begin to provision the cell; this threshold was ring-shaped, which seems to be typical for Osmia species of the subgenus Melanosmia (Müller et al. 2019). Between the outermost cell wall and the innermost wall of the nest plug was an empty vestibule, which ranged in length from 2.75–11 mm (n = 7); in one exceptional case the vestibule was 22 mm long (Fig. 30). The nest plug, which measured 4–9 mm in total length (n = 8), consisted of two (n = 2), three (n = 5) or four (n = 1) one-layered walls made from leaf pulp (Figs 26–28, 30). These plug walls were constructed immediately behind each other, rarely enclosing a few loose clumps of leaf pulp between them. They were thicker than the cell walls and had a maximum marginal width of 1.5–3.5 mm and a minimum central width of 0.5–1.25 mm (n = 23). The outermost plug wall was either flush with the nest opening (n = 3) or slightly recessed by 1–3 mm (n = 3); in two reused nests, it was at a distance of 1.0 cm and 2.1 cm from the opening.

DNA metabarcoding revealed that the brood cell and plug walls of two nests were constructed from masticated leaves of Fragaria spec. and Potentilla spec. (both Rosaceae) (Tab. 1). In addition, very low proportions of sequence reads corresponded to Asteraceae and Viola (Violaceae). Although the percentage of reads obtained by next-generation sequencing is only a rough indication for the relative amount of a plant taxon in the extracted DNA, this low proportions might be due to contaminations.

Larval mortality

Osmia uncinata successfully developed to the imaginal stage only in two of the eight nests that were occupied upon discovery: a three-celled nest produced one male and a six-celled nest produced four females and one male (Fig. 30). In five brood cells of four nests, two females and three males of the sapygid wasp Sapyga similis (Fabricius) developed. Three adjacent cells within the same nest were destroyed by the drosophilid fly Cacoxenus indagator Loew. The content of two neighbouring cells within the same nest was devoured by the larva of a snakefly (Raphidioptera). And in ten cells distributed among four nests the larvae of O. uncinata died for unknown reasons. In summary, a successful development of O. uncinata failed in 20 out of 26 brood cells, which corresponds to a pre-imaginal mortality of 77%.

The Scottish nests

The two nests of Osmia uncinata discovered in the Scottish Highlands were located inside the bark of a Pinus sylvestris stump (Fig. 31). They were constructed by the same female, which entered each nest by an exit hole of the xylophagous beetle species Rhagium inquisitor L. (Cerambycidae). The two exit holes lay 10 cm and 11 cm above the ground and were separated by a distance of less than 5 cm (Figs 31, 32). Upon discovery, one nest had been sealed by the female bee with leaf pulp (nest 1), whereas the other was being provisioned (nest 2). Each nest consisted of a 4.5–5 mm wide burrow, which had a length of 1.2 cm in nest 1 and 4.0 cm in nest 2 (Figs 33, 34). The two nesting burrows were tunneled out by the female at the roof (nest 1) or the upper lateral corner (nest 2) of the beetle boring. Both were directed upwards and they contained one (nest 1) and three (nest 2) brood cells. The presence of numerous small borings of anobiid beetles – probably Ernobius mollis (L.) – in the bark directly above the Rhagium exit areas was likely the reason why the short nesting burrow of nest 1 was not advanced further upwards and why the burrow of nest 2 was started laterally rather than apically within the beetle boring (Figs 33, 34). Nest 1 was sealed with walls of leaf pulp both at the entrance of the short nesting burrow and at the opening of the Rhagium boring at the bark surface. Nest 2 remained unsealed, probably because the female died before the last brood cell was finalized.

Figures 31–34. 

Scottish nests of Osmia uncinata. 31) Stump of a burnt Pinus sylvestris tree containing two nests. 32) Female of O. uncinata on bark with three large exit holes of Rhagium inquisitor, of which the upper two served as entrance to one nest each: nest 1 (left) is sealed, nest 2 (upper right) is being provisioned; note the bark particles that have accumulated below nest 2, indicating the former digging activity of the female. 33, 34) X-rayed bark (left) and dissected nests (right) with nest 1 excavated at the roof and nest 2 excavated at the upper lateral corner of the Rhagium boring; the large dark spots on 33) are the three Rhagium exit holes, the numerous small spots are exit holes of anobiid beetles. The red arrows indicate the entrance to the excavated nesting burrows and the red frames the enlarged section on the opposite image.

