Osmia (Melanosmia) uncinata Gerstäcker is a Palaearctic megachilid bee distributed from temperate and northern Europe eastwards to the Russian Far East. The discovery of over 80 nests in Switzerland, southern Germany and Scotland enabled for the first time a closer investigation of its nesting biology and prompted the assessment of the species’ phenology, distribution and habitat. O. uncinata nested in self-excavated burrows inside the bark of both living trunks and dead stumps of Pinus sylvestris. The nests were excavated at a height of 10–220 cm above ground either on the underside of prominences of longitudinal bark ribs or inside beetle borings and extended more or less vertically upwards. They consisted of a single straight to slightly curved burrow with rarely one to three side burrows, had a total length of 1.2–12.0 cm and contained 1–6 brood cells. The brood cells, which faced downwards with the larval provisions being located in the upper cell half, were separated from each other by one-layered walls of chewed leaves (“leaf pulp”). The nests were sealed with a plug of 2–4 closely adjacent walls of leaf pulp. DNA metabarcoding of cell and plug walls revealed that Potentilla and Fragaria (Rosaceae) served as leaf pulp sources. Pre-imaginal mortality amounted to 77%, partly caused by brood parasites such as Sapyga similis (Sapygidae) and Cacoxenus indagator (Drosophilidae) or predators such as snakeflies (Raphidioptera). At low elevations, O. uncinata needs one year for its development and overwinters as imago inside the nest, whereas in the subalpine zone of the Alps it has a two-year cycle passing the first winter as prepupa and the second winter as imago. O. uncinata starts to emerge between the end of March at low elevations and the end of May at higher elevations qualifying as an early flying bee like the other European O. (Melanosmia) species. The distribution of O. uncinata in Central Europe and Scotland largely coincides with the occurrence of P. sylvestris. As in the pine, it extends over a wide altitudinal range from below 100 m up to 1900 m a.s.l. and encompasses dry and wet as well as warm and cold habitats including open pine forests, inner and outer forest edges dominated by pine and isolated pine groups. At a few locations in the subalpine zone of the Alps, O. uncinata occurs in the absence of P. sylvestris; here, the thick bark of Larix decidua serves as a substitute nesting substrate.
