Osmia pilicornis occurs from western, central and eastern temperate Europe eastwards to the Kemerovo region in western Siberia (Fig. 1). It inhabits a rather narrow belt between 44.5° and 62.5° northern latitude, extending from 3.5° western to 87.5° eastern longitude (Table 1). It is distributed from sea level up to about 950 m a.s.l., with the highest records all lying below 1000 m a.s.l. It has been recorded so far from (west to east) southern Great Britain, northern France, Belgium, western and northern Switzerland, Luxembourg, Germany, Denmark, Austria, southern Sweden, Czech Republic, Slovenia, Hungary, Slovakia, Poland, Romania, Lithuania, southern Finland, Latvia, Belarus, Ukraine and Russia (southern North-West district, Central district, Wolga district, southern Ural district, southwestern Siberia district). Wu (2006) reports O. pilicornis from the Taihang mountains in Hebei province in northeastern China, which is about 2600 km to the east of the easternmost records of O. pilicornis in western Siberia. This record appears to be doubtful and needs verification as already demanded by Ebmer (2010).

Figure 1. 

Distribution of Osmia pilicornis. Records east of the Ural mountains are not shown.

Osmia pilicornis emerges early in spring (Fig. 2). Its appearance coincides with that of other early bee species, such as Andrena bicolor Fabricius, Anthophora plumipes (Pallas), Eucera nigrescens Pérez, Osmia bicolor (Schrank) and Osmia bicornis (Linné). In Central Europe, the earliest records are 22 February for males and 18 March for females. Males have their peak activity from the last third of March to the first third of May and occasionally fly until mid May. The latest male record in Central Europe is 17 May. Females have their peak activity from the first third of April to the first third of June and occasionally fly until the end of July. The latest female record in Central Europe is 4 August. While the males emerge only a few days before the females, their flight period is roughly one month shorter, indicating that the females have a distinctly longer average life span than the males.

Figure 2. 

Phenology of Osmia pilicornis in Central Europe. Central Europe is defined here to comprise Austria, Czech Republic, Germany, Hungary, Poland, Switzerland, Slovakia and Slovenia.

Nesting site

Six nests of Osmia pilicornis were discovered in 2015 and 2016. Five nests were found on a 0.8 ha large and about five years old clear-cutting in a former spruce plantation with numerous plants of Pulmonaria mollis near Crailsheim (Baden-Württemberg) in southern Germany (10.11°E 49.18°N, 450 m a.s.l., Fig. 42). One nest was detected in a recently thinned, very light and oak-dominated deciduous forest with large stands of Pulmonaria officinalis near Ernstbrunn (Niederösterreich) in northeastern Austria (16.35°E 48.59°N, 330 m a.s.l., Fig. 44).

All six nests were burrows excavated by the female bees in dead branches lying on sun-exposed ground (Table 2, Figs 3–11). One branch contained two separate nests built by two females (nesting site 1), another branch also contained two nests, which, however, were probably constructed by the same female (nesting site 3), and two branches contained one nest each (nesting sites 2 and 4). The branches selected as nesting sites had a length of 7.5–85 cm and a diameter of 1.5–6.1 cm, were largely hidden under vegetation and belonged to four different tree or shrub species. The females started to gnaw the nest burrows either in the apical area of branch fracture (n=4), on the side of the branch (n=1) or on the underside of the branch about 10 cm above ground (n=1). The nest entrances were exposed to south-southwest (n=3), southeast (n=1) or northeast (n=1). The hardness of the nesting wood varied from very hard (n=2) and moderately hard (n=1) to soft (n=1).

Figures 3–11. 

Nesting sites of Osmia pilicornis in dead fallen branches (see Table 2): 3–4 Nesting site 1 with nests 1 and 2 in a branch of Fraxinus excelsior 5–6 Nesting site 2 with nest 3 in a branch of Picea abies 7–9 Nesting site 3 with nests 4 and 5 in a branch of Salix spec. 10–11 Nesting site 4 with nest 6 in a branch of Corylus avellana.

