Genus-level entry for the solitary mason bees, named for the mud or clay partitions females use to wall off the cells of their nests. Roughly 140 Osmia species occur in North America, including the native blue orchard bee (Osmia lignaria) and the blueberry bee (Osmia ribifloris). They are cavity nesters that do not excavate their own holes — instead occupying beetle burrows, hollow stems, and gaps in wood, and readily adopting drilled blocks and reed or paper tubes. Active in early spring, mason bees are highly efficient pollinators of Rosaceae fruit trees (apple, pear, cherry, plum, almond, peach) because they carry dry pollen on the underside of the abdomen and forage in cool, overcast weather when honeybees stay in the hive.
Conservation
No genus-wide IUCN listing; the native blue orchard bee (Osmia lignaria) is widespread and not considered at risk. However, several native Osmia species have declined since around 2020, a trend linked to competition and pathogen spillover from introduced congeners such as the hornfaced bee (Osmia cornifrons). Supporting native mason bees means prioritizing native species, providing clean undyed nesting tubes that are replaced or sanitized between seasons, and avoiding the spread of imported bees and their parasites.