Female of Osmia (Osmia) mustelina Gerstaecker, 1869 at her nest located in a fissure between stones (left) and collecting leaf pulp on Helianthemum used for nest construction (right). Fotos A. Krebs.

Nesting biology

According to our present knowledge (Friese, 1923; Malyshev, 1937; Krombein, 1967; Westrich, 1989; O’Toole and Raw, 1991; Müller et al., 1997; Cane et al., 2007), the majority of the Osmiini build their brood cells in preexisting cavities, such as insect borings in dead wood, hollow plant stems or rock crevices. Nesting in empty snail shells is widespread at least among the palaearctic osmiine bees and has probably independently evolved several times (Westrich, 1989; O’Toole and Raw, 1991; Müller, 1992, 1994). Other osmiine bee species excavate their brood cells in plant stems and rotten wood, dig them in the ground or glue them freely to stones. As diverse as the nesting sites is the material used for brood cell construction. Depending on the species, cell partitions, nest plug or entire brood cells consist of mud, pebbles, leaves, resin, flower petals, glandular secretions or mixtures of these products.

Flower preferences

Many osmiine bees are strictly dependent on certain plant species for pollen gathering (Krombein et al., 1979; Westrich, 1989; Sedivy et al., 2008). Among the Central European osmiine bee species, about 55% are supposed to be pollen specialists at the level of plant genus or plant family (Müller et al., 1997). Important pollen sources for these oligolectic species are flowers of the Asteraceae, Fabaceae, Campanulaceae, Boraginaceae, Dipsacaceae and others. The intimate relationships of the osmiine bees to flowers is perhaps best exemplified by those species that are equipped with morphological tools on head or mouthparts for gathering pollen from anthers hidden below an upper lip or concealed within deep tubular flowers (Peters, 1974; Müller, 1995, 1996, 2006).