Nest architecture

The nests of Osmia pilicornis were composed of i) a burrow tunelled out by the female bee, ii) a varying number of brood cells each delimited towards the nest entrance by a cell partition, iii) a (facultative) empty space (“vestibule”) of varying length in front of the outermost cell and iv) a nest plug that closed the nest at the front end (Figs 12–14). There was no basal wall that sealed the nest burrow against the rear end.

Figures 12–14. 

Opened nests of Osmia pilicornis in dead branches (see Table 2): 12 Nests 1 and 2 with three and two brood cells, respectively, each containing a cocoon spun by the O. pilicornis larva 13 Nest 3 with one brood cell containing a cocoon spun by the O. pilicornis larva; at the bottom a pupa of Molorchus minor (Cerambycidae) 14 Nests 4 and 5 with two and one brood cell, respectively, each containing a larval provision; an empty vestibule is present between the outermost cell partition and the nest plug in nest 4, while the cell partition is part of the nest plug in nest 5.

The excavated burrows consisted of a short entrance part, which was 6–7 mm long and 5–6.4 mm wide, and a main part, which ran in longitudinal direction of the branch and contained the brood cells. The main part deviated from the entrance part by an angle of 90–140°. Depending on the number of brood cells, the total length of the burrows was 1.7–3.2 cm. The burrow and cell walls were neither lined with glandular secretions nor with leaf pulp except for a small area of less than 15 mm² in the rearmost brood cell of nest 4, which was covered by a thin layer of leaf pulp. In nest 3, the gnawing female obviously met a preexisting burrow of the cerambycid beetle Molorchus minor (Linné) running in longitudinal direction of the branch and adopted it as part of her own burrow by broadening its diameter from 2.8 mm to 5.7 mm (Fig. 13).

The nests contained one (n=2), two (n=2) or three (n=1) linearly arranged brood cells. Brood cell number could not be assessed in one nest as the nesting branch disappeared in the course of the study for unknown reasons. The brood cells had a length of 9.7–12.4 mm and a maximal width of 5.4–6.5 mm. They were widest in the centre and slightly constricted at both ends. The cell partitions were one-layered and had a width of 1–1.25 mm along the burrow wall.

Three out of five nests contained a vestibule between the outermost cell partition and the nest plug measuring 4–6 mm in length. In the other nests, no vestibule was developed; instead, the outermost cell partition was part of the nest plug.

The nest plugs measured 3–7.5 mm in length. They were multi-layered and consisted of 2–4 and 1–2 mm thick walls, which were built immediately behind each other. The nest plugs were flush with the nest entrance (n=3) or recessed by 2.5–3.5 mm (n=2).

Both cell partitions and nest plug exclusively consisted of leaf pulp. At nesting sites 1–3, females of Osmia pilicornis were repeatedly observed to collect leaf material from Fragaria vesca Linné (Fig. 19).

Figures 15–20. 

Nesting behaviour of Osmia pilicornis (see Table 2): 15–16 Females of nests 1 and 4 tunneling out burrows in dead branches 17–18 Females of nests 2 and 4 transporting small leaf fragments of Fragaria vesca to build brood cell partitions and nest plug 19 Female harvesting nest-building material on a leaf of Fragaria vesca 20 Female of nest 4 returning from a foraging flight with pollen-filled metasomal scopa.

Female nesting behaviour

The females of Osmia pilicornis gnawed their nests in dead wood exclusively with the aid of their mandibles (Figs 15, 16, Suppl. material 2). At nesting site 1, the female was repeatedly observed to burrow for a period of 10–15 sec, before she left the nest for a short circular flight of 1–1.5 m diameter, during which she dropped the carved wood chips that she had held in her mandibles. Occasionally, the female pushed the wood chips with her legs backwards out of the nest. It took about 10 min from the start of carving at the surface of the very hard Fraxinus branch until head and mesosoma had disappeared within the substrate, indicating a remarkably high speed of tunneling.

