Mud Daubers and Behavior - Summer World: A Season of Bounty - Bernd Heinrich

Summer World: A Season of Bounty - Bernd Heinrich (2009)

Chapter 6. Mud Daubers and Behavior

I’M NOT A GLUTTON FOR PUNISHMENT. IN ORDER TO get a kick out of nature—to enjoy insects, even stinging wasps—it is probably not necessary to risk life, limb, and anaphylactic shock. Indeed, I think I enhanced my educational experience, and possibly even advanced the cause of science, at least as much as I did by fooling around with the bald-faced hornets, during the summer of 2006, by sitting on our front porch sipping a glass of red wine with Rachel, my wife. It was early on a balmy August evening when big green darning needles, or dragonflies, were zigzagging back and forth in the clearing between our house and the bog. I was probably half dreaming about experiments I had done years earlier with a colleague, Timothy Casey, in which we had pinned them down and simulated overheating of their flight muscles (that they could normally experience in flight) by focusing a heat lamp on the thorax, and proved that they could stabilize their body temperature by shunting excess heat into their long cylindrical abdomen, which then served as a radiator. I had loved the control, the certainty, and the presumed cleverness of discovering a new phenomenon. Now, reminiscing, I still felt the glow, but in more ways than one, as I lifted the glass, looked out, saw the dragonflies, and then on the porch noticed a wasp dragging a spider.

The handsome blue wasp with dark wings carried the spider up onto a planter and tried to fly off with it, but the weight of the spider apparently pulled the wasp down: the wasp went only a short way before having to climb up a railing to make another short flight. I was wondering if the wasp’s flight muscles were not hot enough to generate sufficient lift to fly, or if the spider was too heavy for the wasp to carry. I didn’t know right away what kind of a wasp it was, except that it was a solitary wasp, unlike the hornets in yet another communal nest under our porch that I had recently offered, along with their nest, to my ravens.

As I was contemplating the wasp carrying the spider, Rachel casually mentioned having seen a wasp of the same description that had also been hovering along the side of the house, but she said that this one was carrying “a long piece of dry grass” and, furthermore, that she saw the wasp drag this grass into “a crack in the wall.” I know that wasps do some amazing things. But they always do the same things—they don’t vary their behavior. In short, I flat-out didn’t believe Rachel. But maybe I should have. I will get to the reason why shortly.

A few days later, under similar circumstances, I again saw the dark blue wasp—a female (male wasps don’t carry any objects)—carrying another spider. This time I followed her and saw her take the spider to a mud nest shaped like a long vertical tube that was plastered onto the south-facing wall of our house. Nearby, there were three of these nests of different lengths, neatly aligned adjacent to one another like the pipes of an organ. I knew from this nest that it was the organ-pipe mud dauber (Trypoxylon politum).

Insects are models that have given us a view into basic mechanisms of behavior and evolution. And birds, because they are emotional animals, provide a bridge to understanding ourselves. Their specialized behaviors—courting, vocalizing, nest building, foraging, habitat preferences, and strategies of parenting—are all deep-rooted in patterns that they are born with, as are those of insects. Insects show us how much can be done with a pinpoint-size brain, and they therefore seem magical. If so much can be programmed into such a small brain, how much more is possible with a brain like a bird’s, which is hundreds or thousands of times larger? Nest building by wasps offers a direct comparison with birds, and when on that August day I found the organ-pipe (or “organ tube”) mud dauber wasp carrying a spider to a mud nest on our house, I paused to ponder the differences between a bird and a wasp.

Eighty million years ago, in the Cretaceous, the birds’ relatives, Maiasaurus, were hollowing out holes in six-foot scrapes to deposit their eggs and then care for their young in them. They nested in colonies, as many birds do now, probably because there is safety in numbers, and maybe also for defense. Undoubtedly, nest building by insects is at least twice as old.

Unlike dinosaurs, small birds had the option of building their nests in trees, and also of hiding the nests, which requires more finesse than digging a crude scrape. As with insects, each bird species makes a nest as distinctive as its plumage and just as circumscribed or encoded in its DNA. The goldfinch’s nest is wedged into a vertical fork and is made of fine grasses and plant down. The oriole’s is a bag made from the fiber of dead milkweed plants and is hung from the tip of a long limb on a spreading tree. The chestnut-sided warbler’s nest is hidden near the ground in dense meadowsweet or raspberry vines and is a flimsy affair made entirely of very thin grass stalks. The tree swallow’s nest in my bird box is made loosely from dried grass, and almost invariably the nests of this species are lined with feathers, preferably white ones. Robins build a hardened mud cup on leaves and debris and line it with thin grass strips. Wood thrushes commonly incorporate a snake skin into their nest. Catbirds line their nests with fine rootlets and use grape bark to garnish the exterior. Ravens and chickadees line their nests with fur. Often the specific items used in the construction seem to have little rhyme or reason, but the nests are always exquisitely “perfect” in functional design and constructed unerringly.