Nests in the Lötschental/Valais

In the subalpine larch forest searched for nests of Osmia uncinata, three abandoned nests and one new nest were discovered in the bark of living trees of Larix decidua, which grew at an altitude between 1630–1780 m a.s.l. and measured 61–79 cm in trunk diameter. The nest entrances were 77–146 cm above the ground and – as in the nests in pine bark – were situated below prominences of longitudinal bark ribs. The four nests were located on the south (n = 2) and southeast (n = 2) side of the trunks. Their architecture exactly corresponded to that of the nests in pine bark described above. The new nest contained two brood cells with an overwintering prepupa each of Sapyga spec. and O. uncinata.

In Central Europe, the imaginal period of Osmia uncinata varies depending on the altitude (Fig. 35, Suppl. material 1). At the lowest elevations below 500 m a.s.l., the flight activity may start already at the end of March and rarely extends beyond the end of June, whereas at elevations above 1500 m a.s.l. in the subalpine zone of the Alps the imagines usually do not emerge before mid June and normally fly till the end of July.

Figure 35. 

Phenology of Osmia uncinata in Central Europe. For each period of five days, the number of female and male records per elevation is given. For a given locality and date, only one record per sex was considered.

Six brood cells originating from two nests built in 2019 at an altitude of 550 m and 700 m a.s.l. contained living adults when they were opened in fall (Fig. 30), indicating that O. uncinata has a one-year cycle at lower elevations and overwinters as fully developed imago within the nest. In contrast, a brood cell in a nest built in 2019 at an altitude of 1630 m a.s.l contained a prepupa of O. uncinata in April 2020, suggesting that O. uncinata passes at least its first winter in the prepupal stage in the subalpine zone of the Alps (see Discussion).

Osmia uncinata has a vast distribution occurring from temperate and northern Europe eastwards to the Russian Far East (Fig. 36). It inhabits a wide belt between 43° and 70° northern latitude, extending from 5° western to 153° eastern longitude. It is distributed from sea level up to about 1900 m a.s.l., with most records above 1500 m a.s.l. being situated in the inner alpine valleys of the Valais, Grisons and South Tyrol. It has been recorded from western Europe (Scotland, Belgium, Netherlands, Luxembourg, northern and southeastern France), central Europe (Switzerland, Liechtenstein, Germany, northern Italy, Slovenia, Austria, Czech Republic, Slovakia, Poland), northern Europe (Denmark, Norway, Sweden, Finland, Lithuania, Latvia, Estonia), eastern Europe (northern Ukraine, central and northern European Russia) and Asian Russia (Western Siberia, Eastern Siberia, Far East).

Figure 36. 

Distribution of Osmia uncinata. See Suppl. material 1 for a complete list of all distributional data. Made with Natural Earth (www.naturalearthdata.com).

Mapping of the 198 Swiss records of O. uncinata onto the distribution of Pinus sylvestris in Switzerland revealed a distinct match between the occurrence of the bee species and the presence of pine stands (Fig. 37). Exceptions were few regions in the Valais (Lötschental, Obergoms), Bernese Oberland (Gadmental) and the Grisons (Val Müstair), where O. uncinata was recorded despite the lack of P. sylvestris. Out of the 198 Swiss O. uncinata records, 26 could not be assigned to a square kilometre and were excluded from the analysis of pine presence (see Methods). Among the remaining 172 records, pine presence was confirmed for 167 records (97.1%) and excluded for five records (2.9%). These five records originated from the Lötschental (Valais; n = 2), Obergoms (Valais; n = 1), Gadmental (Berne; n = 1) and Val Müstair (Grisons; n = 1) and were situated at elevations between 1500 and 1900 m a.s.l.. The identification of the bee individuals collected at these five localities proved to be correct with the exception of the Bernese record, which could not be confirmed as the specimen was not available. The presence of P. sylvestris in a radius of 250 m around the 70 records of O. uncinata reported by German and Austrian bee experts was certain or highly probable for 56 records (80.0%), highly improbable for one record (1.4%) and unclear for 13 records (18.6%).

Figure 37. 

Swiss records of Osmia uncinata (n = 198) mapped onto the distribution of Pinus sylvestris in Switzerland. Grey = sample plots of the Swiss National Forest Inventory (NFI) without occurrence of P. sylvestris; red = sample plots of the NFI with occurrence of P. sylvestris; green = records of O. uncinata with pine presence; blue = records of O. uncinata without pine presence; black = records of O. uncinata not assignable to a square kilometre. Data made available by the Centre Suisse de Cartographie de la Faune (CSCF) for O. uncinata and the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) for P. sylvestris (WSL 2019).