The shape of the female mandible of Osmia pilicornis differs from that of closely related Osmia (Melanosmia) species, such as O. inermis or O. parietina (Figs 21–26). In O. pilicornis, the mandible has a much broader and distinctly more inflated base, is relatively shorter, has less diverging margins and is apically only slightly curved inwards giving it a more compact and chisel-shaped appearance. In addition, the uppermost mandibular tooth is more prominent being as large as the medial tooth so that the apices of all mandibular teeth are almost at the same height forming a regular three-toothed apical edge.

The female of nest 1 needed 25 foraging flights to provision a single brood cell. This female spent 0.6–19.9 min for one provisioning flight (mean 8.7 min, n=25) and remained 10–105 sec within the nest to deposit nectar and pollen (mean 48 sec, n=25). The female of nest 4 returning from a provisioning flight entered the nest head first to regurgitate nectar before she came out, turned around at the nest entrance and entered the nest metasoma first to comb pollen out of the scopa (Fig. 20, Suppl. material 3). The first phase lasted about 25 sec (n=2), suggesting that the regurgitation of nectar and the deposition of pollen require a similar amount of time. Turning at the nest entrance was not observed at nest 1. Here, there was probably enough space within the burrow allowing the female to turn inside the nest. At nest 1, the last provisioning flight took place at 7:27 pm (temperature 22 °C) and the first provisioning flight the subsequent day at 8:49 am (nesting site in full sunlight), indicating a remarkably long daily activity of more than 10.5 h under favourable conditions.

Figures 21–26. 

Female mandibles of three closely related Osmia (Melanosmia) species; left=mandible in front view, right=mandible from below: 21–22 O. pilicornis 23–24 O. inermis 25–26 O. parietina. Figures are not to scale.

Figures 27–28. 

Floral morphs of Pulmonaria mollis in top and lateral view: 27 Longistylous flower with anthers deeply hidden within the floral tube 28 Brevistylous flower with anthers located at the entrance of the floral tube.

The females harvested leaf material of Fragaria vesca with their mandibles (Fig. 19). They transported it as small leaf fragments rather than as masticated pellets back to their nest (Figs 17, 18, Suppl. material 4). As cell partitions and nest plug were built mainly from masticated leaves, the leaf fragments were probably chewed to leaf pulp during their processing inside the nest. The collection of leaf pulp by the female of nest 1 lasted 5–155 sec per flight including flight time (mean 27 sec, n=21) and its processing inside the nest 5–355 sec (mean 75 sec, n=21).

A female of the chrysidid wasp Chrysura hirsuta (Gerstaecker) was observed to visit nest 1 on 11 May 2015 three times in succession (Fig. 37). During the first visit, the wasp lurked at a distance of 2 cm from the nest entrance during 19 min, before she entered the nest headfirst and left it a few seconds later backward. During the second visit, the wasp intruded the nest again headfirst, backed out and immediately entered the nest a second time, but now backward, thereby possibly laying an egg into the brood cell; after about 15 sec she reappeared at the nest entrance and flew away. During the third visit, the wasp was collected for identification. Similarly, females of C. hirsuta were repeatedly observed in the close neighbourhood of nest 6 on four days from 2 to 21 April 2016. In contrast to nest 1, however, C. hirsuta was not observed to approach or enter the nest.

Five out of the six brood cells of nests 1–3 discovered in 2015 were parasitized, suggesting a high pressure exerted by brood parasites on Osmia pilicornis. In all three cells of nest 1, including the cell that was entered by Chrysura hirsuta (see above), females of the ichneumonid wasp Hoplocryptus confector (Gravenhorst) developed. A female of the same ichneumonid species emerged from the rear cell of nest 2, while the front cell contained inside the larval cocoon of O. pilicornis a cocoon with a dead chrysidid wasp larva most probably of C. hirsuta. The only unparasitized cell was the single cell of nest 3, in which a male of O. pilicornis developed.

As a possible further brood parasite we noticed a single triungulin larva of Meloe violaceus Marsham (Meloidae) adhering to the body of a male of Osmia pilicornis.

