Blue Mason Bee
Osmia caerulescens
This record describes Osmia caerulescens (species). Recorded family: Megachilidae. Recorded order: Hymenoptera.
Identification and Appearance

Length 8 to 10 mm. The females have large box-shaped heads and a blue metallic lustre to the abdomen giving rise to its common name, with a black pollen basket on the hind legs. The males are bronze with pale yellow hairs and a shining slope at the front of the abdomen. Most solitary bees and wasps are difficult to identify, and can rarely be identified from photos taken in the field. All red-rated records should include a photo or set of photos of the specimen, illustrating the key characters taken from a standard key, which should also be referenced (e.g. ‘Falk, 2015’). The full set of key characters are generally not visible in field photos and photos are rarely sharp enough. To aid in the verification of your records, please include face shot, side, top and wings. The notes should state whether male or female, and explain how the specimen met the key characters. Although NS may not be able to identify the species even if these reference photos are provided, the photos will be stored with the record and may allow it be identified in future. Alternatively, NS will accept records identified by a recognised local or national expert, or that have been identified via BWARS’ Facebook https://www.facebook.com/profile.php?id=100065021433202 . If you have obtained this advice, please note the name of the person/organisation identifying the record in the ‘determiner’ field (e.g. ‘Stuart Roberts, BWARS Facebook’) rather than just a comment of ‘BWARS Facebook’. Forest edges and clearings, flower meadows, orchards, parks and gardens. Nests in holes, often existing cavities in dead wood and plant stems, rocks and mud walls. The female deposits her egg, adds food for the larvae, builds a small wall out of chewed leaves and creates the next chamber, where she will deposit the next egg. Widespread in England and Wales but more common in the South.
Wikipedia reference for Osmia caerulescens: http://en.wikipedia.org/wiki/Osmia_caerulescens.
Classification
- Kingdom
- Animalia
- Phylum
- Arthropoda
- Class
- Insecta
- Order
- Hymenoptera (Ants, Bees, Wasps and Sawflies)
- Superfamily
- Trigonaloidea
- Family
- Megachilidae (Leafcutter, Mason, and Resin Bees, and allies)
- Subfamily
- Megachilinae
- Tribe
- Osmiini
- Genus
- Osmia (Mason Bees)
Imported taxonomy record. Source availability varies; see the source references and editorial policy below.
Habitat and Range
Habitat
Forest edges and clearings, flower meadows, orchards, parks and gardens.
Seasonal Activity
April to August/September.
Diet and Behavior
Life History
Nests in holes, often existing cavities in dead wood and plant stems, rocks and mud walls. The female deposits her egg, adds food for the larvae, builds a small wall out of chewed leaves and creates the next chamber, where she will deposit the next egg.
Additional species information
Recorded status
Widespread in England and Wales but more common in the South.
About Flies: the wider group
Order-level background for Hymenoptera, the group containing Osmia caerulescens. Species-specific details appear above.
Group overview
Hymenoptera includes over 150,000 described species of ants, bees, wasps, and sawflies. Earth harbors approximately 20 quadrillion (20×10^15) ants, with a combined biomass of ~12 megatons of dry carbon - exceeding the combined biomass of all wild birds and mammals (PNAS, 2022).
Ecological role of the group
About 35% of the world's food crops depend on animal pollinators. The chocolate industry, valued at over $100 billion, depends on pollinating flies and midges. Ants are the dominant soil engineers in most terrestrial ecosystems, turning more soil than earthworms in many habitats.
Economic and practical significance of the group
Pollinators contribute $235-577 billion worth of annual global food production (FAO). In the U.S., bee pollination alone benefits crops valued at $18 billion annually, and honey bees are responsible for $5.4 billion in agricultural productivity. Between April 2023-2024, 55.1% of managed honey bee colonies were lost. The U.S. bee population dropped from 5 million to 2.5 million.
Recorded sources for this group: FAO, USDA, EPA, PNAS (2022), US Fish & Wildlife Service. Imported estimates may describe particular regions or reporting periods.
Photos of Blue Mason Bee (2)
Frequently Asked Questions About Blue Mason Bee
What is Blue Mason Bee?
Blue Mason Bee (Osmia caerulescens) is a species in the family Megachilidae, within the order Hymenoptera.
How big is Blue Mason Bee?
8 to 10 mm. The females have large box-shaped heads and a blue metallic lustre to the abdomen giving rise to its common name, with a black pollen basket on the hind legs. The males are bronze with pale yellow hairs and a shining slope at the front of the abdomen.
Where is Blue Mason Bee found?
Habitat: Forest edges and clearings, flower meadows, orchards, parks and gardens.
When is Blue Mason Bee active?
Blue Mason Bee: seasonal activity - April to August/September..
What is the life cycle of a Blue Mason Bee?
Nests in holes, often existing cavities in dead wood and plant stems, rocks and mud walls. The female deposits her egg, adds food for the larvae, builds a small wall out of chewed leaves and creates the next chamber, where she will deposit the next egg.
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