Nesting in bark – the peculiar life history of the rare boreoalpine osmiine bee Osmia (Melanosmia) nigriventris (Hymenoptera, Megachilidae)

Osmia (Melanosmia) nigriventris (Zetterstedt) is a predominantly boreoalpine megachilid bee species, whose biology is poorly known due to its pronounced rarity all over Central Europe. The discovery of 19 nests in the Grisons and Valais (Switzerland) allowed for the investigation of its nesting biology and prompted the assessment of the species’ host-plant spectrum, phenology and distribution. All nests were in thick bark pieces of Larixdecidua, which lay in grassy vegetation on sun-exposed ground of open subalpine forests dominated by larch. The nests contained 1–26 brood cells constructed within one to several burrows tunneled out by the female bees with their specialized mandibles, which are distinctly stronger than those of related O. (Melanosmia) species known to nest in preexisting cavities or loose soil. The linearly arranged brood cells were separated from each other by three-layered partitions consisting of an interlayer of densely packed small bark particles sandwiched between two thin layers of chewed green leaves (“leaf pulp”). DNA metabarcoding of several nest plugs revealed that Potentilla (Rosaceae) and Helianthemum (Cistaceae) served as source of the leaf pulp. Anthraxanthrax (Bombyliidae) and Sapygasimilis (Sapygidae) parasitized the brood cells of O.nigriventris as shown by DNA barcoding of prepupae overwintering in the host’s nests. O.nigriventris is mesolectic and harvests pollen almost exclusively on Fabaceae (e.g. Lotus, Hippocrepis), Ericaceae (Rhododendron, Vaccinium) and Cistaceae (Helianthemum). Depending on the altitude, O.nigriventris emerges from the beginning of May to the first decade of June and thus qualifies as an early flying bee active in spring and early summer similar to the other European O. (Melanosmia) species. It likely needs two years for its development in the subalpine zone of the Alps and overwinters as prepupa in the first and probably as imago in the second winter. O.nigriventris has a Holarctic distribution, its disjunct Palaearctic range encompasses the boreal zone from Scandinavia eastwards to the Russian Far East, the Caucausus, the Alps as well as scattered locations at lower altitudes throughout Central Europe, where it is regarded as a glacial relict and has probably suffered strong declines during the last decades.