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Dark bees

StelisCleptoparasitic — lays in other bees' nestsProduces no honey

A genus rather than a single species — Around 100 species worldwide, all cleptoparasites, and almost all parasitising mason bees, leafcutter bees and their relatives. General statements below are statements about the group.

Small, dark, sparsely haired cleptoparasites that target the cavity-nesting megachilids — mason bees, leafcutters, scissor bees and resin bees. They are among the parasites that turn up in a bee hotel, and their presence indicates a host population large enough to be worth exploiting rather than a problem to be solved.

Can it sting?

Females have a sting and use it on people essentially never. The hard cuticle is armour against host females, which attack intruders. Males cannot sting.

How to recognise it

Small and dark, sparsely haired with a hard shining cuticle, and marked in some species with pale spots or bands on the abdomen. Females have no pollen-carrying structures whatsoever.

  • Small, dark and shining, with a hard heavily punctured cuticle.
  • No pollen brushes anywhere — the diagnostic character of the cuckoo strategy.
  • Investigating bee hotel tubes and hollow stems methodically, which host bees never do.
  • Never carrying pollen.

Body length 4–11 mm (typically 7 mm).

Biology

Social organisation

Cleptoparasitic — lays in other bees' nests

Builds no nest and gathers no pollen for her own young, laying instead in another bee's provisioned cell. Sometimes called cuckoo bees.

Nesting
Builds no nest

Builds nothing. The female enters a host cell that has been provisioned and lays her egg in it, frequently before the host has finished sealing. The larva kills the host offspring — in some species with enlarged first-instar mandibles, in others simply by outcompeting it — and consumes the provision.

Carries pollen
Carries no pollen for young
On the wing
May to August

Lifecycle

  1. Emergence timed to the hostAdults emerge with their hosts, which is why Stelis appear at bee hotels shortly after the mason and leafcutter bees do.May to August.
  2. SearchingThe female works methodically along tubes and cavities, testing each in turn.
  3. DevelopmentThe larva consumes the host's provision and overwinters on the host's schedule.

Where it lives and what it does

Habitat
Wherever cavity-nesting megachilids occur — gardens, bee hotels, hedgerows, brownfield land, heathland and woodland edge.
Distribution
Worldwide except Australia, following the distribution of megachilid bees.
Behaviour
Slow and methodical rather than purposeful. Because they need no pollen they spend little time at flowers, and the characteristic sight is a small dark bee working steadily along a bee hotel investigating tube after tube — behaviour no host bee shows.
Pollination role
Negligible. They visit flowers only for their own fuel. Their significance is diagnostic.

Incidental only.

What it forages on
Visits flowers for nectar only and is unfussy about which.

Honey

Produces no honey

None. No nest, no colony, no stores.

Relationship with people

How people interact with it
Encountered mainly by people running bee hotels, and usually removed on sight. That is generally the wrong response — a parasite load is normal, and a hotel with no parasites at all usually means very few bees.
Conservation
Several species are scarce and, as obligate parasites, they decline faster than their hosts and recover more slowly.

Sources

  • Field Guide to the Bees of Great Britain and Ireland — Bloomsbury (2015) · Field guide
  • The Bees of the World — Johns Hopkins University Press (2007) · Textbook

How sources are selected and weighted is set out in the sources and evidence policy.

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