Summer World: A Season of Bounty - Bernd Heinrich (2009)
Chapter 5. Bald-Faced Hornet Nests
21 July 2006. I FOUND A NEST OF BALD-FACED HORNETS (Dolichovespula maculate). So far it’s only about the size of a baseball. The hornets—or wasps—who were spooked out of the nest after I jiggled its branch inadvertently flew to attack and afterward went back to it, perched on the outside on the gray paper, and pointed in my direction. They rapidly vibrated their abdomens—they were shivering to keep up a high muscle temperature, necessary for fast flight and for more instant attack (nest defense). I advanced no farther. Luckily I had gotten only one sting. The wasps should have good hunting—it seems to be a good caterpillar year because there is a barely audible, gentle “rain” of caterpillar fecal pellets on the leaves at night. I also found a female black ichneumon wasp in the act of injecting an egg into a young tiger swallowtail butterfly larva on a chokecherry. I got the wasp and sketched the act, then watched the sapsucker station on the birch for an hour. Relative to all the activity last year there was so far not much action; but nevertheless three hummingbirds, two satyr or wood nymph butterflies (Satyridae), about a dozen bald-faced hornets, and a swarm of small flies (and in the evening, seen by flashlight, one flying squirrel) came to feed at the sap lick.
A FRIEND OF MINE BUMPED HIS LAWN MOWER AGAINST a bush that held a nest of bald-faced hornets. He regretted it, and I suspect he now disputes Robert Frost’s eulogy of this wasp, “as good as anybody going,” precisely because its “stinging quarters menacingly work.” He was attacked by dozens of the nest’s occupants, and had to be rushed by ambulance to a hospital to save his life. He would never again (knowingly) make the same mistake, and the mere sight of a patch of the telltale gray paper of a hornets’ nest sets off alarms in him. Nor will he forget these wasps’ striking jet black bodies boldly marked in white stripes.
Fig. 12. Bald-faced hornet. Nests shown in the process of construction from golf-ball size, at left, to basketball size or larger at the end of the colony cycle, when it contains three or more vertical combs filled with eggs, larvae, and pupae.
A colony of bald-faced hornets starts out in the spring with one lone individual, the queen. She would have mated in the previous fall and then crawled underground to hibernate. As she comes out of her subterranean hiding place in the spring she shivers to bring her muscle temperature to above 95°F and then flies off in search of food. She hasn’t eaten anything for at least seven months. And, like the ruby-throated hummingbirds who have just returned from Central America at the same time of year, she will most likely end up feeding on sweet sap alongside hummingbirds, where a yellow-bellied sapsucker has cut away birch bark to get at the tree’s sap. She will feed at the sap lick, and later on in the colony cycle also from the sugary secretions with proteins that her own young regurgitate to her after she has fed them chewed-up caterpillars and other insects.
After she has refueled, the young queen becomes attracted to weathered dry wood, and with her mandibles working from side to side, she scrapes off swaths of fibers, mixes them with her saliva, and—presto—she has a gob of liquid paper pulp. She applies this pulp to the underside of a twig on a tree (or the bushes next to a lawn) to fashion a short stiff rod. Hauling in load after load of paper, she flanges the rod out to the sides and adds at the bottom of it a little battery of hexagonal cells (in shape much like the cups honeybees make out of wax, which they secrete from glands between their abdominal segments). She makes paper envelopes around her nursery of young, and while releasing a little sperm that she has stored from last fall, she also deposits an egg into each of the hexagonal paper cups. The offspring from these fertilized eggs are genetically females, though most of them will remain sterile “workers.” Much later, near the end of the summer, she will lay virgin eggs, and these will become males. Afterward, at the appropriate time, she will again fertilize eggs from her stored sperm of the year before, and they will again produce females, but because of special treatment they will become fertile “queens” rather than mostly sterile workers (who may at times, however, produce sons, but never daughters).
Each larva hatches into a white grub in exactly the spot where the egg was deposited. Bald-faced hornets chew their prey into semiliquid gobs and carry these home to the nest to feed to their white grubs, who are neatly stacked in a nursery where the temperature is controlled to maintain their rapid growth. The nests trap heat, and the adults stay warm inside and remain ready for immediate, fast takeoff, even when the outside temperatures may be close to the freezing point. Throughout this time they indiscriminately defend themselves against potential predators, generally those who would make the contents of their nests—the fat juicy grubs and the pupae that they turn into—a feeding bonanza. After achieving full growth in its respective crib, each larva spins a fine white cocoon around itself with silk from its salivary glands. This silk then seals the larva off in a coffinlike cocoon at the same spot where it started as an egg and then molted into a pupa before molting once more into an adult.