On their search for females, males of Osmia pilicornis patrolled flowers of Pulmonaria in a rapid flight, which was regularly interrupted by short resting periods at sunny spots on or near the ground, such as fallen leaves, flexed grass blades or tree stumps (Fig. 38). These resting periods lasted from 10 to 40 sec. In one case a male returned to his resting place every 5-7 min at least ten times in succession, which suggests that this male patrolled along a more or less fixed circular route. The males occasionally discontinued their patrolling flights to suck nectar on flowers of Pulmonaria (Fig. 39). Males were never observed to search for females at or near nests, suggesting that (potential) nesting sites do not serve as rendezvous places for the two sexes. Three copulations were observed: in two cases the male grabbed the female during her visit to a Pulmonaria flower, in another case the male grasped the female in flight immediately after she had left a Pulmonaria flower. In all three cases the pair fell on the ground, where the copulation was completed within 50 to 80 sec (Fig. 40).

During the study period, Osmia pilicornis was observed at nine different localities in Germany, Austria and Switzerland. At six further localities, where we recorded the species after 1990, its presence could not be confirmed. All localities including the unconfirmed ones were open-structured, deciduous woodland sites rich in dead wood between 330 and 650 m a.s.l. on rather nutrient-rich soil with a well developed layer of herbaceous spring flowers (Figs 42–47). More specifically, we found the species along forest edges (n=5) and broad forest tracks (n=3), on clear-cuttings and clearings (n=4) and in thinned forests (n=2) and coppiced woodland (n=1).

The maximal number of individuals of Osmia pilicornis recorded at the same locality and date was three females and two males, while at most localities only one or two individuals were simultaneously observed, indicating very low population densities at the study sites.

Our discovery of six nests at two localities in Germany and Austria built by at least five different bee individuals clearly show that females of Osmia pilicornis gnaw their nest burrows in dead wood. Thus, our findings contradict the assumption that O. pilicornis constructs its brood cells under loose bark (Amiet et al. 2004) or within preexisting insect borings (Earwaker 2014, Falk and Lewington 2015), but instead confirm observations in Britain from the nineteenth century that O. pilicornis burrows in dead wood (Wallis 1886, Perkins 1891). Our findings also explain why the repeated attempts to establish O. pilicornis in trap nest composed of drilled borings in dead wood or hollow stems all have failed (R. Earwaker personal communication, R. Prosi and A. Müller unpublished data).

Interestingly, the female mandibular shape of Osmia pilicornis differs from that of other O. (Melanosmia) species. Compared with closely related species that also use leaf pulp as nest building material but exclusively nest in preexisting cavities such as O. inermis (Zetterstedt) or O. parietina Curtis (Frey-Gessner 1880, Priesner 1981, Westrich 1989, Else and Edwards 1996, Hicks 2009, A. Müller unpublished data), the mandibles of O. pilicornis are relatively shorter, more compact and apically less curved resulting in a chisel-like shape that appears to be well suited to tunnel out burrows in dead wood. In addition, the distinctly broader and more inflated mandibular base suggests the presence of strong mandibular muscles, which might provide the needed strength to burrow even in hard wood. We thus hypothesize that the peculiar mandibular shape in females of O. pilicornis is an adaptation to the species’ specialized nesting behaviour.

All six nests were in dead branches, which had a diameter of 1.5–6.1 cm, lay on the ground and were largely hidden under vegetation. A nest found by Chambers (1949) was also in a broken branch and we often observed females that searched for suitable nesting sites flying close to the ground and patrolling 2-10 cm thick branches. Nest-searching females were also repeatedly seen to fly or crawl under vegetation or grass tussocks. Thus, dead branches of moderate diameter lying on the ground and partly hidden by vegetation appear to represent the typical nesting site of Osmia pilicornis. The discovery of a nest of O. pilicornis in a tree stump by Blüthgen (1919), however, suggests that dead wood other than broken branches might occasionally also serve as nesting site.

