91ÖÆƬ³§

Full confirmation of this hardy species took five decades of scientific study

A few years ago, Adrian Carper, who is in my department and also the Museum of Natural History at the University of Colorado Boulder, told me about a species of bee that could chew nest holes into stone. It was fascinating, and it stuck with me.

Several weeks ago, David Coppedge posted photos in Flickr of sandstone cliffs riddled with small holes, and he asked if anyone had an explanation for the holes. These threads can be tied together, but we have to go back in time.

Almost 50 years ago, Frank Parker, working at the U.S. Department of Agriculture Bee Lab in Utah, made a discovery that has unfolded slowly. He had found a bee that chewed into sandstone to make its nests. He cut out a few blocks of stone and took them to a museum, where the newly emerging bees could be caught, described and named.

Quite recently, Parker and Michael Orr, at Utah State University, were working in the University Museum in search of bees that had not yet been named. The blocks of stone and the bees that emerged from the stone were properly saved and accessible decades after Parker deposited them.

A female Anthophora pueblo bee (Photo: Michael Orr/Utah State University)

Before announcing their discovery of a new species, Orr and Parker felt that they needed to buttress their paper by discovering more bee populations. They soon found five additional sites, one of them in the Long House Ancestral Puebloan Cliff Dwelling in Mesa Verde. They published their discoveries in 2015 and named the new species Anthophora pueblo.

Females dig a hole in sandstone, chewing a tunnel that may branch to about five alcoves and then depositing one egg per alcove, plus sufficient provisions to nourish a larva until it emerges as an adult. Gnawing into sandstone is a formidable task, but females make the task possible by drinking from a nearby spring or stream and regurgitating water on the surface that they are working on.

The water weakens the carbonate crystals that glue grains of sand together. Gnawing can be heard outside the tunnel, and it wears down mandibles until they are nearly useless. Nest holes are placed in soft sandstone, but the bees cannot tunnel into hard sandstone. Soft sandstones are relatively low in quartz and high in clay. The bees have a preference for the soft mortar used to assemble stone walls in pueblos.

A nest site of A. pueblo is often an expanse of vertical sandstone, facing east, with an overhang above and a reliable source of water nearby. Surfaces facing east are warmed by the morning sun, speeding development of eggs and larvae. Vertical surfaces beneath overhangs cannot be flooded by water sheeting down walls or swept away by swollen and raging streams. A reliable source of water enables females to soak the surfaces of stone that they are chewing through.

Another advantage of nests in stone is the longevity of the structure itself. In contrast to nests placed into the ground, or into soft banks carved by streams, nests in stone last many years.

This affords A. pueblo larvae the option of delaying emergence. When conditions in arid environments are harsh, it is a great advantage to larvae to put off emergence until conditions predict a salubrious season for flowers and bees. For example, some A. pueblo have delayed emergence for four years.

A female emerging from a stone nest may deposit her eggs into the nest that she vacated. Use of her own nest would save her an enormous amount of labor. Energy saved could be devoted to increasing the provisions supplied to each nest, to give bees in the next generation a strong start.

And perhaps generous provisioning of bee larvae would benefit the original female's entire lineage, stretching many generations into the future. The original nest site discovered by Parker is still in use today, 50 years later. The nest holes in the Long House Ancient Puebloan Cliff Dwelling at Mesa Verde are not in use today because the rock surface fell so that it is no longer warmed by morning sun. However, the holes in the rock have lasted more than 700 years; is it possible for nests to be used for many centuries?

One of the other advantages of stone nest sites is that this environment is inimical to the pathogens and parasites that kill bee species that nest in wood or moist dirt. Stone tunnels lack the moisture and food that most pathogens and parasites need.

My literature search for other rock-munching bee species yielded only one species, but I will mention it here because it lives in only one locality, the White Rocks Formation in Boulder.

In 1928, while he was a graduate student at CU Boulder, Clarence P. Custer published a paper describing a bee, Perdita opuntiae, that digs tunnels into sandstone and nests communally. Each nest has multiple openings and is occupied by an average of nine bees.

A. pueblo's story was released relatively recently, and it is not yet known in the general public. I am sure that many nest sites remain undiscovered and I think it would be fun to search for unknown nest sites while hiking on the Colorado Plateau.

Top image: An Anthophora pueblo bee emerzing from its sandstone nesting site in Utah’s San Rafael Desert. (Photo: Michael Orr/Utah State University)