The queen’s first successive batches of larvae need to grow fast and become a crowd of workers and nest defenders. She needs these sterile daughters, who are single-minded in their tasks and undistracted by sex, to help raise males and new queens in the fall at the end of the colony cycle. Keeping her brood warm accomplishes fast growth, and she encloses her brood comb with more and larger envelopes of insulating paper. She builds the paper layers from the top down, by attaching them to the petiole at the tops of the cells and extending it at the bottom edge until only a small entrance hole is left at the bottom. After she has enclosed her brood in her upside-down “bell” of paper, she also adds a long tubular extension, at the entrance hole, like that on a weaverbird nest. This construction reduces convective air movement through the entrance. Now, whenever she is inside the nest with her eggs, grubs, and pupae, she shivers and produces heat. The warm air is trapped in the bell chamber. She thus creates and maintains her own very local tropical “summer” climate despite what the weather may bring.
To the queen hornet, time and temperature are inextricably linked. She is programmed to behave in ways that shorten the development time of her offspring, so that she can raise as many as several hundred of them in the one summer allotted for her life. Whenever the nest cools, the eggs stop developing and the young stop growing. She needs to keep her baby factory going, to raise lots of workers during the summer. Her first batches of workers are slaves that promote her objective of raising many reproducing offspring later. These sterile workers cooperate because their evolutionary “objective” is to help to raise brothers and sisters who will insert the same genes they have into the next generation. Meanwhile, the colony as a whole is in competition with other colonies for resources to fuel its economy.
By middle to late October the shortening photoperiod has activated the enzymes that have transformed the leaves’ physiology and caused them to be shed. The first heavy frosts are expected. On mornings when there is no wind, the remaining leaves on the tree come rustling down as the sun comes up, illuminates them, and melts the last thing that then holds them on the tree—a bit of ice. A week later all the leaves are down, and now many bird nests and wasp nests that were hidden earlier are suddenly revealed. There is little or no more insect prey and the wasps’ colony cycle is completed. It is now safe to examine hornet nests to learn much about what they have accomplished during the summer.
The hornet nests are now the size and shape of a basketball, and they are sturdy structures that have withstood summer rains and produced generations of workers, then a brood of drones, and finally a brood of queens. I pulled one down (on 26 October 2003) to examine the details that would normally not have been accessible earlier, since an angry fleet of wasps would have come out like sidewinder missiles that seldom miss their mark, when one comes a-knocking. I assumed the nest would be empty. To my surprise, however, it contained twenty-eight new, live queens. All were comatose; they were too cold to fly, and they barely buzzed.
The hornet nest contained three horizontal combs that were hung one below the other, and this nursery was enclosed with multiple layers of insulating paper. The cells of the combs gave clues to the minimum wasp population the nest had produced. The oldest (topmost) comb had the smallest cells, which would have been the “cribs” of worker larvae (and their pupae). There were 240 of these, showing the silk tops of the cocoons where the young wasps had chewed their way out. The second two tiers of combs had larger cells; the first of these tiers had 212 cells that probably each produced a drone, and the lowest comb with the largest cells contained 257 large (queen) cells. The twenty-eight comatose queens still in the nest would have come from them, as well as the 220 additional queen cells. This comb had grown from a central point outward to the periphery, since the empty cells from which sixty-eight adults had emerged were in the center, surrounded by a ring of fifty-four now dead pupae (still in capped cocoons), in turn surrounded by 135 cells, also with dead larvae. The largest larvae were located toward the inside of the ring. There had been many casualties in this wasp colony—a loss of 46 percent of the total production of reproductives attempted; the summer had been too short for many wasps. However, since at least 212 drones and 40 new queens had successfully left the nest, it was probably not too short for the life of the colony.
