Western mason bee  (Osmia parietina)

Classified as Rare in the UK.

The mason bees are a family of solitary bees in which the queens construct individual nest cells and do not form colonies. The western mason bee is a medium-sized, predominantly black bee, with ginger-coloured 'fur' round the top of the abdomen.

This bee is found across most of Europe, but is confined largely to hilly or mountainous wooded areas. In the UK it has been recently recorded from Lancashire and Cumbria, and is also present in small numbers in scattered localities in Scotland, and two sites in North Wales.

This species shows a preference for upland herb-rich pasture and woodland glades and clearings with an abundance of its sole pollen resource, bird's-foot trefoil Lotus corniculatus. Dry-stone walls, rocky outcrops and standing dead trees provide essential nesting sites.

Mason bees nest in cavities such as in dry-stone walls and holes in trees. In the UK, this species shows a preference for the former, but sites in rock faces and dead standing trees are also used. The nest cells are constructed with sand grains or mud, bound together with saliva or water collected from a nearby source.The female bees lay one egg in each cell, and provision the cell with pollen and nectar, collected from bird's-foot trefoil. The western mason bee is sometimes the victim of parasitism by the cuckoo wasp Chrysura hirsuta.

Agricultural change, through increased grazing of herb-rich upland meadows with bird's-foot trefoil, and the reduction in the management of woodland resulting in the loss of open, sunny, flower-rich glades and clearings have accelerated the decline of this species. Conversion of meadows to silage, rye-grass leys or arable production has also affected this bee. The loss of sunny, rocky outcrops through scrub encroachment, and the removal or destruction of dry-stone walls and standing dead wood, has limited the availability of nest sites.

The western mason bee is listed in the UK Biodiversity Action Plans (UKBAPs), and included in English Nature's Species Recovery Programme (SRP). Following surveys on several of the sites where this species is known to occur, it has been discovered that the principal conservation measure for this species is the management of bird's-foot trefoil. However, it is also important to establish just how many populations of this bee exist throughout the UK. Work on saving the western mason bee could also benefit two other species of mason bees as well as the parasitic wasp, Chrysura hirsuta.

The UK Biodiversity Action Plan for this species is available at UK BAP.