SERIES: Invertebrate of the Week

Invertebrate of the Week #12 – Osmia bicolor: A Bee That Nests in Shells

Meet Osmia bicolor, this week’s Invertebrate of the Week. Unlike more social HymenopteransOsmia bicolor is not gregarious nor does it have any central monarch to which it answers. This solitary bee prefers to go it alone and it has a fantastic strategy for doing so.

Osmia bicolor mason bee preparing a nest in a snail shell
Osmia bicolor preparing a nest in a snail shell. Source:

Osmia bicolor is Palearctic and found predominantly in England, southern Wales, and Central Europe. There, it occupies grasslands growing atop chalk or limestone soil. The species foraging behavior is polylectic, meaning that Osmia bicolor doesn’t have a preference for any one type of flower. It has observed on flowering plants in the families Primulaceae, Rosaceae, Lamiaceae, Asteraceae, and Liliaceae.

Between late February and early March, the males of the species emerge and seek out early spring flowers. They remain in loose grassland fraternities for a few weeks until the females emerge later. After mating, the emergent females begin to nest. Rather than construct a nest from scratch, female Osmia bicolor repurposes the vacant shells of terrestrial snails. They appear to have a particular penchant for shells of the Banded Snail Cepaea and Kentish Snail Monacha

A female Osmia bicolor moving her snail shell nest site

Once she selects a suitable shell, the female must maneuver it into the proper position. Afterward, she enters the shells and begins partitioning the interior into 4-5 cells.

Each cell is stocked with an egg and pollen before being sealed with a layer of leaf mastic (the product of masticated green-leaf matter), soil, and other debris.  With her shell fully stocked and sealed, the female maneuvers the shell again so that the opening is facing the substrate.

Over the next hour or so, the female collects dead grass and bramble stems from the surrounding area. She returns to the shell and uses the material to thatch together a camouflaged shelter.

Osmia bicolor thatching a nest

The female repeats this entire process several more times over her brief lifetime. All the bees disappear after their univoltine life cycle comes to end by late June.

Want to learn more?  Check out naturalist John Walter’s feature on this wonderful bee on BBC Radio 4.

Acknowledgment: Hat tip to Paul Bee (@solitarybee) and John Walters (@JWentomologist) for bringing this insect to my attention.

This post was updated from an earlier version published in 2014.

Further Reading and References