Megachile genalis is a 10 to 14 millimetre long bee distributed from Western Europe across Turkey to easternmost Russia and Japan. In Central Europe it flies late in the season, from mid-June to early September, and unlike most of its relatives it does not move into ready-made cavities. It constructs its nests exclusively in erect, thick and usually hollow plant stems, and, remarkably, it chooses living rather than dead stems, a habit that sets it apart from every other stem-nesting bee in Europe. The only other bee known to nest in living stems is the North American Megachile montivaga, which nests in green thistle stems but has a broader nesting niche, also using the ground and trap nests.

The species is red-listed throughout Europe, including in Belgium, the Czech Republic, Germany, Slovakia and Switzerland. In Switzerland it is one of the rarest bees of all, recorded only six times in the Grisons between 1932 and 2019, in the Lower Engadine and the Albula Valley. A targeted search in 2018 during the preparation of the Swiss red list of bees failed to rediscover it in the Lower Engadine, leaving the Albula Valley as the only region where the species was believed to survive. The new study set out to map its current distribution, clarify its habitat requirements and shed light on its poorly known nest architecture as a foundation for conservation.

Between 23 July and 5 October 2023, the researchers spent 14 days and 105 hours searching 26 subplots across five study areas, examining every plant stem with an outer diameter of at least 10 millimetres for the characteristic oblong-oval nest openings that female M. genalis gnaw into stems with their mandibles. They found 205 nests, 141 in living and 64 in dead stems, at elevations between 600 and 1270 metres. The Albula Valley emerged as the stronghold with 176 nests, followed by the Lower Engadine with 23, the Domleschg with 5 and the Ruinaulta with a single nest. No nests were found around Chur, most probably because suitable erect stems were almost entirely absent there.

Every single nest was built in the living stem of one plant: Peucedanum verticillare, a monocarpic umbellifer of the carrot family that grows vegetatively for two to four years, reaches two to three metres in height, flowers once and then dies. Its hollow stems grow on fallow land, road and railway embankments, scree slopes, ruderal areas and forest clearings, habitats the plant tolerates precisely because they are neither mown nor grazed. The bee’s nesting habitat therefore coincides exactly with the growth sites of this giant herb, and the distributions of the two species largely overlap in the Grisons. Since P. verticillare also colonises naturally disturbed habitats such as scree slopes and forest clearings, the bee’s occurrence in the Alps may predate the Neolithic opening of the forested landscape by humans.

The nests themselves proved surprisingly variable in architecture. Each consisted of a nest opening gnawed by the female at a height of 22 to 217 centimetres above ground, an average of 8.6 millimetres long and 5.9 wide and predominantly oriented towards the east to south sector; a nest plug built 1.5 to 45 centimetres below the opening from pith particles, leaf fragments, grass fibres or combinations of these; one to 14 brood cells, usually arranged in a single cigar-like series lying up to 99 centimetres below the entrance; and, in many nests, a facultative basal plug of pith or leaf fragments. The brood cells lay on average half a metre below the opening, so females that placed their cells in lower internodia had to gnaw through the thin transverse pith partitions separating the stem segments. The researchers suggest the females gnaw their entrance in the upper, thinner part of the stem, where the hard wall is easier to penetrate, even though the cells must then be placed lower down, where the stem is wide enough.

The female’s mandibles appear tailor-made for this carpentry. Compared with nine closely related Megachile species that nest in preexisting cavities, the mandibles of M. genalis are strongly inflated at the base, shorter relative to their basal width, and barely curved inwards at the tip. The inflated base likely houses powerful adductor muscles capable of exerting high pressure on the stem surface, while the compact, chisel-like shape transmits that force efficiently onto the plant tissue. A striking parallel exists in the mason bee Osmia nigriventris, whose females gnaw burrows into hard wood with similarly shortened, almost straight mandibles, whereas related cavity-nesting species have longer, inwardly curved jaws, supporting the interpretation that the peculiar mandible of M. genalis evolved for penetrating living stem walls.

DNA metabarcoding of the leaf fragments lining 157 brood cells from 20 nests showed that the females built their cells exclusively from leaves of the rose family and, rarely, hazel. Among the Rosaceae, dog rose, raspberry and blackberry dominated, together accounting for 85.2 percent of sequence reads, with rose leaves recorded in 80 percent of nests and rosaceous leaves in all 20 nests analysed. The pollen analysis was equally striking: of 65 brood cells examined microscopically, more than 99 percent of the pollen volume came from the daisy family, and pollen of thistles such as burdock, musk thistle and creeping thistle alone made up 72.2 percent, recorded in every brood cell. This confirms that M. genalis is oligolectic on Asteraceae with a strong preference for thistles, a preference echoed by observations in eastern Germany, Poland, Austria and elsewhere.

The study also uncovered a sobering picture of the bee’s reproductive success. Of 284 brood cells examined, at least 56 percent failed due to parasitism, mould or unknown causes, and reproduction completely failed in 21 of 46 nests. The most harmful enemy was Melittobia acasta, a tiny eulophid wasp that infested 19 percent of brood cells and 28 percent of nests, followed by ichneumonid wasps, the cuckoo bee Coelioxys inermis, a blister beetle and a bee fly. Given an average of six brood cells per nest and assuming females build only one or two nests in their lifetime, the reproductive output averaged no more than two to five offspring per female, far below the 29 percent average brood mortality reported across 147 solitary bee species in a recent metastudy.

The conservation implications follow directly from the bee’s threefold dependence. While the rosaceous leaves it needs for nest building are ubiquitous, both its pollen hosts and, above all, its nesting sites are scarce. Conservation should therefore focus on promoting thistles and, crucially, on maintaining and creating habitats for Peucedanum verticillare close to Asteraceae-rich areas. Because the plant tolerates neither regular mowing nor grazing, many of its stands lie on fallow land at risk of becoming overgrown, as sporadic management is not rewarded by agroecological schemes. The fate of this rare bee in the Grisons will depend on whether a balance can be struck between complete abandonment and annual mowing of the transitional habitats between forest and open land that the giant umbellifer, and with it the bee, requires to survive.

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

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

Article References: Müller, A., Weibel, U., & Lenz, R. (2024). A threefold plant specialist – distribution, habitat requirements and nesting biology of the rare leafcutter bee Megachile genalis in the eastern Swiss Alps (Hymenoptera, Megachilidae). Alpine Entomology, 8, 131-146. https://doi.org/10.3897/alpento.8.137006

Image Credits: AI Generated

DOI: 10.3897/alpento.8.137006

Keywords: threefold, plant, specialist, distribution, habitat, requirements, nesting, biology, rare, leafcutter, Megachile, genalis