Osmia

Genus Osmia

Although their center of diversity is in the American southwest, Osmia are also the most diverse group of bees in the Salish Sea, with 24 species collected thus far in the San Juan and Gulf Islands. These bees are small—half an inch (12 mm) or less—and are typically metallic blue, green, or black. The abdomen is stubby and round with dense setae on the bottom. There is usually also a halo or ruff of fine setae on the thorax, and often tufts of setae on the face and “shoulders”.

Mason bees typically nest in pre-existing tunnels in old wood, such as tunnels left by beetle and moth larvae, plastering the cells with mud. This behavior makes them easy to attract to “bee hotels,” so mason bees are frequently raised commercially and imported by gardeners and farmers, often without sufficient attention to the impact of competition on native bees, or the risk of introducing new parasites and pathogens. Osmia lignaria is the species most widely sold commercially.

Osmia lignaria, Lopez Island