Mason bees are important pollinators of crops, wildflowers and woodland plants.  They are solitary bees that construct individual nests in hollow reeds or in pre-existing cavities and burrows found in dead wood. The name "mason bee" derives from the bees' practice of transporting mud to nesting areas in order to build structures in which to lay eggs. 

Mason bees belong to the tribe Osmini of the family Megachilidae.  Female mason bees carry pollen on scopal hairs located on the undersides of their abdomens, a trait that helps identify them -- and which they share with other members of the Megachilidae family, such as leafcutter, resin and carder bees.  Mason bees possess two other distinctive traits.  Their forewings have two submarginal cells (rather than the more common three)  and their front feet sport an extra part called an areolum. These traits are shown in the photo strip below.


There are various types of mason bees within the tribe Osmini.  Those shown here include the genera (genuses) Osmia, Hoplitis and Chelostoma.  A fourth category of Osmini, the genus Heriades, is shown in the Resin Bee section of this guide.


​OSMIA MASON BEES:   The term "mason bee" is most often used in reference to bees of the genus Osmia.  These bees are fast fliers and energetic and important crop pollinators.  Throughout North America, orchard owners use Osmia mason bees to pollinate spring-flowering fruit and nut trees. Commerical bee houses, like that shown at right, are often used to attract and house Osmia mason bees instrumental in orchard pollination.  


Osmia mason bees are small-to-medium-sized, with robust builds and relatively large heads.  In our area, Osmia  tend to fall into three subgenera (subgenuses):  Helicosmia (usually metallic green or blue with pale hairs and sometimes with pale abdominal bands as well); Melanosmia (usually robust in build and metallic green or black); and Osmia (which includes two imported species, the hornfaced bee and the bull mason bee). 


At least 25 Osmia species are native to New York.  A sampling of mason bees found in Rockfeller Park Preserve and Stone Barns Center for Food & Agriculture is shown below.  In addition to being useful fruit and nut pollinators, wild Osmia mason bees are efficient pollinators of wildflowers and garden flowers. 

HOPLITIS MASON BEES :  Hoplitis mason bees are pollinators of woodland trees and shrubs, garden flowers and berries; they also visit some varieties of commercial fruit. These members of the tribe Osmini have slender bodies markedly different from that of the typical Osmia bee.  Most Hoplitis mason bees of our region are black or darkly-colored, with pale abdominal stripes. 

Hoplitis bees use chewed leaves, dirt, pebbles or wood particles to construct walls between the brood cells of their nests.  Seven Hoplitis species are found in New York.

CHELOSTOMA  MASON BEES:  The genus Chelostoma is represented in New York by only one native species, the mock orange Chelostoma, shown at the bottom of this guide page.  This species has a singular appearance: mock orange Chelostoma  are small, antlike bees with disproportionately long jaws.  

Bees of the genus Chelostoma tend to specialize in particular plants. Thus, the mock orange Chelostoma is principally a pollinator of mock orange. Two other non-native species found in New York are pollinators of bellflowers. 


Osmia Mason Bees

FAMILY: Megachilidae
SUBFAMILY:  Megachilinae
TRIBE:  Osmini
GENUS:  Osmia
SPECIES SHOWN BELOW:  Osmia georgica,
O. pumila, O. bucephala, O. cornifrons, O. taurus

Hoplitis Mason Bees

FAMILY: Megachilidae
SUBFAMILY:  Megachilinae
TRIBE:  Osmini
GENUS:  Hoplitis
SUBGENUS SHOWN BELOW :  Alcidamea sensu stricto

Chelostoma Mason Bees

FAMILY: Megachilidae
SUBFAMILY:  Megachilinae
TRIBE:  Osmini
GENUS:  Chelostoma
SPECIES SHOWN BELOW :  Chelostoma philadelphi



An Osmia mason bee

An Osmia mason bee house

Hoplitis mason bee

A Chelostoma mason bee

Common physical traits of mason bees

Osmia Mason Bees

The beautiful Osmia georgica bee emerges in early to mid-May in our region and visits various woodland wildflowers including wild geraniums, members of the aster family and the blossoms of wild berries.  Osmia georgica mason bees are also documented pollinators of a variety of crops, among them apples, melons, squash and cucumbers.

