Osmia pumila

Osmia pumila (Mason bee)

Taxonomy

Osmia pumila is in the subgenus Melanosmia within the Osmia genus. Bees in the genus Osmia, in the Family Megachilidae, are known as "mason bees" because some of them 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. This is one of several smaller Osmia known from Minnesota and it is the most common small Osmia found in Bee Atlas blocks, primarily in southeastern Minnesota.

Description

Osmia pumila, like 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. They are a smaller bee than O. lignaria, but with a similar chunky body shape. Females carry pollen in a scopa under the abdomen, like all Megachilidae. A close look at the abdomen will show that the scopal hair is light, and pits on the upper surface go all the way to the end of the segments, or very nearly. Males tend to be more brassy green while females are more blue-green.

Nest Structure

Osmia pumila 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 pumila build nests primarily in lower column 3 tunnels (3/16" diameter), and sometimes in upper column 3 (1/8") or upper column 2 holes (1/4") in Bee Atlas blocks. See graphs to the left.

Voltinism

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). They appear to nest just a little later than O. lignaria, and their nests are usually the first chewed vegetation plugs we see in any blocks. Between 2016-2018, nests completion was recorded most often between the middle of May and the end of June. However, some nests may have been completed as early as the last week of April. (see graph in pictures)

References

No information at this time.