Female orange-vented mason bee, Osmia leaiana, showing the bright orange pollen brush beneath the abdomen — *Osmia leaiana*, orange-vented mason bee (worn female). gailhampshire, CC BY 2.0, via Wikimedia Commons.

Osmia leaiana Mason beeSummer-flyingCavity-nestingEngland & Wales

UK Bee Species

Orange-vented Mason Bee (Osmia leaiana)

Osmia leaiana Kirby, 1802 · subgenus Melanosmia · family Megachilidae

The orange-vented mason bee is a compact summer bee best known for the vivid orange brush of pollen hairs beneath the female's abdomen. A cavity nester that seals its cells with chewed green leaf paste, it is one of two small Osmia species that turn up regularly in gardens and bee hotels across England and Wales.

Quick facts

Common name

Orange-vented mason bee

Scientific name

Osmia leaiana

Authority

Kirby, 1802

Family

Megachilidae

UK status

Locally common, England & Wales

Size

Female about 10 mm; male about 6 mm

Active

May to August

Nesting

Cavities in wood, walls, cliffs, bee hotels

Cell material

Chewed leaf paste

Forage

Asteraceae: thistles, knapweed, daisies

Look-alike

Blue mason bee

Temperament

Docile, very rarely stings

Kingdom	Animalia
Phylum	Arthropoda
Class	Insecta
Order	Hymenoptera
Family	Megachilidae
Genus	Osmia
Subgenus	Melanosmia
Species	Osmia leaiana

An orange belly-brush and a big square head

Mason bees of the genus Osmia are solitary, cavity-nesting bees that carry pollen dry on a brush of stiff hairs, the scopa, beneath the abdomen rather than on the legs.^[1] The female orange-vented mason bee is unmistakable at close range: a dark brown bee roughly 10 mm long with a dense, bright orange scopa that contrasts sharply with the otherwise dark upperside.^[2] Like all Osmia it has a large, box-shaped head almost as broad as the thorax, which in the field can make it resemble a stout leafcutter bee.^[3]

Males are smaller, around 6 mm, and lack the pollen brush. They are shining metallic green with bright ginger hairs when fresh and black eyes, and are notoriously hard to tell from male blue mason bees without a microscope.^[2]

The quickest field mark

On a fresh female, the solid orange brush under the abdomen against a dark brown body is the fastest way to separate this bee from the similar blue mason bee, whose females are bluish with a black scopa.^[2]

A dark brown bee with a blazing orange belly: the orange-vented mason bee wears its pollen like a badge.

Where it lives and what it visits

The orange-vented mason bee is restricted to England and Wales, where it is locally common in gardens, woodland edges and clearings.^[1]^[5] It is less frequently met than the closely related blue mason bee but shares much the same habitat.^[1] You can see where it fits among Britain's bees on the UK Native Bee Species Map.

Females collect pollen chiefly from the daisy family, Asteraceae, favouring spear thistle, black knapweed and yellow-flowered composites such as oxeye daisy.^[4] That preference ties the bee to flower-rich summer grassland, brownfield and garden borders where those plants grow.

10 mm

Typical female length, with a full-width orange scopa beneath the abdomen.^[2]

The pesto nest

The orange-vented mason bee nests in ready-made cavities: holes in dead wood, standing dead trees, fence posts, walls and occasionally sand faces, and it readily takes to bee hotels and observation boxes.^[2] It does not excavate its own tunnel. Inside, the female divides the cavity into cells and seals the partitions and final plug with chewed-up leaf material, bright green when fresh and drying to a dark brown, granular finish that has earned it the nickname the pesto bee.^[1]

May to August

The orange-vented mason bee's flight season, a true bee of high summer rather than early spring.^[1]

A note on names

A very similar species, Osmia niveata (formerly O. fulvicornis), occurs on the Channel Islands and has been confirmed at a few sites in southern England. Females can be separated by the shape of the clypeal margin, but males are not easily told apart, so some garden records of “orange-vented mason bee” in the far south are worth a second look.^[2]

Why it matters

As a summer specialist on thistles and knapweeds, the orange-vented mason bee helps pollinate exactly the kind of flower-rich grassland that supports a wide community of other insects. Because it nests in cavities rather than bare ground, it is one of the easiest solitary bees to welcome deliberately: a south-facing bee hotel with a range of hole sizes, near summer composites such as knapweed and oxeye daisy, gives this bee and its relatives somewhere to breed.

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Frequently asked questions

How do I identify an orange-vented mason bee?

Look in summer for a compact, dark brown bee about 10 mm long with a large, square head and a bright orange brush of pollen hairs beneath the abdomen. That orange belly-brush on a dark body is the clearest mark. Males are smaller, metallic green with ginger hairs, and much harder to identify.

What is a mason bee?

Mason bees are solitary, cavity-nesting bees in the genus Osmia. Rather than digging in the ground, each female uses an existing hole, divides it into cells, provisions each with pollen and nectar, lays one egg, and seals the cells with mud or, in this species, chewed leaf paste.

When is the orange-vented mason bee active?

It flies from May to August, making it a bee of high summer rather than early spring. That sets it apart from the red mason bee, which is on the wing much earlier in the year.

Where does it nest?

In ready-made cavities: holes in dead wood, fence posts, walls, cliffs and, very often, bee hotels and observation boxes. It seals the cell partitions with chewed leaf, which dries dark and granular and gives it the nickname the pesto bee.

What flowers does it visit?

Females collect pollen mainly from the daisy family, especially thistles, black knapweed and yellow composites such as oxeye daisy. Planting summer meadow flowers is the best way to support it.

How is it different from the blue mason bee?

The females are easy: the orange-vented mason bee is dark brown with an orange belly-brush, while the blue mason bee is bluish with a black one. The metallic-green males are much harder and often need microscopic characters to separate.

Do orange-vented mason bees sting?

They can in theory, but they are very docile and almost never do, and the sting is weak. With no colony to defend, they are safe around gardens, children and pets, and are a good bee to encourage in a bee hotel.

Does the orange-vented mason bee make honey?

No. Each female stores only enough pollen and nectar to feed her own larvae, never a surplus. Only the honeybee makes honey in quantity. Compare the bee families in the World Bee Atlas.

Related species

Sources & references

BWARS (Bees, Wasps & Ants Recording Society). Species account: Osmia leaiana (Kirby, 1802): flight period, habitat, nesting and chewed-leaf partitions. bwars.com.
Falk, S. Field Guide to the Bees of Great Britain and Ireland, Bloomsbury, and the Steven Falk Flickr collection: Osmia leaiana (orange scopa, female and male characters, separation from O. caerulescens and O. niveata).
NatureSpot. Orange-vented Mason Bee (Osmia leaiana): box-shaped head, dense orange scopa, dark male with ginger hairs. naturespot.org.
WildBristol / Bristol Bees, Wasps & Ants group. Orange-vented Mason Bee: forage on Asteraceae including spear thistle, black knapweed and oxeye daisy, and cavity nesting. wildbristol.uk.
NBN Atlas / GBIF Secretariat. Osmia leaiana (Kirby, 1802): taxonomy and UK distribution. nbnatlas.org; gbif.org.

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