Many kinds of wasps make nests from clay or mud mixed with saliva, as barn and cliff swallows do: the mud hardens, and as long as it stays dry, it stays solid, like concrete. Like birds’ nests, wasps’ nests are shelters for their offspring. Solitary wasps, however, provision their nest not only with their eggs but also with food for the larvae after they hatch, and then seal the nest off to prevent parasites from entering. Some wasps, like the potter wasp, make a nest that closely resembles a narrow-necked jar.

The organ-pipe mud daubers I had been watching are another kind of solitary wasp that uses mud to make nests, but of a very different design from those of the potter wasp. The daubers fashion an upside-down tube with an entrance at the bottom. The tube is plastered against a wall (such as a cliff face or house wall). After making a first small section of tube, the female wasp (no male insects make or help make a nest, or provision one, or sting) collects spiders; jams the prey, still alive, up the tube; inserts an egg; and then makes a partition at the bottom so that the contents, the prey, won’t fall out. She does not have to be concerned that the spiders will crawl out, because after she catches them she injects them with a chemical that keeps them in a zombie-like state of suspended animation. As a result, they don’t struggle when carried, and they will still be alive and fresh when the larvae (which look like white grubs or maggots) need to feed on them days or weeks later.


Fig. 14. Nests of the organ-pipe mud dauber. Left: Exterior of a nest, with ridges resulting from successive loads of hardened mortar. Three adjacent nests have been opened, each showing three cells containing various stages of development, from egg placed on fresh spiders to larvae (“grubs”), and to pupae.

The vertical tube design is efficient, because to have a cell for the next potential offspring, the wasp merely extends the bottom of the tube. And so she may continue to make one cell below another in a lengthening tube that may ultimately extend several feet down. As the summer progresses and the nest grows by being extended in a succession of cells at the bottom end, it eventually contains dozens of cells. The wasp lays only one egg in each cell. The cell that was made first (on top) may already have a pupa while the cell made most recently (at the bottom) is still being provisioned with spiders. An egg is deposited on the last spider put in each cell, just before the cell is sealed.

Using a sharp knife, I cut into the first nest and was surprised by how hard the walls were. This first nest I examined was divided vertically into only two cells. There were no spider remains in either one; the prey had already been eaten by the wasp larvae, down to the last leg. I cut into an adjacent nest, and here the lowest compartment contained eight fully intact spiders; these were of different sizes but were all similar and probably belonged to the same species. As I pulled them out, one by one, they wiggled only slightly. Each raised its front legs briefly in a defensive gesture, then quickly let them droop. However, as long as I didn’t touch them they remained motionless. The largest of the eight spiders had a yellow oblong egg attached to its side. The compartment above this one (sealed off from the one below) also contained eight live spiders of the same type. Only one was dead—it had a collapsed, flaccid abdomen with fluid leaking out, a sign that the wasp larva was eating it. One compartment, the upper one, contained a pill-like pupa; the lower compartment had a dead larva that was as flaccid as the spider being eaten, so I assumed it was diseased and dead.

The following summer, however, I learned that the flaccid, seemingly dead larva probably hadn’t been “diseased” at all. I had saved several nests to rear out the wasps. Three wasps emerged, and to my great surprise they were not organ-pipe mud daubers. Instead, they were scoliid wasps. Scoliids are well-known parasites of scarab beetle larvae, such as those of June beetles that live in the ground; apparently here was a species of wasp that parasitizes a mud-nest-dwelling larva of another wasp.

Although only the females build and provision the nest, the male organ-pipe mud dauber is one of a very few that stay around during the nesting. He is thought to help guard the nest against potential intruders while the female is away hunting prey for feeding the young (when the nest is left open). Apparently males had been absent at the parasitized nests I had found, or else they had been negligent. In this case the male’s apparent negligence resulted in the death of his and the female’s offspring. However, in other instances, when flies enter the nest to lay eggs that feed on the wasp’s prey, the larva’s food, the offspring are not necessary killed. Instead, they grow into miniatures because of food deprivation (O’Neil et al. 2007).

