Summer World: A Season of Bounty - Bernd Heinrich (2009)
Chapter 10. Cecropia Moths
22 June 2007. THE TREES ARE FINALLY FULLY LEAFED OUT and have completed most of their twig growth now, and—more noteworthy—the buds that had produced the twigs with new leaves have in some species also already produced the buds for next year’s growth. And a few of these new red oak buds have already “broken” to produce a second spurt of twig and new leaf growth a year ahead of the others on the same branch. There is now ample foliage, and the giant silk moths have laid their eggs. I pick up a dead luna moth in the woods. Its pure white body “fur” contrasts with its fresh leaf look from its large, pale greenish blue wings. One of its two “tails” has broken off and the edges of its delicate wings are frayed from its hectic flight during the last week’s nights—the only time allotted to it as an adult. This is the only narrow window of time that one can meet these gorgeous creations as adults. This moth’s abdomen is shrunken—it had managed to lay its eggs, and its green caterpillars are probably hatching and starting to feed on the new oak, maple, and birch leaves.
IN HIS MASTER’S THESIS, A STUDENT, FRANK L. MARSH, wrote: “About the middle of March, 1933, the writer chanced upon an area within the limits of southwest Chicago” in which he discovered the cecropia cocoons (structures of silk made by caterpillars to hold and protect the pupa—butterflies don’t have cocoons) visible by the dozens on any one tree. In conversations with several people who had lived in the area for years, he learned that the cocoons had “always been just as thick.” But he wondered why they were not even more common, since each female moth lays 200 to 400 eggs. Marsh surmised that the moth population had achieved and was maintaining a state of equilibrium, in which births equaled deaths. He then proceeded to study possible mechanisms for maintaining such an equilibrium. He concentrated on the causes of death that could be deduced from the contents of the 2,741 cocoons that he collected. His was a project that I can scarcely conceive of, since I feel lucky to find even one of these now very rare cocoons; I have seen perhaps three in the last five years.
With all the abundance of the northern forest caterpillars, it is easy to forget that most of them turn into moths (primarily of the families Noctuidae and Geometridae). Not only are the moths much rarer than their larvae—there is perhaps only one moth per 100 larvae—they are almost all nocturnal. Summer nights belong to the moths and fireflies. The difference is that we see the fireflies. I “see” the moths only in my mind’s eye—especially the big ones of the family Saturniidae, the giant silk moths, which can be easily mistaken for bats when they fly in the dark. We know they are out there mainly because we may find their caterpillars in the summer, and for several species, such as the cecropia moths and the prometheans, also their cocoons in the winter.
The woods of New England contain (or contained) an abundance of half a dozen species of the spectacular saturniid moths. All of them are decorated in striking color patterns and clothed in a fine, though thick, velvety “fur” (modified scales, technically pile) that not only gives them their bright and intricate color patterns but also insulates them after they shiver to warm up to get ready for flight.
The cecropia moth, Hyalophora cecropia, is the largest of the local wild silk moths, and its cocoon is a brown baggy spindle-shaped structure. Although these moths’ caterpillars have been tried for commercial silk production, they have been used much more successfully as laboratory animals in numerous studies that have revealed secrets of the physiological connections of neurons and hormones affecting behavior, development, and metamorphosis. The Harvard biologists Carroll Williams, Jim Truman, and Lynne Riddiford are legendary and strike me as scientific shamans because of their extraordinary clever and revealing experiments, which delved deeply into the mysteries of the reincarnation of a caterpillar into a moth, or presumably any insect’s metamorphosis from larva to adult. Among their numerous discoveries was that behavior patterns are inscribed on neurons and are expressed under the influence of hormones. Their studies also showed that internal and external (environmental) stimuli, filtered through the central nervous system, affect the body profoundly. Our vertebrate line split away from that of the insects at an early stage of evolution, but we still share many basic mechanisms, including those discovered in moths. These mechanisms differ not so much in kind as in degree and in how and where they are applied.
Cecropias emerge from their pupae as adults when they shed their pupal “skin” (strictly, an exoskeleton), and then they crawl out the escape hatch of their cocoons around noon on any one day in May. The freshly emerged moths then hang stationary, expand their soft flaccid wing stubs (the outlines are visibly inscribed on the hard pupal exoskeleton), and inflate them with blood to stretch them until they have expanded to full size. The still soft fresh moth then releases a hormone from the brain that activates a hardening agent, and the wings then become frozen into their final shape. When eclosion—the act of emerging from the pupa, which is triggered by hormones—is finished, the moth then purges its gut. What is purged, the meconium, contains fecal and urinary wastes that had accumulated during the pupal stage (which lasts more than ten months); in the females, the meconium also contains the sexual attractant.
