Tale of Two Trachusa: Confessions of a Nest Voyeur

Last week I killed a bee family.  I had been watching them for months.  I broke into their home while the mother was out and removed her five little ones, one by one.  I don’t feel great about it.  But it was actually pretty cool.

I first noticed “Stacy” bee (Yes, I named her before I killed her family.  Sadistic and shockingly unscientific, I know.) about four weeks ago while scanning a dry, rocky, exposed area near one of my sampling plots for bee activity.  She flew into the area in a hurry, with some sort of package in tow, and disappeared into a previously invisible hole in the ground.  I kept my eyes glued to the indistinct, dusty area where she had disappeared and was rewarded to see her emerge several minutes later, pause for just a split second at her doorway, and then zoom off in the opposite direction.

I set up camp.  Nestled into the sun-baked red rocks about three feet away from the tiny nest entrance hole, I pulled out my field notebook, pen, and camera, and waited for her to return.  A grueling thirty minutes later, once I had started to wonder if I was going too cross-eyed to even catch her arrival and had rechecked several times that I was indeed staring at an actual nest entrance hole, she reappeared on the scene.  I watched her dart left and right around the area, perhaps unsure of the dangers posed by my looming figure, a new addition to her visually recorded neighborhood.  I sat stark still, waiting for her to recognize the subtle visual cues directing her to her nest entrance: the bent blade of grass, the discarded insect skin, the triangle-shaped rock.

Despite my hovering, Stacy-bee finally made a dash for her entrance hole and I was able to catch a glimpse of her package: a small ellipsoid piece of dark green leaf that she clutched underneath her body with her four front legs.  She was building her nest.

Stacy peeking out from her nest entrance, perhaps to see if the coast is clear for another foraging flight. Photo J. Meiners.

Several weeks earlier, somewhere in these hills in a similar soil type and lined with the same type of leaves, Stacy herself emerged from a nest like this.  Perhaps she was one of several larvae packed away vertically into a hole in the ground and had to wait for her siblings to finish developing and then chew and squirm their way out of the nest before she could make her own clean exit from below them.  Perhaps one of her younger, more shallowly nested siblings hadn’t survived the winter and Stacy had to crawl through the empty, grave-like chamber with the sticky remains of her un-metamorphosed brother to find the world above.  Perhaps she was the only one who survived.

As Stacy climbed from her nest cell into her first rays of sunshine, she was likely bombarded by at least one eager male courtier.  Deposited last, and thus highest, by their mothers in the columnar nests, the males of solitary bee species typically emerge before their conspecific mates and spend this head-start searching out other nest entrances of their species.  In the early spring, if you pay close attention to aerial movements along patches of  seemingly barren ground, you may notice male bees patrolling these discovered nesting sites in low, pacing flights.  Once they detect (most likely by scent and visual cues) a good spot to pick up females, they lurk in wait to pounce on adult females during their first emergence into the world, ensuring that they pass on their genes when she builds and fills a nest of her own.

This is the fleeting love tale of, in this case, two Trachusa (my guess of the Genus in question though, without catching and interrupting our nest-builder, it is difficult to be sure).  Sadly, it is a short affair: a brief, unsolicited delivery of gametes (reproductive haploid cells, i.e. sperm) after which Stacy goes on to a life of hard work and little company while her mate likely flies off to try his luck at another nest entrance, then heads off in search of delicious nectar and ends up spending the night in a flower.

Yes, it’s true.  Males are deplorably unhelpful and undisciplined in the world of bees.  They have no work ethic.  They offer no loyalty.  They display a complete lack of commitment or investment in the future of their offspring.  They hunt and attack naïve virgin females and then promptly flee the scene to spend the rest of the day drinking sweet nectar and passing out in flowers.  I’m not just telling you this because I happen to be a Seven Sisters Scholar and a graduate of the country’s first institution of higher education for women, still a single-sex establishment.  It’s just the way it is, scientifically speaking.  But the males never know what they’re missing.  Stacy is about to have a far more interesting life.

