Genus Osmia

Nest of Osmia (Helicosmia) leaiana (Kirby, 1802) in a hollow stem; the brood cell provisions consist of pollen of Carduoideae (Asteraceae) and the cell partitions are made of chewed leaves. Foto A. Krebs.

Biogeography and diversity

The genus Osmia Panzer is distributed in the Palaearctic and the Nearctic region, a few species also occur in the Oriental and Neotropical region (Michener, 2007; Ungricht et al., 2008).

Osmia is the second most diverse osmiine genus in terms of number of species: 347 species have been described so far, 206 of which occur in the Palaearctic. At least 4 Palaearctic species are still undescribed.

Phylogeny and classification

The genus Osmia, whose monophyly is well supported, is sister to the clade Atoposmia+Ashmeadiella (Praz et al., 2008b). It was subdivided into 19 subgenera by Michener (2007). In contrast to Micheners’ classification, i) Hoplosmia is treated here as a subgenus of Osmia rather than as a genus of its own (Praz et al., 2008b), ii) the subgenus Nasutosmia, which was formerly treated as subgenus of Hoplitis (Michener, 2007), recently turned out to belong to the genus Osmia (Praz et al., 2008b), iii) the subgenus Ozbekosmia is newly synonymized with the subgenus Tergosmia (A. Müller, unpublished), and iv) the subgenera Monosmia and Orientosmia are synonymized with the subgenus Osmia (Haider et al., 2013).

The Palaearctic Osmia species belong to the following 12 subgenera:

Allosmia with 9 described species

Erythrosmia with 4 described species

Helicosmia with 68 described and 1 undescribed species

Hemiosmia with 7 described species

Hoplosmia with 21 described species

Melanosmia with 19 described species

Metallinella with 1 described species

Nasutosmia with 2 described and 1 undescribed species

Neosmia with 8 described and 1 undescribed species

Osmia s. str. with 27 described and 1 undescribed species

Pyrosmia with 30 described species

Tergosmia with 7 described species

Subgenus uncertain with 3 described species

The following subgenera most probably form monophyletic clades (Praz et al., 2008b):

Allosmia+Neosmia

Pyrosmia+Diceratosmia (Nearctic)

Helicosmia+Melanosmia (including the Nearctic Acanthosmioides and Mystacosmia)

Cephalosmia+Trichinosmia (both Nearctic)

Nesting biology

Information on the nesting biology is available for 148 Osmia species belonging to 19 subgenera from the Palaearctic, Nearctic and Neotropical region (A. Müller, C. Praz, J. Neff, G. Le Goff and C. Sedivy, unpublished).

The diversity of nesting sites in the genus Osmia is high and encompasses much of the diversity observed in the osmiine bees. In contrast to the genus Hoplitis, there are no cleptoparasitic species and nesting in empty snail shells is more common. Among Osmia, there are species that

nest in excavated burrows in the soil (e.g. some Tergosmia, most Hemiosmia, some Melanosmia) or occasionally in pithy stems or dead wood (e.g. few Melanosmia);

nest in preexisting cavities as insect burrows in dead wood and pithy stems (e.g. Metallinella, many Helicosmia, Hoplosmia, Melanosmia, Osmia s. str. and Pyrosmia), in galls or in the soil; abandoned cells in exposed nests of other aculeates; in empty snail shells (e.g. Allosmia, Erythrosmia, Neosmiamany Hoplosmia, some Helicosmia and Pyrosmia); in fissures, cracks or holes in rocks (e.g. some Osmia s. str., Pyrosmia and Tergosmia); between vegetative parts (e.g. O. (Melanosmia) xanthomelana, O. (Tergosmia) tergestensis);

build exposed nests (e.g. some Pyrosmia).

Similarly diverse is the nesting material used for the construction of cell partitions and nest plug or for building entire cells. Depending on the subgenus or species, mud and pebbles, leaf pulp, whole petals or pith are used alone or in diverse combinations. In contrast to the species of the Heriades group, for which the use of resin is widespread, the only Osmia species known to collect resin for cell construction is the Nearctic Osmia (Mystacosmia) nemoris (Rust and Clement, 1972).

Many Osmia species nesting in narrow cavities use the nesting material only to delimit the linearly arranged cells by walls and to plug the nest. Some of these species, especially when nesting in larger cavities, have the flexibility to arrange cells more irregularly by partially or wholly constructing also the walls of the cells (e.g. many Osmia and Helicosmia). Other species invariably build their entire cells with foreign material (e.g. Tergosmia with whole petals and mud; some Pyrosmia with leaf pulp and sand; Hemiosmia, few Helicosmia and Osmia s. str. with leaf pulp; some Melanosmia with either mud or leaf pulp).

Flower preferences

To our current knowledge, all categories of bee host ranges as proposed by Müller and Kuhlmann (2008) can be found in the Palaearctic Osmia fauna, except true monolecty and eclectic oligolecty:

narrow oligoleges, which restrict pollen collection to one single plant genus only, e.g. to Cerinthe (Boraginaceae), Onosma (Boraginaceae);

broad oligoleges, which restrict pollen collection to one plant family or tribe, e.g. to Asteraceae, Brassicaceae, Ericaceae, Fabaceae, Hedysareae (Fabaceae);

probable mesoleges, which collect pollen on only two to three plant families; e.g. Fabaceae and Ericaceae;

polyleges with a strong preference, e.g. for Fabaceae, Lamiaceae;

polyleges, which collect pollen on at least four plant families.

Several Osmia species are equipped with specialized bristles on the proboscis (e.g. O. (Helicosmia) cinereaO. (Melanosmia) pilicornis) to remove pollen from anthers hidden in the narrow flower tubes of Boraginaceae (e.g. Pulmonaria). Similarly, several Osmia species of the subgenera Erythrosmia and Helicosmia possess a specialized facial pilosity composed of apically twisted bristles which serve to remove pollen from the nototribic anthers of Lamiaceae and Antirrhineae (Scrophulariaceae).