The six nests were carved out in dead branches of Fraxinus excelsior Linné, Picea abies (Linné) Karsten, Salix spec. and Corylus avellana Linné. The nest detected by Blüthgen (1919) was in a stump of Fagus sylvatica Linné and Chambers (1949) assumed the nesting branch he found to probably originate from Quercus. The different kinds of wood selected as nesting substrate indicate that Osmia pilicornis is flexible in its choice of wood for nesting. A similar flexibility was observed with respect to the direction of the nest entrance and the hardness of the nesting wood. The nest entrances were directed from northeast to south-southwest and both hard and soft wood was used as nesting substrate.

The nests of Osmia pilicornis discovered in the course of our study contained only one to three brood cells, which is in line with Chambers (1949), who reported two cells for the nest he discovered. Thus, the construction of few-celled nests appears to be a typical trait of O. pilicornis. As females of osmiine bees usually lay up to 20 eggs during their lifetime under natural conditions (Raw 1972, Hawkins 1975, Correia 1981, Haeseler 1982, Müller 1994), a female of O. pilicornis is expected to construct a number of nests in several branches during her flight period.

In all nests the main part of the nesting burrow, which harboured the brood cells and ran in longitudinal direction of the branch, deviated from the short entrance part by an angle of 90-140°. This suggests that the females gnawed into the wood until they reached a sufficient depth to continue their tunneling in longitudinal direction, which possibly might have been less labour intensive than gnawing perpendicular to the wood fibers.

We identified leaves of Fragaria vesca as material to construct both cell partitions and nest plug. Interestingly, Westrich (2010) observed a female of Osmia pilicornis collecting leaf pulp on the same plant species, suggesting that Fragaria vesca might play a prominent role as source of nest-building material.

The nesting biology of Osmia species of the subgenus Melanosmia is diverse (Cane et al. 2007, Müller 2016, and references therein). While the majority of species construct cell partitions, nest plug or entire brood cells with leaf pulp as O. pilicornis, others combine leaf pulp with wood fibers, pith, mud, sand grains or small pebbles, or exclusively use mud as nesting material, sometimes under addition of small pebbles. Although information on the nesting sites is available only for about 50 of the 135 O. (Melanosmia) species (Cane et al. 2007, Müller 2016, and references therein), most species appear to nest in preexisting cavities, such as insect borings in dead wood, hollow stems, holes and fissures in stones and walls, abandoned nests of other aculeates or deserted galls. Others attach their brood cells to the underside of stones, hide them among dense vegetation and litter or excavate their own burrows in the ground. So far, only two O. (Melanosmia) species are reported to occasionally gnaw burrows in dead wood. The Nearctic O. bucephala Cresson was observed to tunnel out burrows in a maple tree (Packard 1868) and the Holarctic O. nigriventris (Zetterstedt) was found to nest in burrows in bark and decaying wood, which were at least partly excavated by the female bees (Frey-Gessner 1881, Friese 1911, Amiet et al. 2004). However, at least the former species also colonizes preexisting burrows in dead wood or trap nests (Krombein 1967, Sheffield et al. 2003), suggesting a considerable intraspecific flexibility in nest site selection. To the best of our knowledge, excavation of burrows in dead wood is unknown in osmiine bees other than Melanosmia. Thus, the nesting behaviour of O. pilicornis, which constructs its brood cells most probably exclusively in self excavated burrows in dead wood, appears to be unique among both Melanosmia and the Osmiini, contributing a further facet to the astonishingly diverse nesting biology of the osmiine bees.

No brood parasites have been recorded for Osmia pilicornis so far. However, Blüthgen (1919) found larvae of Meloe beetles (Meloidae) adhering to the body of several specimens of O. pilicornis as we did in the present study, suggesting that these parasitic beetles might occasionally develop in nests of O. pilicornis. However, as osmiine bees have never been recorded as hosts of Central European Meloe species (Lückmann and Niehuis 2009), the beetle larvae might have used O. pilicornis simply for dispersal.

