The Great Sunflower Project

Photo by Hartmut Wisch

Osmia - small leaf-cutters or mason bees (family Megachilidae)
By Lisa Schonberg and Mace Vaughan (Xerces Society) and Gretchen LeBuhn (SFSU)

Genus summary: Osmia is a large genus found in Eurasia and the New World, and many Osmia spp. are widespread and abundant. There are approximately 135 species in North America (Hurd 1979), 75 in the northwestern portions of both the U.S. & Canada (Stephen et al. 1969), and 19 in New York State (Danforthg & Magnacca 2002). They are most common in the western United States; east of the Mississippi there are only 27 species (Mitchell 1962). Osmia are rare in deserts (Michener 1994).

Floral relationships: Osmia are most common in spring. Most species are generalist foragers and commonly visit flowering shrubs and small trees in the Rose family, especially fruit trees in orchards [gretchen]. Some species prefer monkey flower, lotus, Phacelia spp., and plants in the families Asteraceae (asters, daisies) and Fabaceae (legumes). Osmia are important pollinators of fruit crops such as almond, apple, cherry, pear, and plum. One to two hives of honey bees (approximately 10,000 to 25,000 foragers) are needed in order to pollinate an acre of apple trees, whereas just 250 female Osmia bees can accomplish this same task. Some are known as “ leaf-cutter bees,” because they cut the leaves or flowers of plants and use the pieces to form nest cells. They are the “small leaf-cutter bees”, whereas the Megachile are the “large leaf-cutter bees.” Other species of Osmia use mud to separate their brood cells and are called mason bees.

Nesting habits: Osmia are solitary nesters. They nest in a variety of sites and have a diversity of nest architectures. Most Osmia do not construct their nests themselves (Michener 2000), but rather build cells in pre-existing narrow tunnels such as beetle burrows found in woody plants, the pithy or hollow centers of some plant stems, crevices between stones, stems, and abandoned wasp or bee nests. One North American species, O.conjuncta, and many Eurasian species, nest in empty snail shells. Osmia are opportunists; if you provide them with artificial nest sites such as wood blocks with holes, paper drinking straws or bamboo, they will readily occupy them (O’Toole & Raw 1999). Osmia construct cells with materials they collect from outside the nest such as chewed leaves, mud and resin (tree sap) (Michener 2000). Some species use a special pair of horns on their head to pack and smooth mud, whereas others use special spurs on their mandibles. The Osmia queen protects the nest by constructing an extra cell adjacent to the entrance and plugging the entrance with mud (O’Toole & Raw 1999). She also will typically lay eggs that will hatch into male bees in the two cells closest to the entrance, protecting the more important female bees in the back of the nest, away from predators and parasites that may attack the foremost brood cells.

Diagnostic characteristics: Osmia are robust, small to medium bees that are between 0.2 to 0.8 inches long. Most species are metallic, and many are brilliant metallic green, blue or even purple (Michener 2000). They usually do not have conspicuous bands of hair on their abdomen (Michener 1994). They have broad heads and wide abdomens (gretchen). Osmia females carry dry pollen in a scopa (brush of hairs) on the underside of the abdomen rather than on their hind legs like most other bees [O’Toole & Raw 1999]. This is characteristic of all females in the family Megachilidae, which also includes the genera Megachile, Anthidium, and Hoplitis.

Similar taxa: Osmia may be mistaken for other Megachilid genera, since they also carry pollen on the abdomen instead of on the legs. Actually, the more iridescent green species can look like some sweat bees, and the blue/purple species may appear similar to some andrenids (and some caliphorid flies for that matter).

Known conservation concerns: Two Osmia spp. are on the Xerces Society Red List of Pollinator Insects. Osmia ashmeadii is listed as Critically Imperiled, and is only known from one specimen collected near The Dalles, Oregon. This rare endemic is probably a specialist forager on flowers in the legume family (Fabaceae), like other species in its subgenus. Osmia cascadica is listed as Vulnerable because its range is limited to the high Cascades bounding the Columbia River Basin.

Interesting fact: In one Osmia sp., O.rufa, larvae must know which direction the nest entrance is in before they become mature larvae and pupate; at this point they are too large to turn around in the nest and will be stuck once they emerge as adults. The oldest bee is the first to emerge from its cocoon; it is normally in the deepest cell, farthest from the nest entrance. It proceeds to nip its neighbor, which then emerges and nips its next neighbor, and so on and so forth until the bee in the first cell is awoken, thus permitting them all to exit the nest. These behaviors are common in other Osmia spp. with similar nest architectures (O’Toole & Raw 1999).

Additional resources:

Key to Eastern Osmia females at Discover Life
Key to Eastern Osmia males at Discover Life