According to, Osmia georgica mason bees are a relatively rare sight in New York.  These bees have appeared, however, for three consecutive springs (2015-2017) in Rockefeller State Park Preserve.  Enhanced trail management at the park has resulted in increased numbers of spring wildflowers in recent years.  In late May, 2016 and 2017, the humble wildflower known as common fleabane (Erigeron philadelphicus) appeared in the park in unusual abundance along trail edges. Wherever this flower flourished, Osmia georgica mason bees seemed to follow; multiple bees often appear on single plants in the Swan Lake area. 

Like mason bees generally, Osmia georgica are solitary.  They nest in hollow stems, in holes found in trees, in abandoned nests of other bees and in cavities found in clay banks. Female Osmia georgica are energetic pollen-gatherers that sometimes exhibit striking behavior when foraging:  when pollinating fleabane, for example, they position their heads in the center of the fleabane flower and whirl around in circles.

Osmia georgica bees belong to the subgenus Helicosmia.  As noted in this guide page’s introduction, mason bees of this subgenus tend to be metallic green or blue with pale hairs.  According to Charles D. Michener's The Bees of the World, female bees of this subgenus characteristically have four tufts of orange hair beneath the clypeus (the bottom part of the face, just above the bee’s jaws).  All of these traits are possessed by Osmia georgica, as shown in the photos at right.

Identification Information:   Osmia georgica bees are fairly small for mason bees.  The two female Osmia georgica bees shown here were between 1/4" and 3/10" long.  Viewed with the naked eye, an Osmia georgica bee appears a dark metallic blue-green with dark wings, legs and antennae.  Viewed minutely, the bee has pale hair on its head and thorax and pale hair bands on its abdomen.  These bands may be difficult for the casual observer to see; they are most apparent when the bee is viewed from the side. The bee’s hair bands help distinguish Osmia georgica from similar metallic mason bees that have solid-colored abdomens. 

Female Osmia georgica bees are fairly easy to identify, because the scopal hairs located under their abdomens are a bright orange, a trait shared by only a handful of mason bees in our area. Most mason bees of the American northeast have either black or pale scopal hairs; notable exceptions are the imported species, hornfaced bees and bull mason bees, shown at the bottom of this guide page – but these bees are considerably larger and brown to black, rather than metallic green, and thus easy to tell apart from Osmia georgica.

Osmia georgica bees also have distinctive growths on their mandibles. When viewed in profile (as shown in the photostrip at right), two toothlike projections can be seen protruding from the bee's face above its jaws.  According to bee expert and biologist Sam Droege, these are probably used by the bees to help carry mud to the nests. As noted above on this guide page, mason bees use mud to form partitions between egg cells.

Seen by the naked eye, Osmia georgica mason bees can be confused with small carpenter bees and dark sweat bees.  The best way to distinguish females is by their habit of transporting pollen on scopae (brushes) located under their abdomen, in the way characteristic of Osmia bees. Mason bees of both sexes are generally hairier than sweat bees and small carpenter bees and have stout, cylindrical bodies. In addition, viewed minutely, Osmia georgica have only two marginal cells in their wings, as shown in the photos at right. This trait is common to Osmia mason bees generally.

Osmia Georgica Mason Bee

Osmia georgica 

1/4" - 3/10" (small)

A female Osmia georgica mason bee with orange scopal hairs

A female Osmia georgica mason bee with growth-like structures and orange tufts of hair above its orange-rimmed mandibles

All photos (c) 2014 - 2016 Sharp-Eatman Nature Photography

Osmia (Melanosmia) Mason Bees

(Osmia Subgenus Melanosmia spp.)
approx. 1/3" - 3/5" (medium-sized)

During spring and early summer, Rockefeller State Park Preserve and Stone Barns Center for Food & Agriculture abound with metallic blue-green mason bees that belong to the subgenus Melanosmia. 


