UK Bee Species

Two-coloured Mason Bee (Osmia bicolor)

Osmia bicolor (Schrank, 1781) · family Megachilidae

A black and ginger spring bee

The female two-coloured mason bee is unmistakable: a medium-sized bee, around 12 mm long, with a black head and thorax set against an abdomen clothed in dense, bright ginger-red hair.^[2] That bold two-tone pattern gives the bee both its names. Males are smaller and plainer, brownish or greyish with paler bands and only a hint of red at the tail, and they are among the very first solitary bees to appear each year, sometimes flying as early as late February.^[2] The only real source of confusion is the much larger and fluffier Osmia cornuta, a recent and very localised arrival in southern England.^[2]

The bee that nests in snail shells

Here is what sets this bee apart from every other British bee. Rather than digging a burrow or using a hollow stem, the female nests inside an empty snail shell, choosing the shells of snails such as the banded snails (Cepaea), the Roman snail (Helix pomatia) and the Kentish snail (Monacha).^[1] Working deep inside the spiral, she provisions a row of cells with pollen, about four or five depending on the size of the shell, dividing them with partitions of leaf mastic, a putty she makes from chewed green leaves.^[1][4][5] She fills the last space with a rubble of tiny shells and chalk, plugs the entrance, and turns the whole shell so the opening faces the ground, sometimes part-burying it.^[1]

A thatched roof of grass

The most remarkable sight comes once the nest is sealed. The female gathers dead grass stems, leaf fragments and small twigs, many of them far longer than her own body, and carries them back one by one to build a loose canopy, a kind of wigwam or thatch, over the shell, binding the pieces with her saliva.^[1][2] The purpose is not certain, but it almost certainly hides the nest from the parasites and predators that would otherwise raid it, such as the cleptoparasitic sapygid wasps.^[2] A small black-and-ginger bee flying low across short turf with a long grass stem held in her jaws is one of the quiet wonders of a chalk hillside in spring.

Where and when you will see it

The two-coloured mason bee is above all a bee of calcareous country: chalk and limestone grassland, downland, old quarries and cuttings, the rides of chalk woodland and calcareous coastal dunes, with the empty snail shells and short, warm turf it needs.^[1][2] Most records lie across southern England, south of a line from the Severn to the Wash, though it is spreading north into the Midlands, using brownfield sites as stepping stones.^[2] Males appear from late February or March and females a week or two later, with the flight running on to early July; both visit spring shrubs such as sallows, blackthorn and hawthorn, and low flowers including bird's-foot-trefoil, kidney vetch, dandelion and violets.^[3] It is a local species, scarce away from its calcareous strongholds.^[6]

A scent, and two colours

The genus name Osmia comes from the Greek for "odour", after the lemony scent mason bees use to mark their nest entrances, while bicolor simply means "two-coloured", for the black-and-ginger female.^[7] It was described by the Bavarian naturalist Franz von Paula Schrank in 1781.^[7] Like all members of the family Megachilidae, she carries pollen not on her legs but in a brush beneath the abdomen, so a foraging female is dusted with pollen along her belly.^[8]

How to tell it apart

The female is one of Britain's most recognisable bees, around ten to twelve millimetres long, with a jet-black head and thorax and an abdomen clothed in bright gingery-red hair, which is why she is also called the red-tailed mason bee.^[8] Males are smaller and a duller brown, easily mistaken for other spring bees.^[8] It is a bee of chalk and limestone country, calcareous grassland and open woodland, found mostly south of a line from the Wash to the Severn.^[8]

Why the two-coloured mason bee matters

This is a flagship insect of one of Britain's richest and most threatened habitats, the unimproved chalk and limestone grassland that supports so many of our scarcer wildflowers, butterflies and bees.^[1] Because it needs several things at once, short warm turf, a good supply of empty snail shells, spring flowers and undisturbed ground, its presence is a sign of a healthy old grassland. Protecting and gently grazing that grassland, and resisting the loss of brownfield mosaics that now help it spread, is the surest way to keep this one-of-a-kind bee on our hillsides.

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Dragos Nistor

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

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