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Species guide - species detail

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Osmia tersula (Mason bee)



Bees in the genus Osmia, in the family Megachilidae, are known as "mason bees" because some species use mud to build their nests. 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. Osmia tersula is one of several smaller Osmia recorded in our state. Nests locations were distributed widely within Minnesota, but many came from northern Minnesota, in the Laurentian Mixed Forest biome.


Osmia tersula look very similar to Osmia pumila at first glance. Closer inspection shows wide, unpitted bands on the top of the abdomen and a bluer sheen, especially in males, as compared to the more brassy green O. pumila males. O. tersula females have black scopal hairs instead of light hairs, and can be slightly larger than O. pumila.

Nest Structure

Osmia tersula uses chewed vegetation to make their nest walls and plugs. They chew up plant leaves and press them together into a solid wall that at first is quite green, and later fades to brown and can pull away slightly from the walls of the tunnel. One way to describe the texture is like home-made paper, where the fibers are visible, but stuck together. Another way to visualize these plugs is as a teeny tiny round hay bale, viewed on the round end, and green rather than yellow/brown. Frequently the bees apply the chewed vegetation in a spiral pattern in their nest plugs. These plugs can be confused with “plant fibers” (which are actually whitish cotton ball consistency) or “grass” (which is dried brown grass and shoved more loosely into holes).

Hole Sizes

Osmia tersula primarily used holes in the lower column 3 (3/16" diameter) in Bee Atlas blocks. They also used upper column 3 (1/8") and column 2 holes (1/4" and 5/16") to a lesser extent (see graph).


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

Activity Period

Osmia are among the first bees of the year (frequently in May). Osmia tersula is active just a little later than O. lignaria, and their nests are among the first chewed vegetation plugs we see in any blocks. Between 2016-2018, nest completion was recorded most often between the second week of May and the end of July (see graph in pictures). Nest completion seems to be later in USDA hardiness zone 3 than in zones 4b and 4a. In 2022 and 2023, nest completion was recorded between the first week in June and the first week in July in northern MN.


No information at this time.

Minnesota Record Map

These data are from the Minnesota Bee Atlas project.

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