Osmia bicornis, the red mason bee.
Pjt56, CC BY-SA 4.0, via Wikimedia Commons

Osmia bicornis Linnaeus, 1758, the red mason bee, is Britain's most familiar solitary bee and the one most likely to move into a garden bee hotel. About the size of a honeybee, it is clothed in dense gingery hair, and the female carries two small black horns on her face, the feature that gives the species its name (bicornis means "two-horned"). It is a solitary, cavity-nesting bee: every female is her own queen, building and provisioning her own nest in hollow stems, holes in wood, soft mortar, and bee hotels, sealing each cell with mud. Gentle, abundant and an exceptional pollinator of orchard fruit, it is one of the easiest and most rewarding wild bees to welcome into a garden. See where it lives among Britain's other wild bees on the UK Native Bee Species Map, or explore the wider world of bees in the World Bee Atlas.

Taxonomy and Classification

Carl Linnaeus described the red mason bee in 1758 in the tenth edition of Systema Naturae.^[1] It belongs to the family Megachilidae, the mason, leafcutter and carder bees, within the tribe Osmiini and the genus Osmia. For most of the twentieth century the British bee was known as Osmia rufa, and a great deal of older literature still uses that name; Osmia bicornis is now the accepted name and O. rufa is treated as a synonym.^[2] Of the eleven Osmia species recorded in England, the red mason bee is both the largest and the most common.

Physical Description

The red mason bee is roughly the size of a honeybee but stockier, covered in dense gingery to orange-brown hair, greyish-brown across the thorax and warmer orange-red on the abdomen.^[3] Females and males are easy to tell apart. The female has a black-haired face bearing two small, inward-curving horns on the clypeus, a feature unique among British Osmia and the origin of the name bicornis. She is the larger sex, at 10 to 12 mm, and carries pollen in a brush of hairs, the scopa, on the underside of her abdomen rather than on her legs.^[4]

Males are smaller, 8 to 10 mm, with no horns, no pollen scopa, a tuft of pale grey-white hair on the face, and noticeably longer antennae.^[4] Females are larger than males because their larvae are fed more pollen; brood-cell temperature also affects adult size, with bees reared in hotter cells emerging smaller.^[5] The species is sometimes confused with the tawny mining bee (Andrena fulva), which is a similar fox-red but has a sharply contrasting black-and-red pattern and nests in the ground rather than in cavities.

Nesting: A Mason That Builds With Mud

The red mason bee is a solitary, aerial cavity-nester. Unlike the bumblebees and honeybees, there is no colony, no queen and no workers: every female is fertile and builds her own nest alone.^[1] She does not excavate fresh tunnels but uses existing cavities, hollow plant stems, beetle holes in dead wood, gaps under roof tiles, soft or crumbling mortar joints, and, very readily, the tubes of a garden bee hotel. More unusual sites on record include keyholes and empty snail shells.

Inside a chosen tube she builds a row of separate cells, end to end. Each cell is stocked with a loaf of pollen and nectar, an egg is laid on it, and the cell is then walled off with a partition of mud. The bee is one of only two British Osmia that build with mud rather than chewed leaf, gathering wet soil as small pellets in her jaws and packing it into place with the horns on her face.^[1] When the tube is full she seals the entrance with a thick, characteristic mud plug. The young develop through the summer, spin tough reddish-brown silken cocoons, and overwinter inside them as fully formed adults, ready to emerge the following spring.^[6]

Life Cycle and Behaviour

The red mason bee is univoltine, completing one generation a year, and is on the wing from late March or April through to June, sometimes into July.^[2] Males emerge first and wait near the nest sites for the females to appear. Once a female has mated, usually only once, she sets about building her nest. She allocates the sexes deliberately: female eggs, which need more provisions, are laid in the deeper cells at the back of the tube, and the smaller male cells nearer the entrance, so the males emerge first the following spring.^[5]

As a forager the red mason bee is polylectic, visiting a wide range of spring flowers including fruit blossom, willow, oak, buttercups, dandelions and many garden plants. It is active in cooler, duller conditions than the honeybee, foraging from around 10 degrees Celsius and flying in light rain and wind, which makes it a dependable early-season pollinator.^[7] Because it carries dry pollen loosely on its belly scopa, pollen brushes off readily onto each flower it visits, making it an unusually effective pollinator per bee.

The Orchard Pollinator

The red mason bee is one of very few wild bees managed commercially in Britain, and it is prized for fruit pollination. For tree fruit such as apples, pears, plums and cherries, individual red mason bees are far more efficient pollinators than honeybees, and they are now reared and sold specifically for orchard and soft-fruit pollination.^[7] Their early flight season, tolerance of cool weather, and belly-carried pollen all add up to reliable fruit set even in a poor British spring.

This usefulness, rather than rarity, is the red mason bee's claim to fame. It is the bee behind the explosion of garden bee hotels, and increasingly part of how commercial orchards are pollinated, a rare example of a wild solitary bee working alongside the managed honeybee in food production.

Red Mason Bees, Honey and HoneyBee & Co.

Like all solitary bees, the red mason bee makes no honey. Each female provisions only her own nest cells with pollen and nectar for her young, with no colony and no surplus to store, so there is nothing to harvest. Its gift is pollination, and a particularly valuable one for the orchards and gardens of Britain.

For us as a honey company, the red mason bee is a reminder that a thriving garden runs on more than one kind of bee. The honey in our jars comes from managed Apis mellifera colonies, but those hives share the same blossom-filled spring landscape with solitary bees like Osmia bicornis, and all of them depend on continuous flowering forage and undisturbed nesting places. Our British Wildflower Honey, gathered in the Midlands from hawthorn, clover, bramble and other native blossom, comes from exactly the kind of mixed forage that supports mason bees too. Planting for bees and offering a bee hotel helps the honeybee and the solitary bee alike. To understand what a world with fewer pollinators would look like, visit Your Plate Without Bees.

Apis mellifera | British Midlands

British Wildflower Honey | £10.99

Raw, cold-extracted honey from the rolling Midlands, gathered from the hawthorn, clover and bramble that garden bees forage too. SALSA-certified British supplier. 280g.

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