Our observation of Chrysura hirsuta approaching, entering or flying in close neighbourhood of nests of Osmia pilicornis as well as the discovery of a dead chrysidid wasp larva inside a brood cell of nest 2 clearly reveals that O. pilicornis is a host of this chrysidid wasp species. Chrysura species are metaparasitoids of osmiine bees (Krombein 1967, Morgan 1984, Kunz 1989). After eclosion, their larvae attach to the body of the feeding bee larva with their mandibles and start devouring its host only after the latter has eaten the entire food provision and spun a cocoon. In the literature, several osmiine bee species are mentioned as hosts of C. hirsuta. However, not all of these records seem to be reliable as they are either based on personal communication rather than direct observation by the authors, are not detailed enough to judge their reliability or rely on the mere coocurrence of wasp and bee at the same place. Three osmiine bee species are confirmed hosts since C. hirsuta has been reared from their nests: Hoplitis(Alcidamea) tuberculata (Nylander) (Trautmann 1918), Osmia (Melanosmia) inermis (Zetterstedt) (Xambeu in Buysson 1891, Evans 1896 as Osmia parietina, Maneval in Berland and Bernard 1938, Morgan 1984, Falk and Lewington 2015) and Osmia (Melanosmia) xanthomelana (Kirby) (Trautmann 1918, Enslin 1920). Given their close relatedness with some of the above hosts and their main occurrence in mountainous or northern regions where C. hirsuta has its main distribution (Kunz 1989), the following species are probably also hosts of C. hirsuta, although no proof for the wasp’s development within their nests exists: Osmia (Melanosmia) nigriventris (Zetterstedt) (Zetterstedt in Berland and Bernard 1938, Trautmann 1927, Stoeckhert 1933, Paukkunen et al. 2014), Osmia (Melanosmia) parietina Curtis (Smith in Trautmann 1927, Dalla Torre in Berland and Bernard 1938, Falk and Lewington 2015) and Osmia (Melanosmia) uncinata Gerstäcker (Forsius in Trautmann 1927, Falk and Lewington 2015). Possible further hosts are Hoplitis (Anthocopa) villosa (Schenck) (Maneval in Berland and Bernard 1938), Osmia (Helicosmia) aurulenta (Panzer) (Benoist in Berland and Bernard 1938) and Osmia (Metallinella) brevicornis (Fabricius) (Trautmann 1927). Although the authors report that C. hirsuta has been reared from nests of these latter three species, the evidence is ambiguous as these records are either not based on direct observation by the authors or lack necessary additional information. Chelostoma florisomne (Linné) was also supposed to be a host of C. hirsuta (Berland and Bernard 1938) as the chrysidid wasp was observed to fly in the vicinity of its nests in dead wood (Frey-Gessner 1887). However, as H. tuberculata nested in the same dead wood as C. florisomne according to the author, C. hirsuta probably parasitized the former species. In summary, the above list of confirmed, probable and possible hosts includes numerous Melanosmia species, suggesting that species of this subgenus are the main hosts of C. hirsuta. Our new finding that C. hirsuta also attacks the nests of O. pilicornis supports this assumption.

In four brood cells of Osmia pilicornis in nest 1 and 2 the ichneumonid wasp Hoplocryptus confector developed. Hosts of this parasitoid wasp are mainly stem-nesting aculeate hymenopterans (Schwarz 2007 and references therein). As the wasp is known to parasitize species of both osmiine bees and chrysidid wasps, it remains unclear whether O. pilicornis served as host or whether the wasps developed as hyperparasitoids on the larvae of Chrysura hirsuta.

Flower visiting females of Osmia pilicornis exhibited a distinct preference for Pulmonaria flowers in the earlier red stage and only occasionally visited flowers in the later blue stage. As both stages were found to offer nectar in similar quantities, the near exclusive exploitation of flowers in the red stage is most likely due to differences in the amount of pollen. In fact, flowers in the red stage contained much more pollen than flowers in the blue stage, which were usually almost devoid of pollen due to their older age and the putatively higher number of flower visits they already had experienced. The forceful opening of Pulmonaria flower buds seems to be a regular behaviour as it could be observed in numerous females exploiting three different Pulmonaria species at three localities. Undoubtedly, this behaviour is highly adaptive as it enables the females to collect large quantities of pollen within a short period of time.

