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Invertebrate of the Week #12 – Osmia bicolor: a shell-nesting bee

Acknowledgement: Hat tip to Paul Bee (@solitarybee) and John Walters (@JWentomologistfor bringing this insect to my attention.

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

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

O. bicolor is found predominantly in England, southern Wales, and Central Europe and occupies environments in which grasslands cover chalk or limestone soils.  Between late February and early March, the males of the species emerge and begin seeking-out early spring flowers.  The species is polylectic (meaning that it doesn’t have a preference for any one type of flower) and has been noted on flowering plants in the families Primulaceae, Rosaceae, Lamiaceae, Asteraceae, and Liliaceae.  The males remain in their loose grassland fraternities for a few weeks until the females gradually begin to appear.

The males quickly mate with the emerging females who then set about engaging in a truly delightful behavior.  Rather than construct a nest from scratch, female O. bicolor seek-out the vacant shells of terrestrial snails (apparently favoring those of the Banded Snail Cepaea and Kentish Snail Monacha.)

Once a suitable shell has been selected and maneuvered into the proper position, the female enters and begins partitioning the interior into a maximum of 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.  Once the shell is fully stocked and sealed, the female maneuvers the shell such that the opening is facing the substrate and then the real work begins.  Over the next hour or so, the female collects dead grass and bramble stems from the surrounding area and returns to the shell where it uses its new found material to thatch together a camouflaged shelter over the shell.

The female repeats this entire process several more times over her brief lifetime with all the bees disappearing 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.


Further Reading and References:

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