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Tunnelers of Love: the Most Boring Bee in America

In a world where such a massive breadth of information—not to mention supporting photo and  video footage—is available at the press of a button, it can be difficult to believe that there’s anything new to discover under the sun. While some intrepid explorers have turned their attention outward, to the vast expanse of space, and others have decided to delve into the ocean depths—more than 80% of which has yet to be explored—there are still secrets to be discovered on land.

With an estimated 18,000 new species of plant and animal life identified each year, and only 1.2 million of the projected 8.7 million species in existence—give or take a million—having been formally identified, there’s a lot left to uncover.

Some of these newly discovered species are a simple matter of taxonomic adjustment, wherein a long known species was incorrectly ascribed to one genus or another, or a minor morphological difference between two species was overlooked. Case in point: the giraffe, which was believed to be a single species with nine subspecies until 2021, when researchers in partnership with the Giraffe Conservation Foundation published findings to suggest there are actually four genetically distinct species represented within the giraffe umbrella.

Giraffes in a thatch of dry grass on a wide open savanna. Image courtesy Mariola Gobelskavia Unsplash.

Other newly discovered species have been long extinct—take the Kem Kem Abelisaur, the fossilized bones of which were believed to be those of a juvenile Tyrannosaurus rex until earlier this year—or are teetering on the brink of extinction themselves, like the Brazilian Dinizia jueirana-facao tree, which can grow as tall as a 12-story building and weigh up to 62 tons. Despite its imposing size, this massive 2017 addition to the legume family escaped the notice of the scientific community for many years because there are only 25 of them left in existence.

The insect world in particular is considered to be fertile ground for new species to be identified. As is true in many instances, the smaller things are, the easier they are to overlook. And sometimes, even after those small things have initially been discovered, they get stuck in a drawer and forgotten for a few decades before someone with the time and inclination to properly classify them comes along.

Such it was with this week’s terrific tunneling species: the pueblo bee.

Inheritors of a proud tradition.

Anthophora pueblo—named in honor of the Ancestral Puebloan* peoples, who occupied the Four Corners region of the U.S. around the 12th century BC—was first recorded and studied in the late 1970s by Federal researcher Frank Parker. Though Parker collected samples from the two separate nesting sites he discovered and even managed to successfully rear some of the bees within from brood pots to emergence, his findings were never published. The samples gathered dust in a museum until 2015 when Utah State University graduate student Michael Orr came upon them.

Orr and Parker retraced the latter’s path through the San Rafael Swell—a 75 by 40 mile geologic feature in south-central Utah—where they rediscovered the original A. pueblo nesting site that Parker had identified all those decades before. It was still active even after 40 years, making it the longest lasting nesting aggregation of wild solitary bees we know of today. Most similar aggregations disappear over time, due to erosion, floods, droughts, or other natural phenomena. In the course of their exploration, Orr and Parker also identified five additional nesting sites, and began to expand their study in earnest.

With the assistance of his colleagues from USU and the Agricultural Research Service—the chief in-house scientific research agency of the U.S. Department of Agriculture—Orr and his associates were able to collect enough data to definitively prove that these determined diggers deserve their own branch on the evolutionary tree. They detailed their findings in a 2016 paper published in Current Biology. Since then, Orr, with the help of other researchers and citizen scientists, has identified more than fifty pueblo bee nesting sites in Utah, Nevada, California, and Colorado.

Dead Anthophora pueblo specimen, with mouthparts extended. Image courtesy Entomology Today.

Most desert dwelling bees are little known ground-nesting species, which share many characteristics, including morphology—being size, shape, and structure—behavior, and habitat. A. pueblo is no exception.

Like many of their counterparts, pueblo bees have fuzzy gray bodies—all the better for activity during the dim twilight hours, when temperatures in the desert tend to be more forgiving—four wings, and a pair of big, black compound eyes. They range from about 0.4 inches to a little over half an inch in length. Where they differ most dramatically from their tunneling relatives is in their choice of nesting substrate. While most species of digger bee seek out soft, sandy soil—easy to chew through when excavating their nests—pueblo bees prefer sandstone.

Yep. Sandstone.

