Please note: this blog post has been reformatted to fit the space. While some of the original images have been used, others have been replaced, and any internal links have been styled to show appropriately but may no longer link to their original destinations.
While honey bees are so-called for their capacity to produce honey, and bumble bees are nicknamed for their loud, iconic buzzing, digger bees are named for—you guessed it—their incredible tunneling capabilities.
With 70% of all 20,000 or so native bee species being ground-nesting—meaning they build intricate systems of tunnels to raise their brood in rather than erecting wax comb in hollow logs or under sheltered eaves—what makes a particular species a “digger?” And just how many digger bees are there?
As for what makes a bee a digger bee, to the best of my knowledge the defining characteristic is simply that a bee digs its own nest. Many species of bumble bee are ground nesting, for example, but wouldn’t be considered digger bees as they adopt any existing cavity that suits their needs—hollow trees, birds’ nests, and rodent holes being a few favorites—rather than spending time and energy constructing their own.
Pretty straightforward, right?
When it comes to the question of quantity, things get a little murkier. The short answer is: it depends who you ask
.It's probably no great surprise that taxonomic classification is an ever-shifting landscape. Properly identifying a species and its lineage can be a contentious topic; one that divides the scientific community and often takes years of extensive research and study to resolve. And even those long-fought, hard-earned resolutions are sometimes called into question, reassessed or recanted as new information arises.
Coming up with common names for a species (or a collection thereof) is likewise laborious and no stranger to dispute, as I discovered when embarking on what I thought was going to be a simple search into the digger bee.
Amegella albigena relaxing on some greenery, courtesy HWall via Shutterstock.
The “digger bee”—modeled above by the lovely European Amegella albigena—is a nickname afforded to many species, including bees in the tribes Anthophorini, Centridini, Emphorini, and Eucerini, though most often referencing Anthophora and Habropoda. These two genera—plural of genus—which fall within tribe Anthophorini, comprise around 500 species in total worldwide, 81 of which have been identified in North America.
That's a narrower selection than it could've been, considering that a few of the resources I referenced for this post estimate there may be up to 900 species of digger bee in the US and Canada combined. Even if we exclude all but those 81 species of Anthophorini, it amounts to roughly ten times the number of species we recognize as “honey bees,” which would take some serious effort to explore in full, not to mention bore most of you to death. (Just because I get very excited about minor morphological differences among bee species doesn’t mean everyone cares to take that close a look.)
Besides which, and perhaps more importantly: focusing only on the species in tribe Anthophorini would rob some of the most accomplished tunneling bees from other North American tribes of their shot at the digger bee crown!
With all that in mind, I’ve decided to take the next few weeks to spotlight some of the most exciting and impressive digger bees across the board, rather than trying to determine which species constitute the “true” diggers. (We’ll leave that matter of distinction up to the professionals.)
First up is one of my personal favorites—and one of three bee species I currently have tattooed on my person—the desert digger: Centris pallida.
There are somewhere around 110 species in the genus Centris, occurring from Kansas here in the US at the northernmost down into Argentina, with the richest concentration of species in tropical Central and South America, and Jamaica. Centris bees range in color from your standard black-and-yellow to umber gold to soft heather gray, with eyes running the gamut from black to green to red and many shades between.
A female Centris nitida bee, looking lovely even in death. Morticia Addams eat your heart out. Image courtesy York University’s Packer Lab Bee Tribes of the World website.
Centris pallida is a solitary ground-nesting species that can be found in the dry, hot climes of Arizona, Nevada, southern California, New Mexico, and Western Mexico. It was first discovered and cataloged by William J. Fox, in 1899 near Phoenix, Arizona, and there continues to be a large distribution of C. pallida—among many other native bee species—in central Arizona today that sees frequent study.
C. pallida is on the lighter end of the coloring spectrum and is sometimes referred to as the “pallid bee” due to its dusky gray fur and light yellow-green eyes. Pallid bees are a larger species of North American bee, averaging a little over an inch in length. In combination with their fuzzy coats, this makes them easy to mistake for bumble bees at a glance.
Giving a whole new meaning to the term “silver fox.” Image courtesy jmbearce via the Global Biodiversity Information Facility, shared under CC BY-NC 4.0
Pallid bees emerge from hibernation in the spring, usually late April or May, and live for about a month on the surface, growing scarce and difficult to find come July. They’re a fairly common bee, considered Least Concern in terms of conservation, and tend to be more active at night when temperatures drop. Not at all surprising, considering that pallid bees live within a few degrees of death, boasting an average internal body temperature of 119 degrees Fahrenheit where 125 would prove fatal.
Newly hatched male pallid bees tunnel their way to the surface first, whereupon they begin searching the previous years’ nesting ground for mates. Partners are much prized in the pallid bee community. A female pallid bee will mate with only one partner before she’s finished for the season. As such, C. pallida’s mating dance is usually more of a fistfight, with male pallid bees scrapping viciously over potential mates and racing each other to every fresh prospect that appears.
Smaller male pallid bees—called “small morph” bees in scientific parlance—hover around nearby food sources, relying on visual cues to alert them to a potential partner and avoiding the bigger “large morph” bees, which patrol along the ground, hoping to catch a whiff of the pheromones that precede an emerging female.
Yes, you read that right. Large morph pallid bees literally sniff out their mates, a useful skill to have evolved in such a highly competitive mating environment.
