Mason bees

How to recognise it

Stocky, dark bees of roughly honey bee size, many with dense gingery or foxy-red hair. Females carry pollen on the underside of the abdomen, which gives a foraging bee a distinctive yellow belly — a completely different look from a honey bee's leg pellets. Males are smaller, with pale facial hair and longer antennae.

• Pollen carried dry on the underside of the abdomen, not on the legs. Unmistakable once seen.
• Stocky, blunt-ended body — noticeably chunkier than a honey bee.
• Seen entering and leaving small holes in walls, bee hotels and hollow stems in spring, carrying mud.
• Active from March to June and gone by midsummer, which rules out most confusions on timing alone.

Body length 6–15 mm (typically 11 mm).

Biology

Social organisation

Solitary

Each female builds and provisions her own nest with no workers and no caste. Most of the world's bee species.

Nesting

Existing cavities and tubes; Hollow stems and plant cavities

Uses existing tubular cavities rather than excavating: beetle holes in dead wood, hollow stems, gaps in mortar, and — very readily — the cardboard tubes of a bee hotel. Cells are provisioned in a line along the tube and separated by mud partitions, with a thicker mud plug at the entrance. Females lay female eggs at the back and male eggs at the front, so the males emerge first and are not blocked in behind their sisters.

Carries pollen

Scopa on the underside of the abdomen

On the wing

March to June

Lifecycle

1. 1Emergence and matingMales emerge first in early spring and wait. Females emerge days later, mate, and begin nesting almost immediately.March to April in temperate Europe, tracking spring temperature.
2. 2NestingThe female works alone for the four to six weeks of her adult life, completing perhaps one or two cells a day. She dies before her offspring hatch and never meets them.Four to six weeks.
3. 3Development and overwinteringThe larva eats its pollen loaf, spins a cocoon, and completes development to a fully formed adult by late summer — then stays inside the cocoon all winter and emerges the following spring. The bee spends the great majority of its life as a dormant adult in a tube.Roughly ten months inside the cocoon.

Where it lives and what it does

Habitat

Gardens, orchards, hedgerows, woodland edge, old walls and brownfield land. They need three things within a short distance of one another: flowers, nest tubes and damp bare mud.

Distribution

Widespread across Europe, Asia and North America. Osmia bicornis and O. cornuta are the common European garden species; O. lignaria fills the same role in North America.

Behaviour

Foraging is fast, low and slightly chaotic compared with a honey bee's — mason bees tumble around inside a blossom rather than working it neatly, and that clumsiness is exactly why they pollinate so well. They are also poor flower-constancy performers, moving between species more readily, which spreads pollen between trees rather than around one. Females return to their own tube using visual landmarks and will investigate the wrong hole repeatedly if a bee hotel is uniform-looking.

Pollination role

Exceptional orchard pollinators. Because they carry pollen dry and loose on the abdomen rather than wetted and packed into a leg basket, a far greater proportion falls off onto the next flower. Published comparisons commonly find that a few hundred female mason bees pollinate an orchard acre as effectively as a full honey bee colony of tens of thousands. They also fly in cooler, duller weather than honey bees, which matters enormously for fruit that blossoms in April.

Per individual, among the most effective fruit pollinators known.

Honey

Produces no honey

None. There is no colony to feed through a winter, so there is no reason to store anything: each cell holds exactly enough pollen and nectar for one larva to reach adulthood, and not a gram more.

Relationship with people

How people interact with it

Increasingly managed, in a way quite unlike beekeeping. Mason bee cocoons are sold, overwintered in a fridge and released to time emergence with blossom, and commercial orchard operations in North America, Europe and Japan use them at scale. For gardeners they are the easiest bee to help: a bee hotel that is actually occupied is nearly always occupied by these.

Management

Bee hotels work, with caveats that matter. Tubes must be 6-10 mm across, closed at one end, in full morning sun, and — the part usually missed — cleaned or replaced, because an unmanaged hotel accumulates parasitic mites and fungal disease and eventually does more harm than good.

Conservation

The genus as a whole is not threatened, though individual species are, and all of them depend on the increasingly rare combination of nest cavities and undisturbed bare ground within flying distance of forage.