Osmia uncinata nests in self-excavated short burrows in the outermost bark layer of Pinus sylvestris trees up to slightly more than two metres above the ground. Although all Central European nests were found in the bark of trunks of living trees, the observation of a female that examined the bark of a stump while searching for a suitable nesting site indicates that nests can also be tunneled out in the bark of stumps or dead trunks. In fact, the two nests of O. uncinata from Scotland were dug out in the bark of a burnt stump and Stoeckhert (1933) mentions pine stumps as nesting sites of O. uncinata. Based on the latter author, pieces of pine bark lying on the ground are occasionally also colonized. However, as O. nigriventris was found to occur at the very same locality, the nest mentioned by Stoeckhert most probably belonged to the latter species, which typically nests in detached bark pieces (Müller et al. 2019).

O. uncinata is one of the few osmiine bee species known to excavate its nests in wooden substrate. Among the Osmiini, such a behaviour appears to be restricted to Osmia species of the subgenus Melanosmia (Prosi et al. 2016). It only has been recorded so far in the Palaearctic O. nigriventris and O. pilicornis and in the Nearctic O. bucephala Cresson (Packard 1868; Prosi et al. 2016; Müller et al. 2019). The mandibles of O. uncinata females are considerably stronger than those of related Palaearctic O. (Melanosmia) species nesting in pre-existing cavities or loose soil, but distinctly weaker than those of O. nigriventris and O. pilicornis breeding in self-excavated burrows (Müller et al. 2019). The latter two species often excavate their nests in hard wood, which requires stronger mandibles than for tunneling out nests in relatively soft bark. Thus, the female mandibles of O. uncinata appear to be well adapted for gnawing in bark but are probably not strong enough to excavate nests in hard wood, supporting the assumption that O. uncinata is strictly specialized to bark for nesting.

The nesting burrows of O. uncinata were invariably oriented vertically upwards with the nest entrance situated at the lower end. Such an orientation is expected to be advantageous as rain runoff descending along the bark surface cannot enter the nest. Interestingly, O. uncinata positions its larval provisions in the upper half of the brood cell resulting in cells that face downwards. Downward-facing brood cells are known only in very few bee species (Danforth et al. 2019), e.g. in the Neotropic Hylaeus tricolor (Schrottky) (Colletidae), in the Palaearctic Anthidiellum strigatum (Panzer) (Megachilidae) and occasionally also in the Palaearctic Heriades truncorum (L.) (Megachilidae) (Sakagami and Zucchi 1978; Le Goff 2003; Westrich 2018). Why the provisions of these species do not run out of the brood cell during and after provisioning is unclear, but might be due to a combination of surface tension and viscosity (Danforth et al. 2019). As a consequence of the upper position of the provisions, the larvae are forced to feed in a hanging, upside down position making them susceptible to fall down to the cell bottom, from where they are hardly capable of reaching the provisions again.

Masticated green leaves of the two closely related genera Fragaria and Potentilla (Rosaceae) served as nest building material for O. uncinata. Both genera are also exploited by other O. (Melanosmia) species as leaf pulp sources, such as O. (Melanosmia) nigriventris and O. (Melanosmia) pilicornis (Prosi et al. 2016; Müller et al. 2019). This finding suggests that the selection of leaves for manufacturing leaf pulp might be far from accidental. In fact, several osmiine bee species belonging to the genus Hoplitis and to Osmia subgenera other than Melanosmia were also recorded to harvest leaf pulp from rosaceous genera, such as Alchemilla, Fragaria, Potentilla, Rosa and/or Sanguisorba (Müller and Richter 2018), supporting the view that the leaves of Rosaceae species might possess particularly favourable properties as nest building material. However, O. uncinata does not seem to be strictly specialized to Rosaceae for harvesting leaf pulp since a female was observed in the Netherlands biting off leaf material from Betula pendula Roth (Betulaceae) (Willems 2010).

The nests of O. uncinata discovered in the course of the present study rarely contained more than four brood cells, which is in line with Scottish nests found to contain one to three cells (Else and Edwards 2018, this study). As the females of osmiine bees usually lay from ten to 20 eggs during their lifetime under natural conditions (Raw 1972; Hawkins 1975; Correia 1981; Haeseler 1982; Müller 1994; Danforth et al. 2019), the females of O. uncinata are expected to build two or more nests during their flight period. The construction of few-celled nests might be a strategy to reduce the risk to lose all progeny after a brood parasite has detected a nest. In fact, the pre-imaginal mortality in 26 brood cells of eight nests of O. uncinata amounted to 77%, which is unusually high for solitary bees (Minckley and Danforth 2019, but see Prosi et al. 2016 and Ivanov and Fateryga 2018). The high pressure exerted by brood parasites may not only explain the low number of brood cells per nest, but also why O. uncinata usually occurs in very low densities even at localities where there are many host flowers and numerous suitable nesting sites.

