GREEN SWEAT BEES

Genus Augochloropsis, Augochlora & Augochlorella

Genus Agapostemon      

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The sweat bee family, Halictidae, is a highly important group of wild pollinators.  Sweat bees are instrumental in the cultivation of an impressive range of commercial crops -- among them squash, legumes, sunflowers, watermelons, strawberries, tomatoes and peppers, to name but a few.  Sweat bees are also essential pollinators of both garden flowers and native flora, and are common visitors to wildflowers at the National Butterfly Center. 

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Sweat bees span 14 genera within the United States and a seemingly endless plenitude of species. Some sweat bees are black or dark with striped abdomens.  Others -- members of the tribe Augochlorini and the genus Agapostemon -- are usually an eye-catching iridescent green.​

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Tribe Augochlorini:  Augochlora, Augochloropsis and Augochlorella 

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Augochlora, Augochloropsis and Augochlorella sweat bees belong to the tribe Augochlorini.  Bees in this tribe are brilliantly colored -- they usually range from golden-green to an intense pure green to a dazzling blue-green.  Augochlorella and Augochlora sweat bees may be copper-colored or even metallic-pink as well. 

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Augochlorini sweat bees tend to build nests in soil or, less commonly, in rotted wood.  Some form large aggregations in the ground, constructing nests in clumps linked together by earthen connections or rootlets. 

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Augochlorini sweat bees sometimes act cooperatively, constructing nests that share a common entrance protected by a guard bee.   Some Augochlorini are capable of shifting between solitary and cooperative behavior, depending on environmental conditions.

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Some Augochlorella and Augochlora sweat bees are considered to be “primitively eusocial.”  That is, they have a yearly life cycle split into spring and summer phases.  In the first spring “foundress phase,” the bees construct an underground nest and provision it for offspring.  When the young emerge, the males leave the nest, while the females remain in the nest  provisioning it for a second brood When this second brood hatches in late summer, the males and females mate. The males die and the inseminated females dig downward into the earth from the lowest parts of their nests.  They later emerge in spring to complete the colony’s life cycle.

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Members of the tribe Augochlorini tend to be pollinator generalists.  At the National Butterfly Center, Augochlora, Augochloropsis and Augochlorella sweat bees visit the flowers of a wide and varied range of plant families.   The brilliant blue-green Aztec sweat bee (Augochlora azteca) alone appears to restrict itself principally to one plant -- alamo vine blossoms -- although this bee has been documented visiting the flowers of esperanza and prickly pear cactus as well.

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Distinguishing among green sweat bee genera 

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Augochlora, Augochloropsis and Augochlorella sweat bees can be distinguished easily from one another, and from bees of the genus Agapostemon, by minute traits that are obvious under a macro lens or microscope.  To the naked eye, bees of these four genera may be hard to tell from one another. 

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Generally, Augochloropsis sweat bees tend to be somewhat larger than Augochlora and Augochlorella and are green or blue-green.  Augochlora at the National Butterfly Center may be green, golden-green, bluish-green or copper-colored.  NBC Augochlorella tend to be light-green, copper-colored or even a coppery-pink, like the one shown at right.

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Here are two useful tips for distinguishing among green sweat bees genera.  (1)  First, examine the tegulae (the nodes where the bee's wings join its body).  The tegulae of an Augochloropsis are metallic-green and D-shaped.  The tegulae of an Augochlora are oval and brownish in color. The tegulae of Augochlorella are a pale reddish color and lack a distinctly oval shape.  (2) Second, if you have a macro lens, you can distinguish green sweat bee genera by examining the marginal cells in their forewings.  Differences in the wings of sweat bees are explained in more detail in the photo strip at bottom right.  This photo strip also shows other more subtle characteristics that distinguish green sweat bees of different genera.

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Tribe Halictini:  Agapostemon 

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Agapostemons are small-to-medium-sized, brilliantly-colored bees that look like flying emeralds.  To the naked eye, bees of the genus Agapostemon may resemble the iridescent bees of the Augochlorini tribe discussed above.  Nonetheless, differences in wing and leg structure, facial characteristics and DNA have led to the genus Agapostemon's placement in a different tribe known as the Halictini.  (This tribe also contains, among others, darkly-colored, dark-and-pale striped and and dully-metallic Halictus and Lasioglossum sweat bees; and parasitic Sphecodes bees.)

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Agapostemons build nests in the ground.  Their nests consist of vertical tunnels, and are usually kept well-hidden under leaves or grass.  Like Augochlorini-tribe bees, Agapostemons are solitary, with each female provisioning her own nest , but the bees may act gregariously, building in close proximity with one another.  Like Augochlorini, Agapostemons sometimes share burrows whose entrances are watched by guard bees.

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Agapostemon sweat bees tend to be larger than green Augochlorini-tribe sweat bees.  Agapostemon species also often show pronounced sexual dimorphism:  females may be an intense iridescent green all over, while males are only partly green, with striped yellow-and-dark abdomens, as exemplified by the Texas Agapostemon shown here.  In other species, such as the honey-tailed Agapostemon, also shown here, males and females both have striped abdomens, but detailed markings and coloration on the bees' legs, faces and abdomens differ.  

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Agapostemons are usually generalist pollinators.  The honey-tailed Agapostemon visits plants as widely diverse as melons, cactus, cabbage, peas and mallows.  At the National Butterfly Center, this species is usually found in association with Berlandier's Fiddlewood and crucita.  Texas Agapostemons also pollinate plants from many different families.  At the NBC, they are usually spotted visiting sunflowers and other composite flowers of the aster family. 

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Male Agapostemons of separate species are differentiated by such traits as the colors of their abdominal stripes; the markings on their hind legs; and the marks on the undersides of their abdomens (their sterna).

 

Female Agapostemon species can be difficult and sometimes impossible to tell apart with the naked eye.  In Texas, for example, the females of two species, Agapostemon angelica and Agapostemon texana, are morphologically identical; they can be told apart only if caught mating with males (which are more dissimilar) or through microscopic analysis or DNA testing.  Some female Agapostemons can be distinguished by wing color; or traits invisible to the naked eye, such as the colors of hairs on their legs; and the patterns formed by tiny pits and indentations on the bees' thoraxes.  These differences are illustrated in more detail in the photographs shown below of individual Agapostemon species.