Males, who search for females solely by their scent, may come from miles away, flying upwind. Mating in this species begins just before sunrise and lasts about fifteen hours. (Most of the “mating” time is actually guarding, in which the male prevents another male from mating with his female.) Egg laying follows immediately, and the female then flies every night for about a week to scatter her eggs in clusters. They are coated with a glue and when deposited stick to the leaf undersides of several species of forest trees.
The larvae hatch after about twelve days, near 1 June, and then they grow through five stages. Each of these “instars” is separated by a molt. The first larval instar is black and is covered with orange and black tubercles. The second has bright orange and black spots. The third is yellowish green with black spots and blue tubercles. The fourth is light green with a broad dorsal band of blue and yellow, coral and black tubercles. The fifth larval instar is also light green but with a dorsal band of only blue. After each molt the larva eats its old, shed skin, except for the spines and tubercles.
By mid-July the mature larvae stop feeding and initiate violent gut peristalsis that empties them. They are then restless and wander, often leaving the food plant. They eventually stop wandering and begin an incessant labor of about a week, by day and night, to spin the cocoon. First a larva makes the outer shell, leaving an exit valve for the moth’s eventual escape. It then makes the inner cocoon layer by rapid zigzagging sweeps of the head—all except for the threads at the exit valve, where it lays down the silk in parallel lines with the long axis of the cocoon. The larva keeps turning from one end of the cocoon to the other, first laying down silk and then saturating the entire structure with saliva that cements the threads together and makes the cocoon tough and waterproof. The cocoons of this species are unique in having two layers, an outer and an inner section, and an escape hatch at one end for the moth to emerge from during the following summer. The double case probably helps to protect the defenseless pupa from predators; a would-be predator has to invest considerable effort to penetrate even the first, outer wall, and if it manages to do so and then finds only another wall, it may leave.
After working ceaselessly for several days to finish the cocoon, the caterpillar orients itself within this cocoon so that its head faces the exit valve. It then becomes quiescent and begins to shrink as it reorganizes its body and finally sheds its last larval skin to become the pupa. In the fall and winter after the leaves are down, the cecropia cocoons are conspicuously revealed because they are attached to bare tree branches. The pupae hibernate and can survive in a frozen state, and the moths emerge the following summer provided that the brain has experienced a prolonged period of cold; cold is necessary to activate the brain, telling it that winter has occurred. As a consequence, this moth can produce only one generation per year—unlike some other species that don’t need chilling and that can have at least two broods in the south, where summers are longer.
This scenario of development from egg to moth is the normal one; however, it is actualized in only a small fraction of the caterpillars that hatch, perhaps about one in fifty to 100. Few survive to the pupal stage, and fewer still to the adult stage. We have little idea about the mortality of the eggs and caterpillars, but the pupae can be collected, and they show whether or not a moth emerged and what happened in those cases where no moth emerged. Of the 2,741 cocoons that Marsh collected and analyzed, 10 percent were chewed and pecked open and the contents had been removed; they were the victims of deer mice and woodpeckers. The other 90 percent of the mortality was caused by parasitic flies and wasps. Three percent of the pupae had been destroyed by flies (of the family Tachnidae). Twenty-three percent were victims of a species of ichneumon wasp, Spilocryptus extrematus (the genus is now renamed Gambrus).
Fig. 22. Cocoons of cecropia, polyphemus, and promethean moths. The first has a double wall, is attached to twigs, and has an exit sleeve at one end. The second has a single wall, is built inside a rolled leaf, and has no exit hole—the moth exits by dissolving a hole with enzymes in its saliva. The third is also rolled in a leaf, but it does have an exit hole and it is attached to a twig by a long silk strap (shown is a cocoon that is at least a year old; the silk ring around the twig has constricted its growth).
Each of the 630 pupae killed by Gambrus hatched not just one wasp but, on average, thirty-three. That is, these pupae represent 630 × 33 = 20,790 individual ichneumon parasitoids, which could potentially produce 20,790 × 33 = 686,000 more parasitoids in the next generation, and this wasp can have more than one generation in a year. One might suppose, then, that the Gambrus wasp population would explode quickly enough to eliminate an entire moth population in a year or two. But it doesn’t, because the Gambrus wasps are themselves also “controlled” by parasites. Marsh found, for example, another ichneumon, Aenoplex smithii, attacking the parasitizing Gambrus inside the cecropia cocoons, while the chalcid wasp, Dibrachys boucheanus, entered behind them to attack the A. smithii larvae. Similarly, a parasitic fly in the cecropia cocoons was also controlled by a hyper-parasite wasp, which in turn was the host of another, hyper-hyper-parasitoid wasp.
