Osmia is a genus of solitary bees in the family Megachilidae (order Hymenoptera), commonly known as mason bees. The name refers to their distinctive nesting behaviour: females gather mud, chewed leaves, or other "masonry" materials to partition and seal nest cavities in naturally occurring gaps — cracks between stones, hollow plant stems, or holes bored into wood by other insects.
With over 300 described species, Osmia is distributed broadly across the Northern Hemisphere. The great majority of species inhabit temperate regions of the Palearctic and Nearctic realms, flying from early spring through late summer. Many species are strikingly metallic — green or blue — although some are blackish and at least one (O. bicolor) is rust-red.
Mason bees differ from most bees in how females carry pollen: rather than using specialised leg baskets, Osmia and other members of Megachilidae bear dense pollen-collecting hairs (scopae) on the underside of the abdomen. This "belly-first" contact with flowers causes large amounts of dry pollen to fall onto the stigma at nearly every visit, making them exceptionally efficient pollinators. They are particularly associated with early-spring flowering plants in the family Rosaceae and will forage even in cool or overcast conditions that deter honey bees.
Several species have been adopted as managed pollinators in commercial agriculture. The orchard mason bee (O. lignaria) and blueberry bee (O. ribifloris) are widely used in North America; the hornfaced bee (O. cornifrons), native to eastern Asia, has been introduced to North American orchards; and the red mason bee (O. bicornis) is used across Europe. The translocation of non-native Osmia species has, however, raised conservation concerns, as introduced populations can spread pathogens and intensify competition with native bee fauna.
Etymology
The genus name Osmia derives from the Greek osmē (ὀσμή), meaning "smell" or "odour," a reference noted in early entomological literature. The common name "mason bee" describes the females' habit of using mud, grit, or other mineral materials — like a mason using mortar — to construct and seal their nest cells.
Distribution
Osmia ranges across the temperate Northern Hemisphere, with highest species diversity in the Palearctic and Nearctic realms. Species are absent from tropical regions and the Southern Hemisphere. Several commercially important species — including O. lignaria (orchard mason bee) and O. cornifrons (hornfaced bee) — have been deliberately moved outside their native ranges for agricultural pollination.
Ecology
Mason bees are solitary cavity-nesters active from early spring through late summer. Females provision cells with a pollen-and-nectar mass, lay a single egg, and seal the cell with mud or plant material. They are particularly effective pollinators of Rosaceae (apple, cherry, almond, blueberry) and forage under cool or cloudy conditions that reduce activity in honey bees. Management of non-native Osmia species for commercial pollination has been linked to the spread of parasites and pathogens to wild bee populations; as of 2020, some native species are in decline, and best practices emphasise use of locally native species, screening for disease, and building up wild populations.
Conservation
Some native Osmia species are in decline as of 2020, partly due to competition and disease spread from introduced non-native Osmia used as managed pollinators. Recommended mitigation practices include prioritising locally native species for managed pollination programmes, raising local wild populations, and enforcing parasite and disease screening of commercial stocks.
