Two-coloured Mason Bee

Fascinating facts about the Two-coloured Mason Bee

• The female two-coloured mason bee is 12 mm long and has a bright ginger abdomen contrasting with its black head and thorax.
• While most bees wait for spring, male O. bicolor can be seen buzzing as early as February, making them among the first pollinators to awaken in western Europe’s chilly months.
• A single snail shell can house up to five tiny brood cells, each sealed with chewed leaf pulp and a sand-gravel barrier, turning an abandoned shell into a fortified nursery for the next generation.
• Females often juggle nests several times a season, building up to seven separate brood sites and cloaking each shell with a self-woven canopy of grass and twigs that can be many times their own body length.
• In Britain the bee nests in shells of species like Helix pomatia and Cepaea nemoralis and is found in grasslands over calcareous soils such as limestone.

At a glance

About the Two-coloured Mason Bee

The two-coloured mason-bee, Osmia bicolor, is a diminutive pollinator barely larger than a thumbnail, its females measuring about 12 mm. Their heads and thoraxes are glossy black, while the abdomen is cloaked in a striking ginger fuzz that is softer in the males. These bees favour calcareous grasslands and open deciduous woodlands that rest on limestone or chalk, stretching from southern Great Britain across western Asia to Turkestan, and from the southern reaches of Scandinavia and Finland down to Spain and Romania. Their life rhythm is tightly bound to the spring: males may already be buzzing in February, but the bulk of activity runs from April through July, when females emerge, mate, and begin the most unusual of all mason-bee tasks – nesting inside empty snail shells. The female selects a shell, positions herself, and fashions a series of sealed cells from chewed leaf mastic, each holding a pollen-nectar ball and a single egg. Up to five cells can line a single shell, which the bee camouflages with grass stems, twigs and saliva, sometimes even burying it partially. Males, meanwhile, occasionally take refuge in abandoned shells during rain or cold nights, turning the shells into temporary shelters. Their diet is polylectic, drawing nectar and pollen from a diverse floral palette that includes wood anemone, bluebell, dog-violet, bird's-foot-trefoil, horseshoe vetch, sainfoin, willow, ground-ivy, daisy and dandelion, ensuring they are active participants in a wide-ranged pollination network. The most surprising trait of O. bicolor is its reliance on the discarded homes of gastropods – a nesting strategy that intertwines the fates of bees and snails in a delicate ecological dance.

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