The Devil’s Garden—a protected area of Grand Staircase-Escalante National Monument, home to  hoodoos, arches, and other sandstone features. Photo courtesy Kris Wiktor via Shutterstock.

Grand Staircase-Escalante National Monument (abbreviated GSENM) is a U.S. national monument in southern Utah, designated as such in 1996 to protect a handful of environmental attractions: the Grand Staircase, a massive sequence of sedimentary rock layers; the Kaiparowits Plateau; and the Canyons of the Escalante, a collection of erosional landforms created by the Escalante River and its tributaries. The almost 2 million acres that comprise GSENM are some of the most remote wilderness in the country and were the last land of the contiguous United States—the parts of the continent that physically touch one another—to be mapped.

GSENM also happens to play host to a huge variety of sandstone features, including high vertical canyon walls, slot canyons, water pockets, domes, hoodoos, and an assortment of natural arches and bridges. Plenty of options for the discerning pueblo bee in search of a home.

But how, exactly, does one dig a nest in sandstone?The same way the old adage tells us to eat an elephant, of course. One bite at a time.

More than a mouthful.

The largest hurdle presented by nesting in sandstone is that sandstone is a much harder substrate than sand or clay, meaning it needs to be softened before it can be manipulated.

Pueblo bees accomplish this by collecting water from nearby sources, which they store in their crop—a portion of the alimentary tract used to store food before digestion, often referred to as the “honey stomach” when discussing honey bees. They carry the water back to their preferred nesting site and use their proboscis like a hose, spitting out water and eroding the sandstone until it becomes soft enough to work. Once softened, the pueblo bee will literally chew through the sandstone, using her legs to scoop loosened particles out of her path as she tunnels further into the rock.

As you can imagine, this is a considerable expenditure of energy that effects a lot of wear and tear on the bee’s mandibles over time, which in turn often shortens her lifespan. Tunneling through a rock face also seems to make bee nests denser, meaning there’s greater competition to claim a nesting site. So, why bother? There are multitudes of substrates that would be easier to work with than sandstone. What makes it worth the effort?

The benefits of hard living.

As previously mentioned, sandstone is hard.

Often composed of quartz or feldspar—which generally measure a 7 and a 6-6.5 on the Mohs scale of hardness, a qualitative scale that determines scratch resistance in minerals graded from 1-10—this sedimentary rock occurs when eroded sand is transported by wind or rivers to places where tectonics has created space for it to accumulate. The sand grains are compacted and lithified—meaning they expel any moisture and become less porous as they compact—over time, gradually becoming solid rock.

The Wave, a sandstone feature in the Coyote Buttes North area of the Utah/Arizona border. Photo by Steve Arrington courtesy Unsplash.  page.

The same characteristic that makes sandstone so difficult to work with is also what makes it attractive to the pueblo bee.

Tunnels in hard rock are naturally disposed to last longer than those made in wood or dirt, meaning they can serve several generations of bees. They’re also less likely to succumb to environmental factors like flash flooding and erosion, and provide a safe, long term occupancy option for bees engaged in bet-hedging—a practice where new bees don’t emerge until habitat conditions are optimally primed to support them. Some bet-hedging pueblo bees have been recorded as delaying their emergence for up to four years.

While denser nesting aggregations can sometimes encourage the spread of parasites and disease, it’s been suggested that sandstone nests, which offer less organic matter for pathogens to feed on, might make for a lower microbial environment. In the case of larger parasites, such as the blister beetle—a common pest of many bee species—the hard sandstone caps a pueblo bee leaves on her brood cells, while dooming the larva in that particular cell to death, may serve to keep the beetle from emerging and ravaging the other cells along the tunnel.

All in all, a pretty decent trade-off if all it costs is some wear on your teeth.

It’s worth noting that while A. pueblo is determined, she isn’t foolish. If the sandstone in a particular area proves too hard, she’ll look elsewhere to build her nest. This hardness threshold is what makes the generational re-use of nesting sites so attractive a prospect, and the competition for workable sandstone so fierce.

In any case, the pueblo bee’s commitment to its habitat is to be celebrated. I’ve never taken such a big bite out of home ownership in my life.

*Also called the Anasazi (Navajo) and Hisatsinom (Hopi).