My observations of nature from our front porch soon led to other, even more startling surprises. You may have guessed it—a few days later, as we were again sitting on the front porch drinking our usual after-supper glass of red wine in the gathering dusk, I thought I saw a light-colored piece of straw about half a foot long “flying” horizontally and then hovering in midair. That caught my attention—I looked closer and saw a black wasp that seemed identical in form to a mud dauber, and it was carrying an object. I jumped up in my excitement, and the wasp was spooked and flew off. The proof eluded me, but it dropped its “prey” onto the porch. I picked it up—definitely a long piece of dry grass!

Expecting the wasp to return, I waited. After about twenty minutes it did return, carrying another piece of grass. This time I was ready with an insect catcher net, and I snagged the wasp along with the grass it carried. The wasp was about 0.6 inch long, and the blade it was carrying was about 2.4 inches long. The wasp looked superficially almost identical to the mud dauber, but its body was black rather than black-blue and its wings were smoky-colored instead of blue-black like the mud dauber’s. It was a different species of wasp, which I later identified as Isodontia mexicana. I also learned later that rather than making clay organ-pipe cavities for its nest and filling them with spiders, this wasp lines preexisting cavities with grass and fills them with paralyzed crickets or katydids.

The next year the organ-pipe mud daubers were gone from our house. The remaining nests on our house had all been pecked open, probably by woodpeckers or chickadees. I checked in a neighbor’s barn, and deep inside, above the horse stable in the ceiling of the haymow, I found numerous mud nests of another species, the blue mud dauber, Chalybion californicum, plastered onto the wooden beams. This species attaches successive cells laterally, next to each other, rather than underneath each other as the organ-pipe dauber does. These nests were also packed with paralyzed spiders, although any one cell contained a variety of species. There were white and yellow crab spiders, brown orb web spiders, and still others. In the western United States this species of mud dauber is renowned for preying on the infamous black widow spider.


Fig. 15. A three-celled nest (detached from a barn beam) of the blue mud dauber, and the spiders out of one of these cells along with the wasp grub found with them.

The summer work of the insects was, as it turned out, not totally benign; one managed to “bug” our heating system. When I went to check out and prepare our outdoor wood-burning furnace, which pipes the heat into our house through a water system, I found it faulty. The water gauge, a vertical tube, stayed empty rather than filling up as it was supposed to when I turned on a valve, so I could not risk making a fire. Luckily I noticed some debris in the bottom of the tube and thought of leaf-cutter bees. These solitary bees make their nests in the galleries in wood excavated by long-horned beetle larvae, or substitutes thereof, and line them with green leaf pieces that they cut from whole leaves. The green leaf then wraps their egg and the pollen for the larva that they add next before sealing the tube nest with mud. I sent some of the remains that I could fish out of the furnace water gauge to Kevin O’Neil, an expert on bees, and he confirmed that a leaf-cutter bee had indeed bugged our furnace.


Fig. 16. The four wasps (not to scale). Center, enlarged: Blue mud dauber carrying a paralyzed spider. Right: Grass-carrying wasp and organ-pipe mud dauber. Left: Nest parasite of the organ-pipe mud dauber.

These four species of “house-barn” wasps, three of which are anatomically similar, plus the bee, have behaviors that are strikingly different. Their behaviors are, like their anatomy, presumably encoded on their DNA but expressed only in response to specific cues. Like most adult insects’ lives, theirs are short—a matter of weeks. Since there is little time for experimenting and learning, such animals must necessarily get everything right almost from the moment that they emerge from their pupa. The newly emerged mud dauber flies off soon after its wings harden, and it finds mud! The dauber “knows” how to pick up and carry mud, where to go with it, and—more amazingly—how to make a nest out of it, of a very specific shape. Of the hundreds of thousands of potential things the wasp could search out, it then looks for specific kinds of prey (or it looks in specific ways so that it finds only those kinds). It responds to very specific, minute details out of thousands that it encounters. Just as we pick out what vegetables to buy at a market, it makes an incredible number of choices every second. Its choices are predetermined by genetic instructions. There is little flexibility. And the next species has entirely different instructions. If I wanted someone to exactly duplicate any wasp’s behavior, I would have to write an encyclopedia of instructions, and even so, he or she would almost invariably make innumerable mistakes. But wasps, with a brain that is smaller than a pinhead, accomplish their specific tasks perfectly and without practice, flying out into a world they have never seen before. Aside from the mystery of how wasps can do so much with so little, there is the mystery of how what they know is passed faultlessly from one generation to the next.

Mutants of “motor” behavior (such as stepping, flight, climbing, walking, and reaching over gaps), learning, and memory are known, proving that behavior is indeed tied to genes. But how we get from protein gene products to programs of behavior consisting of hundreds of precise choices and actions—as opposed to behavioral tendencies—remains one of biology’s great mysteries. And here I met it head-on, right in front of my nose on our front porch.