Bees of the Osmia genus are frequently called “Mason Bees,” and they’re most common in the western United States. They differ from other types of bees because of their unusual nesting habits.
The Xerces Society, a nonprofit conservation organization, is a leader in the effort to conserve North America's native pollinators. Founded in 1971, the society protects insects and other invertebrates through advocacy, education, policy development and applied research projects aimed at protecting and managing critical habitat. Attracting Native Pollinators (Storey Publishing 2011), by The Xerces Society, is a complete action plan and information guide for protecting bees, wasps, butterflies, moths, flies and some beetles by providing flowering habitat and nesting sites.The following excerpt comes from Part 3: Bees of North America.
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Osmia are usually called Mason Bees because many of these types of bees construct walls of mud to divide the nest cavity into brood cells. The genus is large, with approximately 150 species in North America. Many species are widespread and abundant, but Osmia are rare in deserts. They are most common in the western United States; east of the Mississippi, there are only 27 species.
Robust, small to moderate-sized bees, 0.2 to 0.8 inches (5–20 mm) long. They have round, broad heads, and their round, wide abdomens usually lack conspicuous markings or hair bands. Most are metallic, and many are brilliant metallic green, blue, or even purple. Osmia females carry dry pollen in a patch of hairs (scopa) on the underside of the abdomen, a feature they share in common with other females in the family Megachilidae.
Similar Types of Bees
Metallic green Osmia can look like some Agapostemon or Augochlora; the blue and purple species may appear similar to bees in the family Andrenidae. Osmia are generally more robust than any of these, and may stand out because of the pollen packed under their abdomens. Osmia are also often mistaken for flies.
Mason Bees are most commonly seen in spring. Most species forage for nectar and pollen on a wide variety of flowers. They commonly visit flowering shrubs and small trees in the Rose family, especially fruit trees in orchards. This fact has made Osmia (especially O. lignaria, the blue orchard bee) important pollinators of fruit crops such as apple, cherry, and plum.
These solitary bees usually nest in beetle tunnels in dead wood or the hollow centers of plant stems, but some nest in crevices between stones or in abandoned wasp or bee nests. One North American species, O. conjuncta, nests in empty snail shells on the ground. Females will readily occupy artificial nest sites such as drilled wood blocks or bundles of reed stems.
The nest cavity is divided into brood cells with materials collected from outside the nest. Most North American species, including the blue orchard bee, use mud to build these walls; others use chewed-up leaves or petals. Mason Bees protect their nests by plugging the entrance with a thick layer of nest materials.
Two species from these types of bees are on the Xerces Society’s Red List. The critically imperiled O. ashmeadii has been collected only near The Dalles, Oregon. O. cascadica, listed as vulnerable, is limited to the high Cascades bounding the Columbia River basin.
Did You Know?
Mason bees (both Osmia and Hoplitis) may nest in wood blocks drilled with holes, bamboo, garden hoses or faucets, even the ground hole in an outdoor electrical outlet!
To learn more about Mason bees and other types of native pollinators, read How to Attract Native Bees to Your Organic Garden.
"Bee" an expert! Identify more types of bees from Attracting Native Pollinators
Regional Plant Lists for Native Pollinator Gardens
The following charts list plant species that are native to their respective regions and commercially available from nurseries, specialty seed producers, and local native plant societies. The plants listed here tolerate a wide range of soil and light conditions. They are listed by season, to make it easier to design a garden that provides forage in spring, summer, and fall. If particular species are difficult to find in your area, look for closely related ones.
Northeastern United States and Eastern Canada
Southeastern United States
Midwestern United States
Great Plains and Prairie Provinces
U.S. and Canadian Rocky Mountain Region
California and the Southwest
Pacific Northwest and British Columbia
Lower-Cost Ornamentals for Many Regions
This excerpt has been reprinted with permission from Attracting Native Pollinators: Protecting North America's Bees and Butterflies by The Xerces Society, published by Storey Publishing, 2011. Buy this book from our store: Attracting Native Pollinators.