Renter: Snail shells. Osmia bicolor nests are constructed in empty and generally medium-sized snail shells, e.g. Helix pomatia, Cepaea nemoralis, C. hortensis and Monacha cantiana. Depending on the size of the shell, a completed nest may contain from one to four or five cells.
Nesting material: The cell walls and closing plug are constructed with plant mastic, the female using the outside wall of the shell as a palette. Leaves of the following plant species are used: Potentilla verna, Potentilla reptans, Sanguisorba minor, Rosa canina, Polygonum convolvulus and Fragaria, Glaucium, Ononis, Salix, Polygonum, Vicia). The space between the last cell and the outer partition is usually filled with a rubble containing very small snail shells and pieces of chalk, but it may be plugged simply with soil (Chambers, 1949). Westrich, (1989) additionally lists shells of: Cepaea vinobonensis, Arianta arbustorum and Bradybaena fruticum. Andreas Müller (http://blogs.ethz.ch/osmiini/) lists and illustrates a female nesting in a shell of Arianta arbustorum.The shell surface is plastered with patches of leaf pulp. (Amiet, 1973; Banaszak and Romasenko, 2001; Bellmann, 1981; Benoist, 1931; Friese, 1897b, 1923; Graeffe, 1902; Grozdanic and Vasic, 1965; Müller et al., 1997; O’Toole and Raw, 1991; Schmiedeknecht, 1885-1886; Smith, 1844; Stoeckhert, 1933; Westrich, 1989)
The most remarkable aspect of this bee’s nesting behaviour is the manner in which it camouflages its completed nest. On open downland the shell is usually hidden beneath a mound of dry grass stems or fragments of dead leaves, while on woodland edges, it may also be covered in grass stems (pers. obs.) and sometimes scales from beech buds (R.C.L. Perkins, 1916). On the continent, pine needles are sometimes used for the same purpose (Friese, 1897b, 1898). These mounds are distinctive, resembling miniature nest mounds of the wood ant Formica rufa and are described as being from 10-15 cm round the base and from 5-8 cm high (V.R. Perkins, 1884, 1891). Hundreds of grass stems (some up to 14 cm or more in length) are used for a mound, each stem being added separately. Geiser (1988), Bellmann (1995) and Müller, Krebs & Amiet (1997) illustrate opened snail shells to show nest architecture and various stages of nest construction and camouflaging of the snail shell. Westrich (1989) also describes nesting biology and illustates a camouflaged nest, plus provisioned cells.It is not known what advantage a mound confers upon a nest; later in the year the covering of plant fragments is generally blown away, revealing the shell. F. Smith (1855) found the bee nesting in burrows in the soil; this record requires confirmation.