THE CHECKS AND BALANCES THAT MARSH UNRAVELED BY patient rearing of pupae collected in the field give us a glimpse of only some of the links that make up an ecosystem, which extends from the microscopic to the top predators. He also looked further into the mechanisms of the parasite-host relationships that reveal the subtlety of the tactics in the arms races between parasites and their hosts.
Timing is important. Gambrus wasps, for example, are attracted to their hosts, the caterpillars, by the odor of fresh silk these caterpillars exude while spinning their cocoon. The wasps arrive as soon as cocoon spinning starts, and to have a chance to lay their eggs they must be there while the cocoon is still soft. Otherwise they cannot thrust their ovipositor in to insert their eggs. Nevertheless, Marsh counted more than 1,000 Gambrus eggs in one recently spun cecropia cocoon, whereas on average only thirty-three larvae could grow to adulthood in one caterpillar. He therefore concluded that when there are too many larvae, the excess is reduced by cannibalism.
Marsh was correct, but despite the great complexity he discovered he still greatly oversimplified the “real” world of the moth’s population dynamics. At that time he could not have considered an even more fantastic phenomenon of some ichneumon parasitoids: polyembryony, in which one egg can divide to produce multiple genetically identical individuals. That is, the egg clones itself to make as many as 100 or more individuals, enough to consume the whole caterpillar. If two wasps each lay an egg into the same caterpillar, then there will be two clones—each with many individuals—developing simultaneously. And, in a newly discovered twist, some of the larvae of any one clone may be precocious and soon die, but not until they have acted as mobile jaws to kill possible competitors of other clones of both the same species and different species.
The resulting checks and balances of predators, parasites, hyperparasites and hyper-hyper-parasites, cannibalism, and disease organisms ensures that the summer mortality depends on population density (it is density-dependent), so that no one population can completely eliminate the other, and the forest—the other end of the chain of cause and effect—stays green all summer long.
Fig 1a. Crocus flower during the daytime, as opposed to night.
Fig 1b. Bloodroot flower as a result of temperature.
Fig. 2a. Caterpillars eaten by birds. Try to find the six different caterpillar disguises: two leaf-edge mimics, one petiole mimic, one twig mimic, one leaf midrib mimic, and one possible debris mimic. Four of the corresponding adult moths also palatable to birds have been included.
Fig. 2b. Four different “unpalatable” caterpillars and their adults (imagos).
Fig. 3a. A cecropia moth on a cocoon from which it has just emerged. The cecropia is one of the better-known saturniid silk moths, to which the luna (right), the io (lower center), polyphemus, and promethean also belong.
Fig. 3b. Caterpillars of North American saturniid moths.
Fig. 4a. A pair of promethea moths and one of their caterpillars.
Fig. 4b. The ichneumon wasps, Enicospilus americanus, and Gambrus nuncius, which parasitizes promethean moths. Only one of the first develops in a caterpillar, but usually twenty to forty (or more) of the second emerge from a single moth.
Fig. 4c. The big white-faced yellow-bellied tachinid, Belvosia bifasciata, a silk moth parasite that showed up near my camp for the first time in 2006.
Fig. 5. A sampling of longhorn beetles from northeastern North America.
1. Cosmosalia chrysoma. 2. Stictoleptura canadensis. 3. Judolia quadrata. 4. Typocerus confluens. 5. Stenodontes dasytomus (spotted stem borer). 6. Saperda cretata (spotted apple borer). 7. Clytus ruricola. 8. Strophiona nitens (chestnut bark borer). 9. Neoclytes approximates (redheaded ash borer). 10. Urugraphis despectus. 11. Tetraopes tetrophanus (red milkweed borer). 12. Monochamus notatus (northeastern sawyer), male. 13. Oberea affinis. 14. Desmocerus palliates (elderberry borer). 15. Glycobius speciosus (sugar maple borer).
Fig. 6. Life at a yellow-bellied sapsucker’s sap lick. Insects and hummingbirds also are attracted to the birch’s sap.
Fig. 7a. The plants that I picked and sketched on 12 May 2007. The leaves of seven of them are those retained from the previous summer.
Fig. 7b. The common bumblebee (Bombus) species that I expected to see.
Fig. 8a and 8b. A bunchberry (Cornus canadensis) flower in the fall, about five months after the usual time of bloom for this species. Bright red fallen maple leaf and yellow poplar leaf are to the top left and right, respectively. But note the green patch on the old weathered poplar leaf below the flower. It was caused by two caterpillars, one to the right and one to the left of the midrib, feeding inside the leaf and greatly delaying its senescence (see also sketch, where only one caterpillar is inside the leaf). A much larger microlepidopteran caterpillar rolls up the poplar leaves and feeds inside the rolls instead (see chapter 8, “Artful Diners”).