Cliff Mason Bee

The Cliff Mason Bee (Osmia xanthomelana) is the largest of the Osmia species found in the UK and also one of our most endangered native solitary bees.

The female Cliff Mason Bee’s thorax and base of the abdomen are orange, or brown-haired, the rest of the abdomen (including the pollen brush) and head are black-haired.  Males have orange-hairs on the thorax and abdomen, which fade to grey with age.

Both sexes have large, crossed mandibles (jaws) when folded.

This species is found from only 2 locations on the Llyn Peninsula in Wales and is extremely vulnerable to extinction.

This species is also known as the Large Mason Bee as it is the largest of the ten species of Osmia that we have in the UK.

• • Size: Approximately 12-13mm in length
  • Life span:  Annual life cycle. Eggs hatch after about one week; the larvae start spinning a cocoon about one month after hatching. The bees become adults in autumn, but stay dormant until the following spring. Upon emergence adults are thought to fly for around 8 weeks, completing their reproductive cycle and ensuring the next generation in the following year.
  • Diet:  The Cliff Mason Bee feeds almost exclusively on Common Bird’s-foot Trefoil
  • Reproduction: This species is single brooded (have one flight period per year) and the females collect wet mud from cliff seepages along with grit or small stones to construct pot-like nests cells (sometimes in clusters) in sandy soil, usually amongst the roots of vegetation.  Each cell is sealed with a flat lid.
  • When to see:  The peak flight period of the Cliff Mason Bee is May to mid-June, however it can extend from April to mid-July.  Males typically appear first with females persisting the longest.
  • Population Trend:  Declining in the UK, but considered stable in Europe.
  • Threats:  Threats to populations include loss of unimproved, eroding cliffs and landslips to coastal development and especially through cliff stabilisation measures.  Loss of cliff seepages and associated wet mud could also result from local water abstraction and other changes to the local hydrology such as drilling of bore holes for cliff stabilisation.  Improvement of flower-rich cliff top grassland through the use of fertilisers could also be damaging.  The extremely localised nature of the bee at its remaining sites, means that landslips which might normally be beneficial, could now threaten it with extinction.
  • Fun Fact:  Females collect wet mud from cliff seepages to construct pot-like nests cells.

How you can help: 

Buglife is working to increase awareness of invertebrates and the Cliff Mason Bee through specific projects, including B-Lines, but we need your help!

Buglife B-Lines are an imaginative and beautiful solution to the problem of the loss of flowers and pollinators. B-Lines are a series of ‘insect pathways’ running through our countryside and towns, along which we are restoring and creating a series of wildflower-rich habitat stepping stones. Linking existing wildlife areas together, creating a network, like a railway, that will weave across the UK landscape.  More information about B-Lines and how you can help pollinators can be found on our B-Lines & Pollinator Projects pages.

Join a recording scheme and log your finds – send any records/sightings to BWARS, Natural Resources Wales (NRW) and Cofnod (North Wales Environmental Information Service) or download the iRecord app and get recording!

Do remember that we rely on donations to continue our work.  If you have searched, found and learnt about our incredible invertebrates on our website, please do consider Making a Donation, Becoming a Member or maybe even making a purchase in our shop.  For more ideas on how to support our work find out how to Get Involved.  Thank you 🕷

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