Melanosmia mason bees first become visible in the park as early as April, pollinating spring-blooming willows and woodland ephemerals such as trout lilies and claytonia.  In late spring and early summer, Melanosmia mason bees appear in the park feeding on penstamon, wild geraniums, bowman's root and wild blackberries and strawberries.  At Stone Barns, Melanosmia mason bees can be observed throughout spring, pollinating fruit crops such as apples and blueberries. 

Feeding habits among  various New York Melanosmia mason bees differ.  Some New York Melanosmia species specialize in a fairly narrow range of plants, such as the relatively uncommon Osmia inermis, which shows a preference for heather; and Osmia inspergens, which favors blueberries and members of the pea family.  Other Melanosmia, such as the Bufflehead mason bee (Osmia bucephala) and the dwarf mason bee (Osmia pumila), shown below, range on a wider array of plants.

The nesting habits of many Melanosmia species have not been fully explored by entomological research.  Known nesting habits vary considerably among species. Many Melanosmia construct their nests of chewed leaves or rootlets found in the ground abutting nesting sites.  Melanosmia  mason bees may excavate nests in the dead or fallen branches of trees and shrubs; underneath tree bark; in the ground; in stumps or dense grass tussocks; in holes burrowed by other insects into wood; or in holes drilled into human-made wood blocks. Still others, such the species Osmia inermis, attach their brood cells to the undersides of rocks or discarded pottery, or ensconce them in pre-existing holes found in walls.  Although Melanosmia are characteristically solitary bees, some work cooperatively:  for instance, several female Osmia inermis mason bees may live together communally, building their nests together under a single stone.

There are at least 17 species of Melanosmia in New York. (Species documented in the general southern New York area in which the preserve is located include, among others, Osmia albiventris, Osmia bucephala, Osmia collinsiae, Osmia distincta, Osmia felti, Osmia inermis, Osmia inspergens, Osmia pumila and Osmia sandhousae.)  The majority of these are somewhat smaller than honey bees, with shimmering blue-green bodies sparsely covered with pale hairs.  Females usually sport white scopal hairs under their abdomens.  Such bees may be very difficult to distinguish from one another, and species identification may require extremely minute inspection by an expert.

A few local Melanosmia species are easier for the casual observer to identify because they have unusual or distinctive characteristics -- these include the bufflehead and dwarf mason bees shown below.  

A female Osmia (Melanosmia)

An Osmia (Melanosmia) pollinating penstamon

An Osmia (Melanosmia) on Bowman's Root.

​​Osmia pumila is one of several metallic blue-green mason bee species belonging to the above-referenced subgenus Melanosmia.  In this group, this bee is distinctive in part because of its size. Pumila means "dwarf" in Latin:  the male of this species is among the smallest of Osmia bees found in the Northeast; the female is only slightly bigger.

These mason bees emerge in April in the park, when they can be spied pollinating spring bulbs such as trout lilies and claytonia.  Osmia pumila bees linger in the park through the end of June. They are frequent visitors of wild geraniums and the orange milkweed variety known as butterfly weed.  The bee shown here was found feeding during June on wild geraniums.

Wild Osmia pumila mason bees favor forest and wetland habitats.  These bees build solitary nests in the dead pithy stems of plants such as elderberry and roses and in holes found in rotted wood.  Osmia pumila nests are tubular in form, with entrances and outer chambers on either end and brood cells in between.  The bees use chewed plant material such as leaf pulp to form partitions between cells and to plug the ends of their nests.

In addition to visiting many woodland wildflowers, Osmia pumila mason bees are useful pollinators of apples, blueberries, strawberries, and caneberries. These small mason bees also pollinate woodland trees and shrubs, among them willow, redbud, dogwood, sassafras, viburnum and hawthorn. 

Osmia pumila are sometimes preyed on by the cleptoparasitic bee Stelis lateralis and by the cleptoparasitic wasp Sapyga centrata.  Parasitism by such species may increase where food sources for the mason bees are in short supply.

