Nature · Insects & other invertebrates

Red mason bee

Univoltine, flying late March or April to June · 8–12 mm

Osmia bicornis — Megachilidae (leafcutter and mason bees)

A gingery-red solitary bee of spring gardens and orchards, and the bee most likely to move into a bee hotel. The female does not dig a nest: she finds an existing cavity — a hollow stem, a soft mortar joint, a gap under a roof tile, even the inside of a door lock — and walls off a row of cells with mud pellets, tamping them into place with a pair of small horns on her face. Those horns give her the name bicornis. She is an excellent pollinator of apple, pear and plum blossom.

How to spot them

▸Densely gingery-red or foxy-brown hair over thorax and abdomen; black head
▸Female has two small prong-like horns on the face, used to tamp mud into place
▸Male smaller, with a tuft of white hair on the face and long antennae; emerges before the females
▸Spring only — late March or April to June, in one generation
▸Nests in existing holes: hollow stems, bee hotels, air bricks, crumbling mortar; cells sealed with mud

How to know it

The female carries a pair of small prong-like horns on her face, and uses them to tamp mud pellets into her nest

Red mason bee — hence Osmia bicornis, 'two-horned'.

It never digs its own burrow: it lines up cells inside an existing hole and walls each one off with mud carried in its jaws

Red mason bee — one of only two British Osmia that build with mud rather than chewed leaf pulp.

Gingery-red solitary bee of spring orchards, an outstanding pollinator of apple, pear and plum blossom

Red mason bee — a single female visits enormous numbers of blossoms.

The bee most likely to take up residence in a garden bee hotel, and also found nesting in soft mortar and even inside door locks

Red mason bee — the colloquial name is red mortar bee.

It overwinters as a fully formed adult inside a tough reddish-brown silk cocoon, and the males emerge first

Red mason bee — protandry, so males are waiting when the females appear.

It used to be called Osmia rufa, and its family is the one that also contains the leafcutters

Red mason bee — Osmia bicornis, family Megachilidae.

Quick-fire facts

•Osmia bicornis, formerly Osmia rufa; bicornis means 'two-horned', for the prong-like extensions on the female's clypeus
•BWARS: females nest in existing cavities, from soil burrows and dead wood to gaps under roof tiles, fissures in soft mortar and the insides of door locks
•Cell walls are built from mud brought to the nest as pellets held in the mandibles and tamped into position with the facial horns
•It is one of only two British Osmia species that use mud for their nests; the rest use leaf pulp
•Univoltine: one generation a year, on the wing from April (occasionally late March) to June or sometimes July
•The winter is passed as a fully formed adult bee inside a tough reddish-brown silken cocoon
•BWARS records it as widely distributed through much of Britain as far north as Perthshire, common except in parts of northern England and Scotland, and a recent arrival in Ireland
•It is solitary: every female is fertile and builds her own nest, though many may nest side by side in the same wall
•Its nests are invaded by cleptoparasites and inquilines including the ruby-tailed wasp Chrysis ignita and the fly Cacoxenus indagator

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Sources

Bees, Wasps and Ants Recording Society (BWARS) — Osmia bicornis (Linnaeus, 1758)
Paul D. Brock — Britain's Insects: A Field Guide to the Insects of Great Britain and Ireland