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View from near Stacy’s nest entrance at Pinnacles National Monument. Photo J. Meiners.

The day I first met Stacy, I could only stay and watch her work for approximately ninety minutes.  During this period, she returned to the nest site three times with leaf pieces, each carefully selected and meticulously cut using her strong and precise mandibles, a defining anatomical feature of her taxonomic family of bees, the Megachilidae, which includes the appropriately named “leaf-cutter bees”.  She spent about four minutes inside the nest each time, presumably arranging the leaf pieces into a series of pill-capsule-sized cells that would each protect and house one of her offspring over the coming year.  But before she got to this stage, before I first encountered her, she had to excavate this hole in the ground herself, pushing out tiny rocks and dirt bit by bit and digging the nest deep enough to fill it with as many offspring as she was likely to have time to provision for in her short, busy, several-week life.  It was crucial that she make the nest deep enough to fit as many eggs as she had time to lay, yet shallow enough that her offspring were not presented with a daunting depth from which to crawl when they themselves begin to emerge as adults.  She must make her nest at the right angle into the ground, on the preferred slope aspect, and in the correct soil type to ensure that her young are incubated at the proper temperature and humidity over the changing seasons during their development over the coming year.  If she miscalculates any of this (or has the misfortune to build her nest under the intrigued eye of a new bee scientist) the odds that she will succeed in passing on her genes to the next generation plummet.  I never saw Stacy in this excavation stage, but I have witnessed other species of bees making tireless, repeated circular trips to and from their nests to pick out a single pebble from their nest tunnel and fly it a few feet away to discard on the hillside.  I have also seen the opposite action, at the end of the nesting process, when the female bee collects tiny rocks, carefully selecting them one by one from her nearby surroundings, with which to slowly plug up and conceal her nest tunnel.

The nesting site with my rock cairn marking above.

The nesting site with my rock cairn marking above. Photo J. Meiners.

At the end of this first observation period, I made a little rock cairn uphill and to the left of Stacy’s nest entrance so that I could find it and check on her progress later.  About two weeks after this initial encounter, I returned to check on Stacy’s progress.  I sat and waited eagerly a few feet from her nest entrance.  After several minutes I was thrilled to see her arrive with a bright yellow load of pollen on the underside of her abdomen.  Megachilid bees, including Trachusa Stacy, are known for having their scopal (pollen-collecting) hairs on the underside of their abdomen instead of arranged into ‘pollen baskets’ on the hind legs where we are used to seeing many other bees, such as honey bees, carry their pollen.  Their ability to collect and carry the dynamically-shaped pollen grains using these elongated, branched hairs on the ventral surface of their abdomens makes leaf-cutting female bees with bright yellow ‘bellies’ relatively easy to identify on the wing.

At this middle-aged stage in her life, Stacy is busy stocking the pantry of the carefully constructed leaf capsule she built in the previous chapter.  Once she has completed digging her nest to the appropriate depth, has made several trips to collect leaf pieces from just the right plant (likely Ceanothus cuneatus or “Buck Brush” in Stacy’s case), and has glued them together with oral secretions into the perfectly shaped, waterproofed leaf capsule nursery, she beginnings provisioning it with food.  Many solitary bees are considered “specialists” and are picky about the type of pollen they feed to their young.  Stacy has been collecting pollen, mixing it with nectar, and forming it into a “pollen ball” or “bee bread” as many melittologists like to call it.  She will leave this packet of goodies (similar in purpose to the yolk of a chicken egg) for her young to consume as it grows from larva to pupa to adult all alone in its sealed leaf capsule underground.  Stacy will not be there to supervise.  She must get everything prepared ahead of time.

