Mason bee is a name now commonly used for species of bees in the genus Osmia, of the family Megachilidae. Mason bees are named for their habit of using mud or other “masonry” products in constructing their nests, which are made in naturally occurring gaps such as between cracks in stones or other small dark cavities; when available some species preferentially use hollow stems or holes in wood made by wood-boring insects.
Species of the genus include the orchard mason bee Osmia lignaria, the blueberry bee O. ribifloris, and the hornfaced bee O. cornifrons. The former two are native to the Americas and the latter to Japan, although O. lignaria and O. cornifrons have been moved from their native ranges for commercial purposes. The red mason bee, Osmia bicornis, is found across the European continent. Over 300 species are found across the Northern Hemisphere. Most occur in temperate habitats within the Palearctic and Neartic zones, and are active from spring through late summer.
Osmia species are frequently metallic green or blue, though many are blackish and at least one rust-red. Most have black ventral scopae which are difficult to notice unless laden with pollen. They have arolia between their claws, unlike Megachile or Anthidium species.
Historically, the term mason bee has also been used refer to bees from a number of other genura under Megachilidae such as Chalicodoma, most notably in "The Mason-Bees" by Jean-Henri Fabre and his translator Alexander Teixeira de Mattos in 1914. 
When the bees emerge from their cocoons, the males exit first. The males typically remain near the nests waiting for the females, and some are known to actively extract females from their cocoons. When the females emerge, they mate with one or several males. The males soon die, and within a few days the females begin provisioning their nests.
Osmia females typically nest in narrow gaps and naturally occurring tubular cavities. Commonly this means hollow twigs, but can be in abandoned nests of wood-boring beetles or carpenter bees, in snail shells, under bark, or in other small protected cavities. They do not excavate their own nests. The material used for the cell can be clay, mud, grit, or chewed plant tissue. The palearctic species O. avosetta is one of a few species known for lining the nest burrows with flower petals. A female might inspect several potential nests before settling in.
Within a few days of mating the female has selected a nest site and has begun to visit flowers to gather pollen and nectar for her nests; many trips are needed to complete a pollen/nectar provision mass. Once a provision mass is complete, the bee backs into the hole and lays an egg on top of the mass. Then, she creates a partition of "mud", which doubles as the back of the next cell. The process continues until she has filled the cavity. Female eggs are laid in the back of the nest, and male eggs towards the front.
Once a bee has finished with a nest, she plugs the entrance to the tube, and then may seek out another nest location.
Within weeks of hatching the larva has probably consumed all of its provisions and begins spinning a cocoon around itself and enters the pupal stage, and the adult matures either in the fall or winter, hibernating inside its insulatory cocoon. Most Osmia species are found in places where the temperature drops below 0°C for long durations and they are well-adapted to cold winters; chilling seems to be a requirement for maturation. Some species of mason bees are semi-voltine, meaning that they have a two year maturation cycle, with a full year (plus) spent as a larva.
Solitary bees produce neither honey nor beeswax. They are immune from acarine and Varroa mites, but have their own unique parasites, pests, and diseases. The nesting habits of many Osmia lend themselves to easy cultivation, and a number of Osmia are commercially propagated in different parts of the world to improve pollination in fruit and nut production. Commercial pollinators include O. lignaria, O. bicornis, O. cornuta, O. cornifrons, O. ribifloris, and O. californica. They are used both as an alternative to and as an augmentation for European honey bees. Mason bees used for orchard and other agricultural applications are all readily attracted to nesting holes - reeds, paper tubes, nesting trays, or drilled blocks of wood; in their dormant season they can be transported as intact nests (tubes, blocks, etc.), or as loose cocoons. As is characteristic of solitary bees, Osmia are very docile and rarely sting when handled (only under distress such as when wet or squeezed), their sting is small and not painful, and their stinger is unbarbed.
- Dogterom, Margeriet (2002). Pollination with Mason Bees: A Gardener's Guide to Managing Mason Bees for Fruit Production. Beediverse Publishing. ISBN 9780968935705.
- Bosch, Jordi and Kemp, William J. (2001). How to manage the blue orchard bee. Sustainable Agriculture Network Handbook Series. p. 98. ISBN 978-1888626063. Retrieved 3 October 2017.
- Michener, Charles D. (2007). The Bees of the World. The Johns Hopkins University Press.
- Sedivy, C.D.; et al. (2013). "Host range evolution in a selected group of osmiine bees (Hymenoptera: Megachilidae): the Boraginaceae-Fabaceae paradox". Biological Journal of the Linnean Society. 108: 35–54.
- Fabre, Jean-Henri (1914). The Mason-Bees. New York: Dodd, Mead and Company.
- Sedivy, C.; et al. (2012). "Evolution of nesting behavior and kleptoparasitism in a selected group of osmiine bees (Hymenoptera: Megachilidae)". Biological Journal of the Linnean Society. 108: 349–360.
- Holland, Jennifer S. (October 2010), "Flower Beds", National Geographic, 218 (6).
- Zurbuchen, A.; et al. (2010). "Long foraging distances impose high costs on offspring production in solitary bees". Journal of Animal Ecology. 79: 674–681.
- Rozen, Jerome G.; et al. (2010). "Nests, Petal Usage, Floral Preferences, and Immatures of Osmia (Ozbekosmia) avosetta (Megachilidae: Megachilinae: Osmiini), Including Biological Comparisons to Other Osmiine Bees". American Museum Novitates. 3680: 1–22.
- Rozen, Jerome G.; et al. (2009). "Biology of the Bee Hoplitis (Hoplitis) monstrabilis Tkalcu and Descriptions of Its Egg and Larva (Megachilidae: Megachilinae: Osmiini)". Zoological Journal of the Linnean Society. 167: 28–42.
- Kemp, Bosch, J., W.P. (2002). "Developing and establishing bee species as crop pollinators: the example of Osmia spp. (Hymenoptera: Megachilidae)". Bulletin of Entomological Research. 92: 3–16.
- Sheffield, C.S.; et al. (2008). "Diversity of cavity-nesting bees (Hymenoptera: Apoidea) within apple orchards and wild habitats in the Annapolis Valley, Nova Scotia, Canada". Can. Entomol. 140: 235–249.
|Wikisource has the text of the 1905 New International Encyclopedia article Mason Bee.|
- Osmia Identification Guide (female)
- Osmia Identification Guide (male)
- List of Species
- Worldwide Species Map
- Palaearctic Osmiine Bees
|Wikibooks has a book on the topic of: Beekeeping/Solitary Bees|