A colony that faces a short summer, as in the arctic, has little option but to start making the drones and queens soon after the colony is founded and without building up a large worker force. However, in New England a colony could invest more time and energy in workers, the infrastructure of what Edward O. Wilson has called the “fortress factory,” if there is assurance that it can reap the benefits of making good on the investment later on. Rearing many workers before switching over to the desired “product” is gambling that the weather will hold out. But a hornet (wasp) queen cannot be overambitious and push her luck too far in the summer, so that she ends up with a nest full of hundreds of larval young that will then all die when the cold sets in and the summer food runs out.
Of five other nests that I examined, four had switched over to make drones and queens in time to get all of them out of the nest to adulthood, but although they had many fewer casualties than the high producer, they also had less output of “product.” On the other hand, the colony that waited the longest to build up its strength before switching to make its product lost 20 percent of its offspring, but it had the largest overall output—about 900 drones and queens—and this is what ultimately matters to wasps. Thus, its more modest “gambling” for a long summer season had paid off despite the loss of some offspring.
My interest in hornet nests is probably matched only by that of a bird, the red-eyed vireo. This bird uses the paper from hornet (wasp) nests to decorate its own nests. I have examined many dozens of vireo nests over the years and have seldom failed to find at least a patch or two of hornet paper conspicuously attached to the outside surface of each nest. Given what hornets can do, I felt that this bird has a strange taste for nest decoration, because the wasp paper was not serving as insulation on the birds’ nests. Furthermore, the apparently obsessive behavior of putting one or more patches of flimsy wasp nest onto the outside of the birds’ nest involves costs. Hornet paper is hard to come by. One can search for a hornet nest for many days and not find one, even in winter when such a nest is conspicuous from as much as 100 feet away. It must often be a burden for a vireo to find this material in the summer when the old wasp nests are not only rare but also nearly invisible, and when the new nests are vigorously defended. There must be an advantage to using the wasp paper that offsets the costs of getting it. I wondered if the wasp paper, which might proximally serve as a decoration for the bird, might ultimately serve as a prop that fools potential predators into avoiding the vireo nest.
Fig. 13. Red-eyed vireo pair and their nest, which is typically decorated with several pieces of hornet paper.
When vireos build their nests in late June the hornets’ nests are still small—about the size of a baseball, the same as the vireos’ nests. From a distance, from underneath, both look like gray blobs hanging from a twig. Might a crow, blue jay, chipmunk, or squirrel initially confuse one with the other? If these predators have experienced a wasp’s nest defense before, then a mere glimpse of wasp paper on a shape that looks like a wasp nest may, as in some of us who have had an experience with these wasps, be sufficient to prevent them from coming nearer to make a close inspection.
Birds are quick learners. Like us, they stay clear of even a single wasp. And we know that this has been so for millions of years, because several different families of flies, as well as some moths and even some beetles, have members that mimic wasps so closely in shape, size, coloration, sound and flight and other behavior, that a non-entomologist is unlikely to detect the difference and would be easily fooled by them. All these totally different insects ended up looking, acting, and sometimes also sounding like wasps because birds have avoided eating them and whatever resembled them. Squirrels, chipmunks, and deer mice routinely raid many bird nests, as do other birds. For the mammals who rely less on sight than bird predators, a wad of paper could also be a repellent simply because of its smell. There was thus scope for experiments.
I wanted to observe the learning curve of wasp-evasion in my tame ravens, and to that end invested effort trying to procure a currently occupied bald-faced hornet nest. In the summer of 2006 I succeeded in finding only one (instead of its finding me). The nest was in raspberry bushes in a heavily overgrown field near my camp in Maine. I wanted to introduce this wasp nest with its occupants to my ravens in Vermont, and decided to “hive” the wasps with their nest into a big plastic bucket. After donning a bee veil and putting on heavy gloves, I approached the nest with bush cutters. The occupants shot out at me as soon as I wiggled the first raspberry twig. One immediately stung me on the wrist. I retreated to wait a bit for the pain to ease up, and after about five minutes I came back for another try. This time I had also brought my bee smoker, and I cranked it up until it was belching big gray puffs. Did the wasps retreat? Not in the least. I was surprised, since honeybees are almost instantly calmed by smoke. The wasps, in contrast, were not fazed. They came after me whenever I made even the slightest move. I retreated again, planning to come back at night.