Our results on the proportion of longistylous and brevistylous flowers visited by females of Osmia pilicornis are ambiguous. The females preferred brevistylous over longistylous flowers during the first observation period, but exhibited no preference for one of the two floral morphs during the second observation period. We hypothesize that this incongruency might possibly be due to Anthophora plumipes, which was common at the study site during the first but entirely lacked during the second observation period. Like O. pilicornis, females of A. plumipes are morphologically adapted to remove pollen out of the narrow Pulmonaria flower tubes (Müller 1995). Since O. pilicornis and A. plumipes are among the few flower visitors that can efficiently exploit longistylous flowers for their deeply hidden pollen, longistylous flowers should be more attractive to these two species due to their higher standing crop of pollen compared to brevistylous flowers, which can be exploited for pollen by a larger proportion of flower visitors. Thus, the numerous females of A. plumipes that visited the Pulmonaria flowers earlier in the season might possibly have preferentially emptied longistylous flowers, thereby indirectly forcing the few females of O. pilicornis to exploit a larger proportion of brevistylous flowers. Later in the season, when A. plumipes was no longer present, a higher proportion of unemptied longistylous flowers was available for O. pilicornis resulting in a more balanced visitation rate of longistylous and brevistylous flowers.

Females of Osmia pilicornis collected pollen on nototribic flowers of the Lamiaceae with the aid of their facial pilosity. In contrast to numerous other bee species, which are equipped with a specialized pollen-collecting apparatus on clypeus and/or frons to harvest pollen on nototribic flowers (Müller 1996, Rightmyer et al. 2011), the facial hairs of O. pilicornis are not specialized and correspond in length, density and shape to those of other O. (Melanosmia) species, such as O. inermis or O. parietina, which do not harvest pollen from Lamiaceae but instead exploit mainly Fabaceae (Westrich 1989, Müller 2016).

In summary, females of Osmia pilicornis are highly adapted to efficiently collect pollen in several respects. On flowers of Pulmonaria, they use specialized bristles on the proboscis to brush pollen out of the corolla tubes, they almost exclusively exploit pollen-rich flowers in the red stage and they often steal pollen from still closed flowers by forcefully opening buds. On flowers of Lamiaceae, they remove pollen from the nototribic flowers by applying a specialized behaviour that involves the facial pilosity.

Although still only fragmentarily known, the mating strategy of the males of Osmia pilicornis seems to correspond to that of other osmiine bee species, such as Hoplitis zandeni (Teunissen and Van Achterberg) or Osmia spinulosa (Kirby). In these two species, both of which nest in empty snail shells (Müller 1994, Müller and Mauss 2016), the males also patrol flowers of the female host plants in a fast flight along more or less fixed circular routes regularly interrupted by short resting periods on the ground. As in O. pilicornis, they do not search for females at potential nesting sites, probably since the habitat of the former species contains huge numbers of empty snail shells making nest localization difficult and the females of the latter species exhibit strict pollen specialization making female detection straightforward. A similar discrepancy in the probability of finding females at Pulmonaria flowers and the widely scattered and inconspicuous nesting sites, respectively, might be the reason why flowers seem to serve as exclusive rendezvous places for the two sexes of O. pilicornis. In fact, the amount of dead fallen branches was often large in habitats occupied by O. pilicornis making it almost impossible for the males to localize nests. As the males of O. pilicornis were not observed to defend their flight routes against conspecifics as for example in O. (Melanosmia) maritima Friese (Haeseler 1982), their mating strategy might be best described as scramble competition polygyny (Thornhill and Alcock 1983).

The males of Osmia pilicornis are characterized by numerous long hairs along the entire posterior margin of their antennae (Fig. 41). The function of this specialized pilosity, which is unique among osmiine bees and gave O. “pilicornis” its species epithet, is unknown. As these hairs are absent in the female sex, they might possibly help in the perception of the females or play a role during copulation or courtship.