​​​Identification Information:   Osmia pumila mason bees are between roughly 3/10" and 2/5” in size; males tend to be are smaller than females.  Both are a dull metallic blue-green on close inspection, although from a distance the bees appear dark and even black when not illuminated by sunlight.  Males appear a shiny yellowish olive-green when illuminated by sunlight; females are a bluer green.  Osmia pumila mason bees have pale hairs on their heads, thoraxes and abdomens.  Females have white scopae under their abdomens, used to transport pollen.


In the Northeastern United States, there are several mason bees species of the subgenus Melanosmia that possess all of the above traits, and that thus closely resemble Osmia pumila.  Differentiating among these species and Osmia pumila requires expert assistance and may hinge on such subtle differences such as the width and appearance of bands rimming the bee’s abdominal segments.

Seen by the naked eye, Osmia pumila mason bees can be confused with small carpenter bees and dark sweat bees.  The best way to distinguish femaleis by their habit of transporting pollen on scopae (brushes) located under their abdomen, in the way characteristic of Osmia bees. Mason bees of both sexes have stout, cylindrical bodies and are generally hairier than sweat bees and small carpenter bees. In addition, viewed minutely, Osmia pumila have only two marginal cells in their wings, as shown in the photos at right. This trait is common to Osmia mason bees generally. 

A female Osmia pumila mason bee

Dwarf  Mason Bee

Osmia pumila )

1/4" (males)  - 3/10" (females) (small)

All photos (c) 2014 - 2016 Sharp-Eatman Nature Photography

​Bufflehead mason bees are among the most distinctive-looking mason bees of the above-referenced subgenus Melanosmia. These singular bees appear in the park and at Stone Barns Center for Food & Agriculture in early April, when temperatures are still chilly and occasionally dip into the thirties; they remain in our area until mid-June.  These bees are singularly hardy in cold temperatures; Osmia bucephala are found as far north as the Yukon.

Bufflehead mason bees nest in the cavities of dead wood or in the hollow dead stems of pithy plants such as elderberries. The bees construct walls between their egg cells with leaf or wood pulp scraped from the cavities’ interiors. In our experience, bufflehead mason bees tend to lurk around flowering plants at ground level, stopping to rest on bare earth.  When visiting flowers, bufflehead mason bees often pause to rest on leaves beside the blossoms, sunning themselves.

These bees are solitary, like the other mason bees on this page.  Nevertheless, Osmia bucephala may build nests close to one another.  Females have been known to establish nests beside those of other species of mason bees as well, in bee houses set out by orchard growers to accommodate commercially purchased Osmia such as blue orchard bees.  (An example of such a house is shown near the top of this guide page.) 

Osmia bucephala are pollinators of early spring wildflowers such as bluebells and trout lilies, and of native spring-blossoming trees and shrubs, among them ninebark, black chokecherry, redbud and dogwood.  Bufflehead mason bees also can be found pollinating garden azaleas, false blue indigo and lilacs. The male bee shown here was feeding on blueberry blossoms in the park during late April.  The female bee was found foraging on false indigo flowers in Stone Barns gardens during early June.

Identification information:  “Buffle,” according to the Merriam-Webster Dictionary, is short for “buffalo.”   Apparently the entomologist who named this mason bee thought that the female bee's large shaggy head made it look like a buffalo.  The most salient characteristic of this species is its robust size, unusual in mason bees of the American northeast.


Male and female bufflehead mason bees differ considerably in appearance.  ​​Females are more robust than males, and males lack the buffalo-like heads.  Males have a strong metallic blue sheen, while females appear black from a distance -- although under intense light, females' heads show metallic blue highlights.


The thoraxes of male bufflehead mason bees sport ruffs of pale hair, and their faces are covered with tufts of light hairs, as shown in the photo at top right. The first two segments of the male bee’s abdomen are metallic blue with sparse light-colored hairs, and the rest of the segments are covered with black hairs or dark hairs intermixed with pale ones. The male bee's black legs are also distinctive:   segments of the bee's tarsi (the bottom parts of its legs) flare outward.

