Interior Decorator Bees
It's the wallpaper that makes the room...
I like to split logs for firewood. I like it not only because it provides fuel for delightful winter fires, but also because to do it well is to perform a kind of dance: the end of the handle of my 12 lb. maul is in my right hand. I lean forward and begin the maul’s arc backwards to the left side of my body, and as the maul makes the circle, I slide my left hand down the shaft toward my right hand, strongly arch my back, and as the maul passes through the apogee, I bring all the force in my arms and arched back to accelerate the maul’s head toward the log section before me. A satisfying “thwock” signifies success as the two parts of the log are flung to either side. Repeated hundreds of times, this dance provides us with a shed full of drying firewood, and a winter’s worth of cozy fires.
But the trees whose wood I split are dead, and dead trees are vibrant habitats for a multitude of creatures, and my splitting zeal often reveals bark beetles, ambrosia beetles and long-horned beetles making burrows and cavities in the wood according to what the Lord meant them to do. Sometimes, a split log reveals a cavity filled with what looks like a primitively constructed cigar (not that I have any expertise in the construction of cigars). I added a few of these flaky items to my Found Objects collection, and soon identified them as the work of megachilid, or leafcutter bees. Megachilids are among the hundreds of species of bees and wasps that nest and reproduce in pre-formed cavities in various solids (even holes drilled in cement or brick).
My megachilid “cigars” appeared to be constructed of pieces of leaves, and indeed they were. I eventually connected these nests to something I have noticed for a long time--- the edges of the leaves of many soft-leaved plants look like caterpillars have nibbled clean ovals out of them. Once you cue to this image, you see it everywhere in the leaves of many different plants and even flowers. But why would a caterpillar only nibble leaf edges (he asked, rhetorically)? Well, they wouldn’t. These missing oval bits from the edges of leaves were cut by megachilid (aka leafcutter) bees, not for food, but for the purpose of lining the nests that my log-splitting had revealed.
Having cut a piece of leaf, the female bee (males have no role in this aspect of reproduction) flies it back to the cavity in which she is nesting, and places it as a lining, like wallpaper. Each subsequent piece overlaps a previous piece, giving rise to the shingled appearance in the image above. Close inspection of the redbud leaves above reveal that most of the leaf segments are oval. These served as the cell lining, but a few (six in the left image and four in the right) are perfect circles that the bee used to cap cells. To cut the pieces, the bee must have gripped the edge of the leaf, head inward, and rotated around its “center of gripping”, cutting the leaf with the mandibles, like scissors. I once found a grape vine that had been so heavily pilfered that I felt compelled to make a drawing of it (below).
Having produced a bee-length lining, the bee changes tasks and forages for nectar and pollen, carrying the pollen in the long hairs on the bottom of her abdomen. She provisions a cell with a pollen ball, lays an egg and closes the cell with circular pieces of leaf. She then repeats this sequence until the total nest is of a length that qualifies as “enough” in her tiny hymenopteran mind.
In old nests the leaves turn from green to dried and brown, as in my Found Object. Megachilids also use flower petals, and when they do, their nests (at least initially) are lovely to behold. In the western USA, poppy bees make nests of brilliant orange poppy petals, and elsewhere, flower petals occasionally add panache to dull green. But why use leaves at all for lining nest cavities? Do leaves provide some special service that is absent from a naked cavity? Do they provide protection from pathogens, from desiccation, from abrasion? Some leaves seem more popular than others, such as redbud and grape, perhaps because they are thinner and easier to cut, but others, like gopher apple seem pretty tough. Is there some magical property that we don’t understand?
But no matter. The finished nest reveals what a sense of geometry the bee has! She is a single individual building a house with a complete knowledge of what material to use, what goes where, in what order, at what time, and how often. There is no teamwork here as there is in ants or many human crews, just one skilled, knowledgeable bee. What’s more, after searching for and cutting suitable leaves, she finds her way back to the tiny opening of the hole in a tree, a fallen log, or a dirt bank where her nest is located, in a world that is hopelessly complex, three-dimensional and huge. The bits of leaves she carries back do not all come from the same plant, so she navigates a different return route from each plant. Are insects really just dumb robots as they are so often popularly characterized? Perhaps our mammalian souls can’t comprehend the mind of an animal so distantly related? Perhaps our bias determines what we see and what we don’t?
Once I understood the oval cut-outs the bees made, another idle, seemingly unconnected observation became imbued with new meaning—-previous knowledge from one phenomenon suddenly explained another. I use colored vinyl flags on wire stakes to mark locations in the field, flags that I often recovered with holes and scalloped edges. It took me a while to realize that the holes and missing pieces were the work of megachilid bees. In the image below, a bee had removed pieces from the flag but seems to have rejected the ones littering the ground below. Was vinyl was not the upscale material that she expected?
I searched my bundles of used vinyl flags and found quite a few with obvious cut-outs made by leaf cutter bees. I began to wonder if the bees had a color preference, that is, had an interior decoration sense, but this question isn’t as easy to answer as it might sound. Although I used yellow, red, orange, blue and white flags, I don’t know how many of each I placed, nor did I record how long they were exposed and in what season. What’s more the flags were often far enough apart to deprive the bees of a direct color comparison before choosing. So, deciding if the bees preferred some colors over others isn’t possible without more data (the most common refrain among scientists).
What was clear was that more than one species of leafcutter bee was cutting vinyl pieces, for the cut-outs were greatly different in size (below), suggesting that there are large leafcutters and small ones. This should be no surprise, as Florida has 63 species of megachilid bees in seven genera, and of course they come in many sizes.
Whatever magic leaf pieces perform for the bee, it seems unlikely that discs of vinyl will do the same. Would an all-vinyl nest in all its sleek modernity better fulfill the reproductive imperative of the bee? If yes, then given time and natural selection, the leafcutter bee of tomorrow might produce something like the nest below. If I am still splitting wood then, wouldn’t I be dazzled?
