Home is where the heart is

By Athayde Tonhasca

Most of the 20,000 or so known species of bee build their nests in the ground, excavating tunnels and constructing chambers where they lay their eggs. But one group of species, the mason bees from the genus Osmia, took another path regarding housing. Most of them occupy or expand naturally occurring cavities – such as crevices under or between stones, cracks in a wall, holes in dead wood, hollow stems and tree bark – to transform them into cosy, safe environments in which to raise their young. 

Mason bees are solitary, i.e., each female builds and keeps a nest on her own. But they often nest close to each other, and in large numbers. They release scents to mark their nest entrances, which allow each bee to find her home among many others nearby (the genus Osmia was named after the Greek for ‘odour’; osmonosology is the branch of medicine dealing with organs of smell and olfactory disorders). 

Mason bees are quite resourceful in converting a cavity into a nest: depending on the species, they can use mud, chewed leaf material, pebbles, petals, pith and resin in diverse combinations to build chambers, walls and a plug to seal off the nest. 

A bee house with a section removed to reveal red mason bee (O. bicornis) nest chambers built with mud. The yellow dust is pollen.

Different species of mason bees have specific building skills, which allow them to occupy a variety of habitats. The mountain mason bee (O. inermis), one of Britain’s rarest species, is essentially a boreo-alpine denizen: it lives on wind-swept, open slopes, with not many accommodation options. That’s not a problem for this bee: it nests in crevices on bare, well-drained rocky surfaces.

A mountain mason bee © Arnstein Staverløkk/Norsk institutt for naturforskning, Wikimedia Commons, and its habitat.

An equally rare close relative, the pinewood mason bee (O. uncinata), is at home in areas of Scots pine (Pinus sylvestris) at lower altitudes. Here, the bee makes use of a material in ample supply: tree bark. Females drill their nests in the bark of living trees and dead stumps of Scots pine.

A female pinewood mason bee at her nest entrance © Müller et al., 2020. Alpine Entomology 4: 157–171, and her habitat © Richard Webb, Wikimedia Commons.

The lodging needs of mason bees can be quite specific: the hairy-horned mason bee (O. pilicornis) nests in fallen dead branches, while the large black-bellied mason bee (O. nigriventris) gnaws out its nest in pieces of larch (Larix decidua) and Scots pine lying on sun-exposed ground. 

Several mason bees and related species habitually nest inside empty snail shells. These structures are conveniently shaped to allow the arrangement of brood cells in a row, and easy closure at the shell aperture. Besides, shells are abundant, resistant and long-lasting housing units. In Britain, the gold-fringed (O. aurulenta), the two-coloured (O. bicolor) and the spined (O. spinulosa) mason bees are helicophiles (snail-lovers), and they go to a lot of effort to move shells to a good spot, build and provision the brood cells, seal the nests and sometimes camouflage them. You can watch a two-coloured mason bee at work here.

Opened nests of O. notata with brood cells side by side (14); O. pinguis with brood cells in a row (15); O. aurulentacovered with leaf pulp (16); Hoplitis fertoni with brood cells side by side (17, photo G. Le Goff); O. rufohirta with a single brood cell (18, photo P. Westrich); O. bicolor with a barrier of pebbles and earth (18, photo A. Krebs) © Müller et al., 2018. Journal of Hymenoptera Research 65: 61-89.

While many mason bees look for walls, bark, rocks or shells to find a home, a North American species, O. integra,goes for other more convenient, abundant and easy to work building materials. A female may nest on soft coastal dunes, or inside dried cow pats. Cow dung may seem an unusual choice, but it is soft, has no roots or stones to impede excavation, does not break apart easily once dried, and has good insulation. What’s not to like?

Osmia integra © Laurence Packer, Discover Life, and her home © Karora, Wikimedia Commons.

There are approximately 500 species of Osmia in the world and 12 in Britain. They are excellent pollinators, and some species are being widely used to complement or substitute the efforts of the European honey bee (Apis mellifera) in fruit tree orchards and other crops. These bees’ range of nesting options and their flexibility to adapt to local conditions make them dependable pollination agents.