This time I crept up slowly, lunged forward with a wad of toilet paper in my hand, and successfully plugged up their nest entrance hole before they had time to react. I then snipped the raspberry twigs holding the nest, dumped it into the plastic bucket, and sealed it, all in less than ten seconds. Within about that time the volume of the wasps’ buzzing increased. The wad fell or was pushed out, and the hornets were exiting from their nest and hitting the inside wall of the bucket. It sounded as if someone was peppering it with dried peas.
During the five-hour drive from my cabin in Maine back to Vermont that night the wasps were undoubtedly jostled and agitated. But by the next morning the bucket was silent. I brought it into the aviary and cautiously lifted the cover, and as I did so only one wasp burst out to attack me. The rest were on the bottom, either barely crawling or dead.
My experiment was now a different one from what I had planned; it had morphed into trying to find out if ravens would attack an undefended bald-faced hornet nest. The results were clear. In minutes the ravens (who were naive with regard to wasp nests) tore the hornet nest to shreds and gorged themselves on the larvae and pupae that they found in the combs. I had achieved a positive result, as I hoped I would: wasp grubs are indeed prized bird food. Apparently wasp nests do require active defending, and in retrospect maybe it doesn’t require an experiment to prove that a raven would back off a hornet nest when the defenders come streaming out to hit and sting with precision.
I saved the torn-apart wasp nest left over from the raiding ravens, and wanting to put it to good use, I wondered what our vireos (there is a pair near our house every summer) might accomplish with a superabundance of wasp paper. Might they plaster the whole of their nest with paper? I made three bundles of the paper by wrapping wire around it and hung them in the trees near the house. The vireo sang vigorously in the summer, but I found no nest until it was revealed after the leaves came down in November. I eagerly retrieved it, and to my surprise it had about the same amount of wasp paper as many of the other nests I had seen. Do vireos, after having incorporated a little bit of wasp paper into their nest, then find that their strange urge is satisfied?
In the next year our neighbor, while sitting on her porch in a thunderstorm in July, was suddenly attacked by a wasp that went for her eye. She suffered swelling down to her chest. After she located the nest I offered to help her get rid of it, and at night I went with a net I had made of wire mesh that was large enough for wasps to get through easily, but this mesh was covered with gauze that the wasps could not get through and that I could remove later. I scraped the rim of the net along the top of the nest to try to get it to drop into the net, and I had a piece of cardboard with me to slip over the net, in case the nest really did drop in, as I hoped. I would then, still at night, transfer the captured wasps in their nest into my (now nearby) raven aviary, where the wasps would settle down to rest. Later, still in the dark, I would remove the outside gauze, and in the morning the wasps would all be at their nest in the aviary.
The results showed that ravens respect wasps: the transfer of the wasp nest appeared to be a success; in the morning the wasps were flying into and out of the nest. No raven went near it. But gradually there were more wasps leaving the nest than coming back. After two days there were no more wasps at the nest, and the ravens then destroyed it and ate all the larvae and pupae with gusto.
Word then got out that I wanted wasp nests, and another neighbor offered a wasp nest that was under her porch. I tried the same approach, except that this was a much larger nest and it just barely fit into my net—and then, to my displeasure, I failed to get the cardboard seal over the top. In seconds after I dislodged the nest, dozens of wasps leaked out, and as I made a run for it my hands and arms felt as if they were on fire from the many stings. I flung the open net with the wasp nest into the back of my pickup, jumped into the cab, slammed the door shut, and spun off down the driveway. Later, back home in the dark, I maneuvered the nest into the aviary. As before, the wasps gradually left. After they had all cleared out, the ravens again destroyed the nest and ate the contents.
Although it would have been satisfying for me to get experimental results proving that the vireos’ paper nest decorations indeed repel blue jays, chipmunks, red squirrels, and crows, negative results would not have proved that the origin of the paper is unrelated to that function. That’s because the proximate results are not necessarily coupled to the ultimate results. It is possible that the paper on a vireo’s nest now serves no useful function, and is more like our appendix, indicating to a previous function in ancestors. Interesting analogies with our cultural practices abound. We often behave in mysterious and enigmatic ways that no longer make sense but that originated from logical, functional antecedents—such as drinking white wine with fish, dipping a slice or two of raw onion into linseed oil to make a famous varnish, or adding a pinch of ammonium chloride to antirust paint, as Primo Levi illustrates in the chapter titled “Chromium” in his book The Periodic Table.