feature

Bee Bank

Text and photographs by Edward S. Ross

Female solitary bees dig brood chambers for their eggs in steep coastal banks such as this one in Cambria, California. Image by Barbara R. Tapp

On a California bank overlooking the chilly Pacific Ocean, hundreds of bees meticulously work their way through a stand of weedy wild radish flowers. Each buzzes heavily from one bloom to the next, collecting nutritious pollen and sugary nectar along the way. Fully laden, with her stocky body covered in sticky yellow grains, she makes a beeline back to the bare vertical face of the bank and disappears into the depths of marble-sized holes arrayed like pegboard notches in the soil’s surface.

Despite appearances, these insects aren’t bumblebees—they are bumblebee impersonators, Anthophora bomboides stanfordiana, one of about 20,000 species of solitary bees whose habits differ greatly from those of social bees such as honeybees and bumblebees. Because such bees (and paper wasps such as yellowjackets) mount fierce nest defenses, at times flying out en masse to sting potential invaders,* their appearance is frequently imitated by harmless solitary bees, wasps, flies, and even beetles. As stanfordiana mimic aggressive bumblebees, they have little, if any, need to use their stingers. Their nests can be closely approached, and they seldom sting even if handled.

The disguise of this false bumblebee suggested its scientific name. Anthophora means “flower carrier” in Latin, and bomboides refers to the buzzing characteristic of bumblebees. Its subspecies name, stanfordiana, was proposed by T.D.A. Cockerell in 1904 for a few specimens collected on the campus of Stanford University. Stanfordiana is found throughout much of California and Oregon, while other subspecies of bomboides range throughout the United States and Canada.

Like most solitary bees, stanfordiana nests in the ground. Because damp conditions increase the danger of mold spoiling nest provisions, they often use banks which provide good drainage. For this reason, solitary bees are most numerous in semiarid regions.

The black and yellow bees flying about the bank are busy at the primary task of all organisms—reproduction. Not having much more on their “minds” but mating, males establish territories near the bee bank, ever-ready to ward off any rival males. They ignore similar-appearing bumblebees because they don’t “smell” right. After establishing his domain, a male will pursue any female foraging for pollen and nectar in his zone.

After mating, a female stanfordiana begins excavating a nest, favoring warm, sunny, southern exposures of eroded gullies. This favored nesting habitat may harbor a great density of nests, sometimes described as a “village.” Scientists think that bees nest in aggregations because young females “memorize” visual landmarks, or they may be attracted to smells emitted by other female bees. Steep, hard-surfaced banks offer greater protection by making it difficult for hungry animals such as skunks or raccoons to dig out and eat the bees’ broods.

The first task of a nest-building female is to soften the hard bank’s surface. She sucks water seeping from the base of the bank into the same gut chamber she later uses to store nectar. She may make as many as 80 trips a day to obtain enough moisture to construct the nest. As she digs, dog-like, into the bank, the excavated wet soil assumes the shape of an arched cylindrical turret. This structure may discourage parasitic bombyliid flies from flying over the nest’s entrance and “skip bombing” their eggs into the nest. (These flies will even drop eggs into holes punched in the ground with a pencil.)

After digging a hole several inches deep straight into the bank, the bee starts constructing the first of about four larval cells which branch off like curled fingers from the main entrance passageway. She coats each cell’s walls with a gray, waterproof, waxlike secretion from glands in her abdomen. This may prevent molds from infiltrating the cell.

After spending the night in her burrow, warmed by the morning sun, she heads out to collect pollen and nectar. She may visiting several thousand flowers, on the 20 or more foraging trips she makes each day. Gradually, she fills the lower two-thirds of the cell with the mixture she has gathered from nearby flowers. The semiliquid substance, which has a rather unpleasant, cheesy, rancid odor, will serve as food for her young.

Once she has filled a cell to the proper level, the bee lays a single, C-shaped egg about two millimeters long which she “floats” atop the food. Like their relatives the ants and wasps, bees can select the sex of their offspring. Any egg she fertilizes with sperm that has been stored in her body since mating will become a female. Males develop from unfertilized eggs. Bees typically lay female eggs first and provide more ample food supplies for daughters than for sons.

Finally, the mother bee caps the cell with an earthen plug before starting on another one. After completing all the cells, she packs the main entrance burrow with dirt and flies away. She will never see her offspring, for she dies soon afterward with the approach of fall.

About a week after an egg is laid, it hatches. After eating all the food in its cell, the larva defecates and enters an inactive prepupal stage. With cooler weather, its metabolism idles, and the prepupal larva remains immobile from late summer to March, living on stored fat.

As cold weather sets in, the bee bank’s activity falls quiet as adult stanfordiana die one by one. Throughout fall, winter, and early spring, all that remains of the once bustling bee community are hundreds of fully-fed prepupal larvae in their underground cells. The snug confines of their earthen burrows will keep the infant bees safe and dry throughout the winter.

As the bee bank warms early in March, each prepupa sheds its skin and becomes a glossy white pupa, bearing in relief the shape of its future adult structure. By mid-May, the black body of the adult begins to show through the pupa’s translucent skin. Mature bees emerge soon after, gnawing their way to the bank’s surface via the plugged entrance shaft.

In spring, adult male Hornia bite their way out of their larval skin and chew their way into adjacent stanfordiana cells in search of a female blister beetle. Soon after mating, the female may lay as many as 1,300 eggs in a pocket just within stanfordiana’s cell plug. Because all Hornia will have completed their life cycle before all stanfordiana finish nesting, the parasite is never able to exterminate a bee colony.

Beyond parasitic insects and other natural enemies, stanfordiana and the many other solitary bee species face a more recent threat. Sadly, the nesting habitat available to solitary bees has been seriously reduced by man’s environmental disturbances. Planting vast fields with a single crop puts local solitary bees at risk because the plant’s flowers all wither at once. Weeding destroys nearby alternative, wild sources of food. Pesticides can kill beneficial solitary bees along with crop pests, while urban development continues to pave over land once colonized by solitary bees.

The loss of solitary bees would be a tragedy. In addition to often being the only pollinators of many species of native plants, which beautify the world, some species of solitary bees are far more efficient at pollinating alfalfa, almonds, and other crops than honeybees. Some farmers have even begun to create artificial nest sites for solitary bees by drilling holes in wood blocks sheltered in open-sided sheds built alongside crop fields. The honeybee is not the only insect pollinator. All of us should welcome stanfordiana and the other solitary bee species as essential elements of our agricultural and aesthetic environment.

Edward S. Ross is curator emeritus of entomology at the California Academy of Sciences.