A threefold plant specialist – distribution, habitat requirements and nesting biology of the rare leafcutter bee Megachile genalis in the eastern Swiss Alps (Hymenoptera, Megachilidae)

The leafcutter bee Megachile genalis Morawitz, which nests in thick, erect and usually hollow plant stems, is rare and endangered throughout Europe. In Switzerland, it was recorded only six times in the Grisons between 1932 and 2019. In order to create the necessary basis for the conservation of this rare bee, its distribution, habitat requirements and nesting biology were investigated in the eastern Swiss Alps by systematically searching for its nests, by DNA metabarcoding of the cell-building material, by analysing the pollen contained in abandoned brood cells and larval faeces, and by deducing aspects of the species’ nesting biology from collected old nests. A total of 141 new and 64 old nests were discovered at elevations between 600 m and 1270 m in – with decreasing frequency – the Albula Valley, the Lower Engadine, the Domleschg and the Ruinaulta. The nests were exclusively built in living stems of Peucedanum verticillare (Apiaceae), an up to 2–3 m tall umbellifer that grew on fallow land, road and railway embankments, scree slopes, ruderal areas and forest clearings. The leaf fragments used by the females of M. genalis to construct the brood cells in 20 selected nests were largely from shrubs and trees of the Rosaceae (e.g. Rosa, Rubus, Prunus, Sorbus), rarely also from Corylus (Betulaceae). The pollen in 65 brood cells was exclusively collected on thistles (e.g. Arctium, Carduus, Cirsium) and other Asteraceae. The nests were characterised by a surprisingly high variability in their architecture. They consisted of i) an approximately 6 mm x 9 mm large nest opening gnawed by the female bee at a height of 22–217 cm above ground with her mandibles, which are well adapted to penetrate the hard stem walls due to their enlarged base indicating strong adductor muscles and the short and chisel-like shape; ii) a 5–25 mm thick nest plug built 1.5–45 cm below the nest opening from pith particles, leaf fragments and/or grass fibers; iii) 1–14 brood cells usually arranged in one cigar-like series and lying 11–99 cm below the nest opening; and iv) a facultative 4–40 mm thick basal plug consisting of pith particles and/or leaf fragments. Brood mortality was high: at least 56% of 284 brood cells were unsuccessful due to parasites, mould or unknown reasons, and reproduction completely failed in almost half of 46 nests. The most harmful brood parasite was Melittobia acasta Walker (Hymenoptera, Eulophidae), which infested 19% of the brood cells and 28% of the nests. Measures to conserve the population of M. genalis in the eastern Swiss Alps should focus on the conservation and propagation of suitable habitats for P. verticillare in close vicinity to Asteraceae-rich areas and on the promotion of thistles.