Once the bee bread is in place, Stacy will lay a small, oblong-shaped egg delicately upon it and carefully seal away her young, never to see it again.  She will not see this bee grow and she will not be alive when it crawls its way into the sunshine of this rocky, sun-baked hillside.  But such is the tale of a Trachusa, and hundreds of other solitary bee genera.  Stacy is building her own time capsule, leaving her legacy.  She will construct and provision as many of these leaf capsules, each containing one new Trachusa bee, as she can in her short life.  Once finished here, she may try to start another nest if her energy remains, but one day she won’t return from her foraging flight to finish this task and the inhabitants of that unfinished nest, left unconcealed, may not survive.  Whereas the production of honey by social bees is a strategy to sustain the active colony throughout the winter (not a gift to humans as we like to think), solitary bees stuck (evolutionarily) to a simpler strategy of shorter life cycles and just one generation per year.  By not making honey or orchestrating division of labor among themselves, solitary bees have no chance at surviving the winter and must arrange everything ahead of time for their young to endure a long period of very minimal activity.  Stacy’s young will remain underground in isolation and silence for the next eleven months, under rain and frost and thaw, while they consume the bee bread and grow and wait.  That is, they would have if I had been able to let them.

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Beginning nest excavations…for science! Photo T. Lamperty.

On June 15, 2012, I returned to visit Stacy for what would be the last time.  I found the cairn marking the nest and settled in to wait for her to arrive and show me the labors of her next project.  But upon closer inspection of her nest, I discovered that it had been sealed up, impressively filled in and concealed with leaf pieces and grass and dirt.  Whatever her fate at that time, Stacy had finished her work here and would not be returning to greet me.

After several pictures and a moment’s sincere appreciation of her work, I dug out my forceps and digging tools and began to slowly unravel her life’s work (for science!).  One by one, I peeled back a couple centimeters of carefully layered fresh leaves, each precisely cut with Stacy’s mandibles.  Then came a thin layer of dirt and the first leaf capsule containing her youngest larvae.  With some guilt, I used my forceps (sharp tweezers) to tease apart the intricate leaf capsule and reveal a tiny white grub perched on a generous pollen ball.  Not wanting to squander this sacrifice, I snapped a few pictures and added the nest contents to a collecting jar.  Kept at the proper conditions, there would be a small chance for this larva to continue its meal and develop in the lab.  The next lowest capsule contained a slightly larger white grub, feasting on a slightly smaller pollen ball.  To reach the final three, and oldest, leaf capsules I had to dig further down into the nest through some hardened dirt where Stacy had laboriously tunneled out and beautifully smoothed the inside of her drinking-straw shaped nest cavity.  I saved two of the middle capsules undisturbed in my collecting jar and opened the deepest one to find, as expected, the largest larva and only a small remaining portion of Stacy’s first pollen/nectar concoction.  After about an hour of carefully chipping away soil clumps bit by bit in the 100 degree heat and blistering sun, I had collected all five of Stacy’s offspring and excavated all 15 centimeters of her custom built home, constructed at about a 30 degree angle into a west-facing slope and insulated with the choicest materials.

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Removing the first leaf piece plugging up the nest entrance. Photo J. Meiners.

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Piece of the nest cavity inner wall. Photo J. Meiners.

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A leaf capsule with developing larva and pollen ball inside. Photo J. Meiners.
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A disassembled leaf capsule and the most developed (lowest placed) of the larva. Photo J. Meiners.

Thus concludes this tale of two Trachusa, though Stacy far outshone her momentary mate in this story.  Her brief life was rich with exploratory flights, prized goods, meticulous craftsmanship, and motherly care.  It amazes me how specific and disciplined are the actions of a female solitary bee: fresh into the world, having known only darkness and solitude followed by a brief encounter with a male of her species, she embarks on a daunting quest with precision and artistry to continue the cycle of wild bee pollination and propagation.  I like to think that in lauding her accomplishments here, her efforts were not in vain.  Thank you Stacy, for letting me into your world.

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About Joan Meiners

PhD student, science writer, bike racer, nomad. Focusing on native bee community ecology, literary nonfiction outreach, and exploring the world on two wheels. Currently at the University of Florida. Formerly at Utah State University and Mount Holyoke College. Twitter @beecycles.
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