Contrary to the literature (Else and Edwards 1996, 2018; Edwards 2001), the Scottish females of O. uncinata do not place their brood cells in pre-existing beetle borings of Rhagium inquisitor. Instead, they use the Rhagium exit holes only to enter the bark before they excavate their own nest inside the bark. Thus, the only difference between the Central European and the Scottish nests of O. uncinata is that nest excavation started below bark prominences in the former and inside Rhagium borings in the latter. This difference is unlikely to be geographical but might rather be related to the thickness of the bark. We hypothesize that Central European females occasionally also enter the bark through exit holes of xylophagous beetles particularly when the bark lacks strong longitudinal ribs with transversal cracks, and that Scottish females sometimes also use the underside of bark prominences as starting point for excavating nests in case that the bark is thick enough.

Like other European representatives of the subgenus Melanosmia, which are early flying bees active in spring and early summer (Prosi et al. 2016; Westrich 2018; Müller et al. 2019), Osmia uncinata emerges in phenological spring, i.e. between the end of March at low altitudes and the end of May at higher altitudes. The overwintering of O. uncinata in the imaginal stage as found in the present study for elevations below 800 m a.s.l. is in line with this early flight period. In contrast to its one-year cycle at lower elevations, O. uncinata appears to need two years for its development in the subalpine zone of the Alps, where a nest at 1630 m a.s.l. still contained a prepupa in April. As the time span between the end of winter and the onset of the early flight period is hardly long enough at this altitude to allow for the metamorphosis to the imaginal stage, we hypothesize that O. uncinata passes the first winter as prepupa and the second winter as diapausing adult at higher elevations. Such an astonishing flexibility in adapting the duration of development depending on climatic conditions is also known from other osmiine bee species (Forrest et al. 2019).

The range of Osmia uncinata in Central Europe largely coincides with the occurrence of Pinus sylvestris as revealed by the finding that pine presence was judged to be highly improbable for only a tiny fraction of the Swiss, German and Austrian O. uncinata records. The bee’s distribution extends over a wide altitudinal range from below 100 m up to 1900 m a.s.l. and encompasses both dry and wet as well as both warm and cold habitats corresponding well to the distribution of P. sylvestris, which is characterized by a very wide ecological amplitude (Brändli 1998; Houston et al. 2016). This coincidence between the occurrence of O. uncinata and P. sylvestris is not restricted to Central Europe. It also applies to Scotland, where O. uncinata is closely associated with remnants of the ancient Caledonian Forest dominated by P. sylvestris and older pine plantations (Falks and Lewington 2015; Else and Edwards 2018), and probably even to the entire range of O. uncinata. In fact, almost all known records of O. uncinata fall neatly into the distribution of P. sylvestris as given by Caudullo et al. (2017). Possibly for climatic reasons, however, O. uncinata is absent from several southern regions, where relictual stands of P. sylvestris occur, such as northern Iberia, southeastern Europe, the Crimea, northern Turkey and the Caucasus. Interestingly, numerous records of O. uncinata in northwestern Germany, the Netherlands, Belgium and northern France lie outside the native range of P. sylvestris (Caudullo et al. 2017), suggesting a rather recent colonization after the widespread introduction and naturalization of this pine species all over western Europe during the last few centuries.

At higher altitudes above 1500 m a.s.l. in the subalpine zone of the Swiss Alps (Lötschental, Obergoms, Val Müstair) there exist a few reliable records of O. uncinata at localities where P. sylvestris does not occur. Dominant tree species at these localities are Picea abies (L.) Karsten, Pinus cembra L., Pinus mugo Turra and/or Larix decidua. Since the former three conifers have only a thin bark, they are unsuitable as nesting site for O. uncinata. In contrast, the bark of L. decidua is similar to that of P. sylvestris in both thickness and consistency. Therefore, we strongly assume that O. uncinata uses the bark of L. decidua as a substitute nesting substrate particularly at elevations above 1400 m a.s.l., where P. sylvestris becomes rare and L. decidua common (Brändli 1998). This assumption is supported by the discovery of several O. uncinata nests in Larix bark at 1630–1780 m a.s.l. in the Lötschental/Valais.

The pronounced preference of O. uncinata for thick-barked P. sylvestris trees appears to be the main factor determining the bee’s habitat in Central Europe. Pines that are well exposed to sun and weather form particularly thick bark (A. Rigling, personal communication) possibly explaining why O. uncinata mainly occurs in open pine forests, along outer and inner forest edges dominated by pine or around isolated pine groups without any obvious clear preference for altitude, humidity or thermal conditions.