Let me freshen up. GIF courtesy GIPHY. If anyone is aware of a better source for this media, please let me know and I’ll update accordingly.
Bees, like most insects, use their antennae to pick up scent, in addition to a multitude of other ambient sensory information such as temperature, noise, and the movement of the air around them. These appendages prove to be a huge advantage when it comes to matters of the olfactory. Unlike our static human nostrils, a bee’s antennae can move independently of one another, swiveling on a geniculate joint—meaning it can bend like a human elbow—which allows the bee to triangulate the source of a scent with pinpoint accuracy.
Large morph male pallid bees have been observed scenting emerging females through more than an inch of soil, whereupon they help said females dig their way out in the hopes of proving themselves a satisfactory mate. That’s like a six-foot human possessing the ability to smell a potential partner through a three-foot wall.
To be certain that it was the male pallid bees’ sense of smell guiding them, rather than, say, faint sounds of movement beneath the surface as the females tunneled toward freedom, behavioral ecologist John Alcock—a leading authority on C. pallida—devised an experiment.
He would bury some freshly dead female pallid bees where the large morph males were patrolling, hoping that the lingering pheromones on the female bees’ bodies would be enough to alert the males to their presence. A little morbid, perhaps, but when the male bees were able to identify the dead females with no trouble, it confirmed that their sense of smell was to thank.
Sometimes, male pallid bees aren’t so lucky, digging up other species of bees or other ground-nesting insects entirely in their mating fervor; which is to say nothing of the females, who emerge to hundreds or even thousands of willing partners literally beating each other back to get to them. On occasion, a male digger bee will tunnel to meet an emerging female and throw himself on her before she even makes it to the surface.
Female pallid bees aren’t entirely without control in this interaction. While a female pallid bee can’t do much about her current mating prospects, she exerts her influence over the next generation of pallid bees by determining how she wants to raise her sons.
Female pallid bees spend their lives provisioning a nest for their offspring—digging various chambers, gathering food, and laying eggs. The larger the tunnel a female pallid bee digs, and the more bee bread—a mixture of honey and pollen—she leaves in a brood pot, the larger her offspring. In years where resources are scarce, or there hasn’t been much rain, C. pallida females might err on the side of caution, apportioning their resources to ensure smaller offspring. In years when water is abundant and food rich on the ground, they may be willing to risk bigger, heartier brood which will require more resources to thrive.
Close-up of some beautiful palo verde blossoms. Courtesy the University of Arizona’s page.
Finding food in places like the Sonoran desert isn’t always an easy task, made harder still when your entire diet is reliant upon the flowering plants in an arid climate seeing enough rain to flourish, but Centris pallida manage.
Pallid bees are pollinating generalists, meaning they visit a variety of plant species, though they favor palo verde—several species of flowering trees in the pea family, which are sometimes referred to as “green stick” trees due to their vivid green branches. Because their diet is so rich in palo verde nectar and pollen, C. pallida’s bee bread is often the same bright orange color. To supplement their diet, pallid bees will also visit orchids, desert willow, ironwood, creosote, and some species of cacti; a necessity for the males, which consume around 3.5 times their body weight in nectar and pollen every day to make up for the energy they expend seeking mates.
Starvation isn’t the only threat looming over our tunneling treasures. Pallid bees make attractive snacks for a variety of desert birds and lizards and have been known to fall victim to parasitic blister beetles, but far and away the most dangerous foes they face are the temperatures and the rain.
Sand is great at reflecting the heat of the sun—which is why daytime temperatures in sandy, arid climates go soaring—but doesn’t do a whole lot to retain it. The lack of humidity in desert biomes doesn’t help, nor does the dearth of vegetation, and many deserts experience temperature drops of 60 degrees Fahrenheit or more as the night descends. While pallid bees spend their days a few degrees away from suffocating to death via paralysis from heat exhaustion, if they haven’t sheltered themselves somewhere warm come sundown, they run the risk of freezing overnight.
Even more worrisome are the monsoons. When the high surface temperatures coming off the sand meet the cooler atmospheric temperatures above, the interaction often coalesces into sudden, violent rainfall that can drown a bee in its tunnel in a matter of moments.
Yet still, these devoted diggers persist. It’s a good thing they do, too!
True, C. pallida’s nesting habits can leave unsightly mounds of sand and holes all over your lawn, but consider the benefits: aside from pollinating some of the most iconic and exciting desert flora, their tunneling aerates the soil, allowing for better water flow to nearby vegetation, and their nitrogen-rich feces act as a natural fertilizer. They’re also generally non-confrontational, meaning they’d rather run than fight—at least when it doesn’t involve mating—and while they do possess the capacity to sting, it’s relatively mild.*
Overall, probably not worth the effort it would take to run them all off; though if you’re eager to flood your yard over and over, be my guest.
Join me next week for a special Halloween post: The Mellified Man and Other Spooky Stories, and the week after for part two of Tunnelers of Love, featuring the most boring bee in America!
*I have never been stung by a pallid bee and so can’t speak to what it feels like, personally, but I am assured by reliable sources that its sting is relatively mild in comparison to other bee stings.
All content on this page is copyright Rebecca Mehnert and/or
the clients for which it was produced, unless otherwise noted.
Please do not use, replicate, or reproduce.