On first glance, the heftier female bufflehead mason bee can be mistaken for a bumble bee, because it  has a yellow-and-black-striped body.  Yellow hairs cover the top of the bee’s head and thorax and the first two segments of the bee’s abdomen; black hairs cover the remaining segments.  The scopal hairs on the underside of the bee's abdomen, which it uses to collect pollen, are black.  The large head of the female bufflehead bee has a distinctive groove across the base of the mandibles.  The bottom of the female bee’s clypeus is thickened and protuberant, as shown in the photostrip at right.

Bufflehead Mason Bee

Osmia bucephala 

1/2" - 3/5"   (medium-sized)

A male bufflehead mason bee

A female bufflehead mason bee

All photos (c) 2014 - 2016 Sharp-Eatman Nature Photography

Hornfaced bees belong to a different subgenus than the Osmia mason bees featured above on this guide page -- the redundantly named subgenus Osmia.  In New York, only two mason bees -- the hornfaced bee and the bull mason bee shown below -- represent this subgenus, and both are invasives originating in Asia. 


In the 1960s, the United States Department of Agriculture hit on the idea of importing hornfaced mason bees from Japan, believing these insects might be better pollinators than honey bees. Why the USDA pursued exotic mason bees when we have a nice supply of our own remains a good question.  Over the next fifty years, hornfaced bees, as they are now commonly called, established themselves along the eastern coast of America, pollinating fruit trees with extraordinary efficiency and escaping from orchards into woodlands.  This species is now endemic to many northeastern states, among them Pennsylvania and New York.


Shipments of  hornfaced bees also contained unintended stowaways -- the bull mason bee and a parasitic Monodontomerus wasp.  Osmia cornifrons can now be purchased easily anywhere in the United States by mail. There remains concern among entomologists that these bees may have a negative impact on native mason bee species such as the blue orchard bee (Osmia lignaria).  


Nevertheless, it is notable that hornfaced bees are docile and rarely sting. And, their formal status as an exotic species notwithstanding, hornfaced bees have fulfilled their promise as pollinators. In Japan, hornfaced bees were used principally as apple pollinators at the time of their importation here. The bees have been described since by scientific teams in the United States as being 80 times more effective than honey bees in pollinating American apples. A hornfaced mason bee can visit fifteen flowers per minute and set 2,450 apples per day.  


The hornfaced bees shown here were found in woods bordering Stone Barns Center for Food and Agriculture.  Although Stone Barns cultivates many fruit tree varieties, the Center did not introduce these bees -- they arrived on their own, setting themselves up in a rotted tree trunk a few yards from a self-seeded cherry tree in a wooded area, and about one hundred yards from Stone Barns' fruit tree groves. 


These bees can be spied exiting and entering small holes in rotted tree trunks, where they nest.  Although the bees are solitary, they like to inhabit nests near one another. Males can  be observed  basking on rocks and leaf litter in early spring. They do this to warm their body temperatures and to lie in wait for mates.  Females tend to emerge 2 to 3  days after males, or slightly later, depending on weather.  Mating begins immediately after females emerge. The females then commence gathering pollen and establishing nests in holes in trees or hollows in dead plant stems. 


Identification Information:   The male Osmia cornifrons mason bee shown here has a black body covered with tan hairs.  From a distance, the bee has a furry appearance; its abdomen looks encircled with bands or sashes of light brown fur.  The bee has a wide, bulky head and a tear-shaped thorax that joins its abdomen in a narrow waist.  Osmia cornifrons bees hold their wings in a distinctive way, in a rough V-shape with the wing tips angled outward at the back. Males have white hair on their faces, as shown in the photostrip at right.  The faces of female Osmia cornifrons have hornlike protrusions in front.


These bees are easy to confuse with the stowaway species, Osmia taurus, which arrived with them in the U.S. in the 1960’s.  Osmia taurus bees can be distinguished, because their hairs are rust-colored, while those of hornfaced bees are tan or light brown.  Observed minutely, hornfaced bees also show some black hairs intermingled with the lighter ones, while Osmia taurus bees lack such black hairs. 


