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Allyson Martin, Carlton, Christopher E.
The steel-blue mason bee (Osmia chalybea) is a solitary native bee found in the southeastern United States, including in Louisiana. Bees within the genus Osmia are commonly called mason bees because of their tendency to build their nests out of mud, plant material or even pebbles. Like many other mason bees, steel-blue mason bee adults possess metallic bodies and vary in hue from dark gray to blue or green. The wings are dark and slightly translucent. Pale yellow hairs (setae) cover the area on the head between the eyes (the vertex), the thorax, the legs and parts of the abdomen. The eyes are large and oblong and vary from black to light green, depending on the angle and lighting. Both males and females have prominent scissor-like mandibles.
Females are 12 mm to 15 mm in body length while males are slightly smaller, 10 mm to 11 mm. Females can be distinguished from males by the presence of orange hairs on the face and mandibles. Males lack these orange hairs, instead only having pale yellow hairs on their faces. Additionally, females have a dense patch of branched, black hairs (scopa) on the underside of the abdomen. The scopa is used by females to collect pollen to provision their nests. Scopae are absent on males.
All bees within the family Megachilidae are solitary. Solitary bees do not form colonies with a caste system typical of social bees and wasps, though they may nest gregariously. The steel-blue mason bee is a cavity-nesting species that uses hollow pithy stems or existing tunnels in wood substrates created by other species for nests. They can utilize artificial nesting sites composed of wood blocks with many drilled tunnels, ideally 7 mm wide and 18 mm deep. The nesting tunnel is divided into cells, each of which houses one individual offspring of the foundress female. One female may construct multiple nesting tunnels throughout her adult life.
In Louisiana and other southern states, adults appear as early as March and can often be seen on thistle flowers collecting pollen and nectar. After emerging during early spring, adult females forage for food, mate, search for suitable nesting sites and construct nests. Beginning at the back of the nest, each female deposits a food pellet made of pollen mixed with nectar. She lays a single egg into each food pellet and then caps the cell with plant material that has been masticated and dried into a cardboard-like texture. She continues building these cells along the tunnel, moving from the back to the front. Up to 12 cells may be completed, depending on the length of the nesting tunnel. Male offspring are located closer to the tunnel entrance, and females are deeper within the tunnel. When the nesting tunnel has been filled with cells, the female covers the tunnel entrance with a thicker cap of plant material and/or mud.
Many details of the steel-blue mason bee’s life cycle, such as the exact timing, are not well documented. The phenology likely varies regionally depending on local climate conditions, but the general pace of the life cycle is likely similar to that of other Osmia species. In the related Osmia lignaria, eggs hatch into larvae within a week of being laid (May), and after approximately two months, the larvae spin a cocoon and pupate (July). These bees reach adulthood during early autumn (September) and overwinter as dormant adults until they emerge the following spring. Cold overwintering temperatures are required for successful emergence of adults. Adults chew their way out of their cocoons and through the nesting tunnel. Newly emerged adults mate just outside the nest, often multiple times. Mated females feed on nectar while they seek a potential nesting site collect pollen for provisions.
Steel-blue mason bees are native to southeastern United States, occurring from Florida to Texas and as far north as North Carolina. The species is a thistle specialist, most often found on thistle flowers during spring (March through May). In Louisiana, it is most strongly associated with the purple-flowered bull thistle (Cirsium horridulum). Records also suggest it may utilize flowers of Carolina desert chicory (Pyrrhopappus carolinianus).
Some Osmia species, such as Osmia lignaria, are gaining recognition for their beneficial role as pollinators of specialty crops like blueberries. Hence, management protocols are being developed. The steel-blue mason bee fills an important ecological niche as a thistle specialist and has little agricultural potential as a managed pollinator because it is not known to interact with flowering crops.
Bees within the genus Osmia pose no threat to human health. Like with all bees, only females possess stingers, but they never sting people unless they are handled roughly. Stings are reportedly less painful than that of a honeybee. These bees and other solitary bees should be observed respectfully from a distance and left alone when encountered.
Contact Us: For advice about pest identification or diagnosis, contact the LSU AgCenter Department of Entomology. Reach the department through the Contact Us webpage.
P4020
9/16/25
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Osmia chalybea visiting thistle flower (Allyson Martin Ewert).
Osmia chalybea, dorsal and lateral views, museum specimen (Louisiana State Arthropod Museum).
