Osmia
Mason Bees

In Minnesota, there are twenty-two described species within the genus Osmia. Only one of these species—Osmia caerulescens—is not native to North America. Osmia construct solitary aboveground nests usually in plant stems or cavities in wood. However, two species, Osmia inermis and O. inspergens, construct clusters of masticated leaf brood cells attached to hard surfaces such as under a flat rock. One species, Osmia conjucnta, nests in snail shells. Most Osmia line and partition their brood cells with masticated leaves (leaves chewed up by the female). Some species combine masticated leaves with wood fiber or use pebbles to cap their nest. The commonly observed Osmia lignaria (orchard mason bee) lines, partitions, and caps its nest with mud.  

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Osmia are metallic blue (less often metallic green), dark gray, or black bees, small- to medium-sized with a rounded form; the dark or black species are primarily found in northern Minnesota. Osmia bucephala resembles a small bumble bee having a black abdomen and light yellow hairs on the thorax. Osmia lack yellow or red markings and a pygidial plate, but like other genera in the family Megachilidae, have two forewing submarginal cells. Osmia have an ariolum (pad between the tarsal claws) but lack a transverse carina on the first tergite (T1). Females collect pollen on hairs on the underside of their abdomen. Bees in this genus range in length from 5 to 17 mm (0.19 to 0.66 inches).