DIGGER BEES
Genus Anthophora

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Anthophora, commonly called digger bees, are entertaining, unusually noisy insects with a distinctive way of flying. They zip speedily around flowers and then stop abruptly, hovering in front of a blossom, sometimes angling their long tongues down a flower’s throat without letting their feet touch the petals.  Or they buzz raucously while clinging to the flowers, and thrust their faces so deeply into them that when the bees emerge, their heads and thoraxes are coated in pollen.

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Anthophora are solitary – that is, each bee builds and provisions its own nest.  Nonetheless, digger bees are often gregarious, preferring to construct their nests close by one another, sometimes forming large aggregations that number in the hundreds or even thousands.  

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As their name suggests, digger bees typically nest in the earth, either in level ground or vertical banks.  A few Anthophora species make tunnels in pithy stems and rotted wood, lining their nests with oils collected from plants. 

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Female Anthophora construct ground nests by digging with their front legs and using their mandibles to loosen dirt.  In some species, such as Anthophora abrupta, females use their front legs to haul mud through the air; they then work the mud to moisten the dirt of dry nesting areas, softening it up for better digging. 

 

When expanding underground lairs, some Anthophora females pile excavated soil outside their holes in tall thin stacks called tumuli. Female Anthophora  vigorously defend their nests against predators. Anthophora females have been observed dragging parasitic bees bodily from nest holes.

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Male digger bees, like the Anthophora californica shown here, may have thickly-built hind legs equipped with wide blades.  Such leg modifications are used for grasping female bees during mating. Male Anthophora often gather outside of nest tunnels dug by females.  The males sometimes pile together in scrums outside of nest entrances, awaiting the emergence of a female.  When she appears, they jump on her en masse.

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Various species of Anthophora  have been documented engaging in boisterous behavior.  In 1929, the entomologist Phil Rau wrote of Anthophora abrupta: “They are neither timid nor aggressive, but …how conspicuous they are as they noisily swing their ponderous bodies to and fro on the wing, arrive home and scramble into their burrows or come tumbling out headlong and dash off into the sunny fields, with all the exuberance of boys just out of school. They have none of the shy, stealthy ways of maneuvering, whereby some of the smaller and daintier varieties of bees and wasps hold their own in a competitive world.”

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Entomologist Robbin Thorp recorded in 1969 that if one strolled through a nest colony of Anthophora  edwardsi,  "swinging an insect net about a meter above the ground, the females would aggregate in the plane of motion.  Thus it was possible to capture 25 to 50 females in five sweeps".  Thorp noted also that objects thrown over or onto the nesting site were likely to be followed aggressively by large numbers of females.  He found their  behavior to be remarkable, because solitary bees rarely act collectively in this manner.