UK Bee Species

Blue Mason Bee (Osmia caerulescens)

Osmia caerulescens (Linnaeus, 1758) · subfamily Megachilinae · family Megachilidae

A dark blue bee with a bright metal sheen

The blue mason bee was described by Carl Linnaeus in 1758 in the founding work of zoological naming.^[3] Its species name, caerulescens, is Latin for "becoming blue", after the striking metallic dark blue sheen of the female, and the source of both its English name and its wider Continental name of steel-blue mason bee.^[3] The genus name Osmia is Greek for "odour", after the citrus-and-honey scent some mason bees use to mark their nest entrances, a feature shared across the genus.^[7][4] Females are ten to eleven millimetres long, dark and dense-bodied, with a black scopa of pollen-collecting hairs slung underneath the abdomen; males are smaller, at eight to nine millimetres, slimmer and lit by pale hair on a bright copper-green body.^[2]

Mason bees, and how they work

The blue mason bee belongs to the Megachilidae, the family of mason, leafcutter and resin bees.^[7] Members of the family share a signature feature: they carry pollen not on their legs, as most other bees do, but in a brush of hairs, the scopa, slung beneath the abdomen, so a foraging female shows a broad belt of pollen along her belly.^[7] Mason bees, more narrowly, are members of the genus Osmia, and take their common name from their habit of using "masonry" to close off their nests: chewed leaf pulp, mud, or a mixture, packed into the entrance of a burrow or stem.^[7]

Chewed leaves, not mud

Where the more familiar red mason bee seals its cells with mud, the blue mason bee uses a paste of chewed leaves, sometimes mixed with a little mud or pebbles.^[1] A female returning to her tube can be seen with a small ball of green leaf pulp clamped in her jaws, and the completed nest plug is often a distinctive muddy-green rather than the brick-red of a red mason.^[7] Nests are dug in whatever cavity is at hand: old beetle burrows in dead wood, drilled holes in wooden blocks, hollow stems of brambles or bamboo, sometimes cracks in mortar or the old nail holes in a garden shed.^[1]

Two broods a year

In warm parts of Britain the blue mason bee is bivoltine, running two generations in a single summer.^[1] Males of the first brood emerge from mid-April, followed by females; a second peak flies through August, before overwintering as adults inside the sealed cells.^[1] Adults visit a wide range of flowers, showing a marked liking for members of the pea family such as bird's-foot trefoil and vetches, along with campion, brambles and garden herbs like sage and lavender.^[5]

Where and when you will see it

Blue mason bee is widespread across England and Wales, thinning northwards and reaching central Scotland, but has not been recorded from Ireland.^[1][6] It is a bee of many habitats: coastal cliffs and dunes, private gardens, urban brownfield land, sunny woodland edges and old buildings.^[1] Its main natural enemy is the striking black-and-white sapygid wasp Sapyga quinquepunctata, a specialist parasite that lays its own eggs in the mason's sealed cells so its larvae can eat the pollen store.^[1] A wider Palaearctic species, it also occurs across Europe, North Africa, central Asia, and, apparently as a human-assisted introduction, parts of North America.^[6]

How to tell it apart

The blue mason bee is unlikely to be confused with any other native bee once seen well. The female's steel-blue metallic body, box-shaped head and jet-black belly scopa are collectively distinctive, and males, with their bright green-bronze thorax and pale hair, look almost like a different species.^[5] At a glance, though, dark-metallic Osmia females can be mistaken for the smaller, greener two-coloured mason bee male, which has a similar coppery sheen but is a very different bee: much smaller, with a bright orange abdomen in the female, and specialised on snail shells for nesting.^[7] If the bee you are watching is a plain metallic dark bee working a bee hotel, blue mason is the safest guess in most of southern and central England.^[1]

Why the blue mason bee matters

Blue mason bees are among the most efficient pollinators of the many flowers they visit, because, like all Megachilidae, they carry pollen dry and loose on the underside of the belly rather than packing it into wetted balls on the legs. This means far more pollen brushes off onto each flower they visit.^[8] Their fondness for cavities in old wood, walls and stems is a reminder that a well-managed garden is often the messier one: dead standing stems, sunny fence posts, a bee hotel and a few square metres of long grass will support blue masons and dozens of other solitary bees at very little cost.^[5]

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Dragos Nistor

Founder, HoneyBee & Co. · Guest Lecturer, University of Greenwich

HoneyBee & Co. draws on six generations of family beekeeping heritage, with honey from the Nistor family apiaries and from carefully chosen British producers. Our British honey supplier holds SALSA Certification, and we offer a 15% NHS Discount. Every jar is raw, unfiltered and traceable to the hive.