Our observations confirm other authors, who assume Osmia pilicornis to be strongly restricted to woodland habitats (Stoeckhert 1933, Westrich 1989, Amiet et al. 2004, Falk and Lewington 2015). This clear habitat preference, which appears to be valid across the species’ entire range (Figs 48, 49), may primarily be explained by the need for dead fallen branches as nesting sites. O. pilicornis never colonizes closed and dark forests but instead exhibits an exclusive preference for semi-open, sunny, rather warm and deciduous woodland habitats rich in dead wood below 1000 m a.s.l. The fact that the species usually colonizes mesophilous rather than xeric woodland is probably due to its need of a rich supply of pollen hosts flowering in early spring, which requires nutrient-rich edaphic conditions. The spectrum of open-structured woodland types colonized by O. pilicornis is surprisingly wide and ranges from clearings and clear-cuttings over forest edges and forest tracks to thinned and coppiced woodland, suggesting that dead fallen branches and a rich spring flora in combination with a rather warm microclimate are the only requisites needed by the species. Interestingly, the majority of woodland habitats currently colonized by O. pilicornis in Central Europe owe their origin to human forest use. This raises the question about the primary habitats of the species in Europe in prehistoric times. We hypothesize that the primary habitats of O. pilicornis were either windfalls or woodland sites kept open by the former grazing megafauna (A. Nilsson personal communication, Bunzel-Drüke et al. 2008).

Populations of Osmia pilicornis at our study sites were remarkably small and often contained only single individuals. Such low population densities seem to be typical for O. pilicornis throughout its range. Of 516 records of O. pilicornis that could be assigned an exact date and contained information on individual numbers (see list of all distributional data in the Suppl. material 1), 91.1% had 1-2 individuals, 7.7% 3-5 individuals and 1.2% 6-11 individuals recorded at the same locality and date. The highest number of females simultaneously observed at the same site was 7 and the highest number of males was 10. These overall very low population densities are probably not due to food limitation as Pulmonaria often occurs in large stands and O. pilicornis is able to collect pollen from a variety of plant taxa. Nor do they seem to be explained by special characteristics of the wood used for nesting as O. pilicornis was found to nest in dead branches of different species and of varying hardness. Instead, we hypothesize that a high pressure by brood parasites might possibly be an important reason contributing to the low population densities. In fact, five out of the six brood cells of nests 1-3 were found to be parasitized. A high brood-parasite pressure would also explain why the females of O. pilicornis construct nests that contain only few brood cells, thereby possibly reducing the risk to loose all progeny after a brood parasite has detected a nest.

Populations of Osmia pilicornis have regressed in many regions of Europe over the last decades resulting in the species’ inclusion in numerous regional or national red lists (Amiet 1994, Winter 1994, Dathe and Saure 2000, Saure 2005, Westrich et al. 2000, 2008, Van der Smissen 2001, Theunert 2002, Mandery et al. 2003, Burger et al. 2004,Burger 2005, Straka 2005, Tischendorf et al. 2009, Esser et al. 2010, Earwaker 2012). In our study, O. pilicornis was no longer found at six out of 15 localities, although the former were still colonized 3-25 years ago, supporting its status as a declining and threatened species. Although this recent decline is poorly understood, changes in woodland management practices might be an important reason, in particular the abandonment of the area-wide but little intensive former human forest use, which led to the closed and dark forests now prevailing in most regions of Central Europe at lower elevations. Due to the species’ overall rareness and low population densities, its widely scattered and often highly localized occurrence and the reported recent declines, current Central European populations of O. pilicornis need regular monitoring and deserve protection. Measures to conserve and foster O. pilicornis should focus on the preservation of the semi-open structure and the rich supply of dead wood in currently colonized woodland habitats as well as the new creation of such habitats by thinning or clearing of woodland on nutrient-rich soils at warm but not xeric sites, preferentially in close neighbourhood to existing populations of O. pilicornis.