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University of Minnesota Extension

Species guide - species detail

Osmia sp. (Mason bees)



Bees in the genus Osmia, in the Family Megachilidae, are known as the mason bees because many of them build their nests from mud. Some members even build free-standing nests. They overwinter as adults and are the first bees active as soon as the snow is mostly melted.


Many Osmia sp. have a metallic blue-green shine to their exoskeletons. You may need to get them in the right light to see this color, otherwise they may look mostly dark.

Nest Structure

Many Osmia sp. build mud nests, and some use chewed leaves.

Hole Sizes

No information at this time.


Univoltine, meaning they complete one generation per year in Minnesota. Both males and females overwinter as adults inside their cocoons.

Activity Period

Very early spring. These are among the first bees out (frequently in May).


No information at this time.

Minnesota Record Map

These data are from the Minnesota Bee Atlas project.

Back to Bee species guide