Osmia cornifrons bees appear in the woodlands bordering Stone Barns very early in spring -- in late March -- and disappear by the end of April.  The bees produce only one generation each year.  Hornfaced bees are also known by the name Japanese Orchard Bee.

Hornfaced Bee

Osmia cornifrons (Subgenus Osmia)

3/10"  - 1/3"  (medium-small)

All photos (c) 2014 - 2016 Sharp-Eatman Nature Photography

As noted in the above guide entry, bull mason bees came to the United States in the 1960's as stowaways on shipments of hornfaced bees imported from Japan by the United States Department of Agriculture. Like hornfaced bees, bull mason bees are native to Asia, but have established themselves in woodlands and orchards in the American Northeast.


These bees appear in woodland areas bordering Stone Barns just as apple trees begin to flower, around the third week of April.  Osmia taurus bees are documented pollinators of apples and berries.  They are also known pollinators of the spring woodland bulb known as the trout lily, which blooms in the same period as apples in our area. 


Like hornfaced bees, bull mason bees produce only one generation each year.  They tend to nest in cavities in trees and logs.  Since the 1970's, Osmia taurus has made its way progressively south through the Mid-Atlantic states.  Its expansion is of some ecological concern because Osmia taurus bees may be disruptive to other cavity-nesters and out-nest native Osmia species.


Identification Information:   The bee shown here is an Osmia taurus female.  This bee is the size of a typical honey bee and has a robust build.  The bee is similar in appearance to the hornfaced bee in the above entry, but the Bull Mason Bee has distinctive rust-red hair on its legs and thorax.  From afar, the bee has a furry appearance; its head, thorax and abdomen are covered with hairs, and it has long yellowish-orange scopal hairs on the underside of its abdomen (which it uses to transport pollen). 


As shown in the photostrip at right, the female Osmia taurus bee has  two bullhorn-like protruberances on its face, positioned over a pair of formidible mandibles.  Males lack horns but have unusually long antennae.  Males also have white hair on their faces.

Bull Mason Bee

Osmia taurus  (Subgenus Osmia)

3/10"  - 1/2"  (medium-sized)

All photos (c) 2014 - 2016 Sharp-Eatman Nature Photography

Hoplitis Mason Bees

Hoplitis Mason Bee

(Subgenus Alcidamea sensu stricto)

1/4" (small)

Like Osmia mason bees, Hoplitis mason bees belong to the tribe Osmiini of the family Megachilidae As noted in the introduction to this guide page, Hoplitis bees are insect-masons that construct walls between the brood cells of their nests.  Hoplitis females carry pollen on scopal hairs located under their abdomens.

​​Like all  the Osmia species shown above on this page, Hoplitis bees are solitary. According to entomologist Charles D. Michener, the nests of Hoplitis bees take many forms. Some Hoplitis bees nest in the stems of pithy plants and make partitions between cells with plant pulp, sometimes mixed with pebbles or wood particles. Other Hoplitis bees construct cell walls of resin mixed with plant bits inside the pre-existing cells of abandoned wasp nets.  Still others nest in the ground and build nest partitions using  soil or clay, sometimes lining cell walls with flower petals.

There are seven species of Hoplitis mason bees in New York. Hoplitis mason bees in our area tend to be between small in size and slender with long bodies markedly different from that of the typical Osmia mason bee.  Most are black or dark with pale abdominal stripes.

The Hoplitis mason bee shown at right belongs to the subgenus Alcidamea sensu strictothree New York species, nearly identical to the naked eye, belong to this subgenus (H. producta, H. pilosifrons, H. truncata).  These bees form nests in plants with pithy stems, such as elderberry, and partition their brood cells with materials made from chewed leaves mixed with wood and soil debris. The bee shown here was found emerging from the dead branch of an elderberry shrub in early June, before the bloom season for the park's wild elderberries. 

Hoplitis bees of the subgenus Alcidamea sensu stricto forage on a vast array of native wildflowers and shrubs, garden flowers and shrubs.  They are generalist pollinators that feed on such plants as sumac, dogbane, roses, cherry trees, caneberries,  blueberries, melons and squash.


