Scottish Mason Bees

Mason bees (Osmia species) are solitary bees that nest in pre-existing cavities in wood, hollow stems, walls, cliffs and even empty snail shells. These bees are excellent pollinators and are 3-4 times more efficient at pollinating than honey bees. Females have a pollen brush beneath their abdomen, and generally have large boxy-heads with powerful jaws for chewing up leaves, or horns for excavating and manipulating mud to line their nest chambers. Twelve species of mason bees are known from the UK, with six species recorded in Scotland. The Red mason bee (Osmia bicornis) is currently spreading across Scotland and will happily colonise urban gardens that provide nesting and foraging opportunities.

The Scottish Mason Bees project focused on three rare species of mason bees, the Mountain mason bee (Osmia inermis), Wall mason bee (Osmia parietina), Pinewood mason bee (Osmia uncinata), and a ruby-tailed wasp Northern Osmia ruby-tailed wasp (Chrysura hirsuta), which is known to parasitise the nests of the three bees. The main forage plant of all three mason bees is Common bird’s foot trefoil. All four species are rare and are on the Scottish Biodiversity List.

Mountain mason bee (Osmia inermis)

The Mountain mason bee is generally found in montane locations and has an extremely localised distribution in the UK, being known from only a couple of upland locations in Scotland. Like many other montane invertebrates, this species is likely to be under recorded. Potential threats include summer grazing by sheep, climate change and associated land-use changes such as agricultural improvement and afforestation.

Wall mason bee (Osmia parietina)

The Wall mason bee is the most widely distributed of the three mason bees identified in this project, however, many historic Scottish records have turned out to be of the Pinewood mason bee or Mountain mason bee.  At present the species appears restricted to the Isle of Colonsay, Whithorn in Dumfries & Galloway, and Killiekrankie in Perthshire. The species is associated with unimproved grasslands, clearings in woodlands and machair.

Pinewood mason bee (Osmia uncinata)

The Pinewood mason bee is only known from northern Scotland in the UK. At present most records are from the Strathspey area, Deeside and east Inverness-shire. The species nests in old longhorn beetle holes in dead Scots pine trees in open areas with bare ground and lots of bird’s-foot trefoil.

Northern Osmia ruby-tailed wasp (Chrysura hirsuta)

This species of cuckoo has a restricted distribution in the UK and is thought to parasitise the nests of all three mason bees mentioned above.

This project ran surveys with the help of volunteers to visit several sites to look for the three species of mason bee and the ruby tailed wasp. Additionally this project has run training workshops introducing solitary bees and how to survey and monitor them.