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Mason bee is a name now commonly used for species of 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 , 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 Osmia ribifloris, the hornfaced bee Osmia cornifrons, and the red mason bee Osmia bicornis. The former two are native to the Americas, the third to eastern Asia, and the latter to the European continent, although O. lignaria and O. cornifrons have been moved from their native ranges for commercial purposes. Over 300 species are found across the Northern Hemisphere. Most occur in temperate habitats within the Palearctic realm and Nearctic realm realms, and are active from spring through late summer.
Osmia species are frequently metallic green or blue, although 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 arolium between their claws, unlike Megachile or Anthidium species.
Historically, the term mason bee has also been used to refer to bees from a number of other genera 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 in hollow twigs but can be in abandoned nests of wood-boring beetles or , 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 Osmia avosetta is one of a few species known for lining their 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 toward 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 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 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 voltinism, meaning that they have a two-year maturation cycle, with a full year (plus) spent as a larva.
Some farmers currently manage populations of Osmia to facilitate efficient pollination on their farms. However, using non-native Osmia species as managed pollinators has ignited the spread of disease, introducing invasive bee species that increase competition for native bees. In some areas, native Osmia species are in decline as of 2020; practices to minimize the impact of non-native pollinators on wild species include prioritizing the use of native bee species, raising local bee populations, and enforcing parasite/disease screening.
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