Identification Information:   In New York,  Hoplitis bees of the subgenus Alcidamea sensu stricto are slender and dark with pale hairs that may form pale stripes.  Females tend to have robust, three-toothed mandibles.  Male H. producta and H. pilosifrons have distinct antennae; the segment at the tip of each antennae is hooked. 

The female Hoplitis bee shown here is black with a long, parallel-sided body.  The bee's antennae and legs are black, and its eyes are dark olive-green. There are four bands of white hairs on its abdomen -- the fifth abdominal segment lacks a band, and on the first two segments, the bands are interrupted in the middle (on the top side of the bee).  The bee was transporting dry yellow pollen on white scopal hairs underneath its abdomen.

Hoplitis bees can sometimes be confused with small leafcutter bees.  A simple way to distinguish them is that North American leafcutter bees lack ariola (small projection) on their tarsi (claws), while Hoplitis bees' claws have ariola.  A Hoplitis ariola is visible in the photostrip at right.

A female Hoplitis mason bee 

All photos (c) 2014 - 2016 Sharp-Eatman Nature Photography

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REFERENCES:  For a comprehensive list of references used in compiling this guide, click here:

Chelostoma Mason Bees

Like Osmia and Hoplitis mason bees, Chelostoma mason bees belong to the tribe Osmini of the family Megachilidae.  And like females of those genera, Chelostoma mason bees carry pollen on scopal hairs under their abdomens.  


According to the Xerces Society's guide, Attracting Native Pollinators, Chelostoma mason bees are solitary and build their individual nests in hollow stems and in pre-existing holes dug by beetles.  The bees divide their nests into series of chambers, fashioning partitions between them from a cement-hard mixture of fine pebbles and plant resins.


There are nine native species of Chelostoma in North America, and only one in New York -- Chelostoma philadelphi, shown here.  This is a small, antlike bee that specializes in pollinating mock orange (Philadelphus), a flowering plant genus that includes several varieties native to the United States.  Mock orange plants have white flowers with bright yellow stamens and leaves which, when crushed, produce a strong citrus scent. Mock orange blossoms in Westchester County in the second week of June.  At Stone Barns, mock orange Chelostoma mason bees arrive in large numbers to pollinate the flowers as soon as they open.  

Despite these mason bees' preference for mock orange, they do occasionally pollinate other flowers.  One of the bees shown here was found foraging on chamomile in the Dooryard Garden of Stone Barns, not far from a mock orange bush.  Mock orange Chelostoma also have been documented feeding on salvia, lilacs, caneberries, geraniums, hawthorne, holly, waterleaf and members of the mustard family.

Identification information:

"Chelostoma" means "claw mouth".  As shown in the photo strip at right, bees of this genus have spectacular long jaws with ridged, toothed mandibles.  Like all members of the tribe Osmini, Chelostoma mason bees have only two submarginal cells in their forewings, and their front feet sport an unusual extra part called an areolum between the tarsi (toe-like claws).

Mock orange Chelostomas can be mistaken easily for small flies or winged ants -- they are black, long-bodied bees with large heads.  They are also miniscule and easily escape notice.  The best way to observe these bees at work in nature is to watch mock orange shrubs during their blooming cycle.  

Female mock orange Chelostomas have white scopal hairs under their abdomens.  The bees' undersides often appear to be bright yellow -- this is because the pollen of mock orange is an intense yellow.

Similar species:  Two invasive species of Chelostoma mason bees, originally from Europe, are found in New York:  C. campanularum and C. rapunculi. Both have somewhat smaller jaws than the mock orange Chelostoma but are otherwise similar in appearance.  Nevertheless the invasive species can be fairly easily distinguished because they specialize on bellflowers.

Mock Orange Chelostoma

Chelostoma philadelphi
Subgenus Prochelostoma

females:  1/4" - 3/10" - males:  1/5"  (small to very small)

All photos (c) 2014 - 2017 Sharp-Eatman Nature Photography

A female Chelostoma philadelphi mason bee 

A female Chelostoma philadelphi mason bee 


The mock orange Chelostoma is a slender, antlike bee.