Summer World: A Season of Bounty - Bernd Heinrich (2009)
Chapter 11. Calosamia Collapse
IN 1903, W. J. HOLLAND, THE MOTH MAVEN OF NORTH America, wrote this about the promethean moth, Calosamia promethea, in his “bible” of the American moths: “Every country boy who lives in the Atlantic states is familiar with the cocoons, which in winter he has found hanging from the twigs of the spice-bush, the sassafras, and other trees.”
I am lucky indeed to have made the aquaintance of these giant silk moths and their cocoons. But country living is apparently not what it used to be—I have not met any country boy, and only the very rare biology graduate student, who has even an inkling of what a promethean moth is. Most students don’t know a cocoon from a pupa. Perhaps that’s not surprising, because they haven’t seen any, either in their everyday lives or in school.
Prometheans were still in abundance recently, near my camp in western Maine, where almost every summer their handsome blue-green caterpillars fed on the leaves of ash and cherry trees. Like many other saturniid silk moth caterpillars, these are strikingly beautiful when viewed from up close. They are decorated with four bright crimson tubercles on their front end and have splashes of yellow along their pale bluish green sides. The adult moths are beautiful, too. The females have fiery orange-red wings with white margins—hence the name promethean, for Prometheus, the Greek god who stole fire from the heavens. The males, in contrast, are deep purplish black.
In northern New England, the adults fly only for a week or two in early June, mate, and lay their eggs the next day. The caterpillars hatch and grow to their full size, about the size of one’s little finger, by late July or August. After they stop feeding, they wander; and after their gut has emptied, they stop under a leaf of almost any species of tree and start to make their cocoon by continually exuding silk from their salivary glands. Like all silk moths, they weave, waving their heads back and forth to opposite sides of the leaf as silk is exuded and attaches and pulls the two leaf edges together. After the caterpillars become wrapped in the green leaf, they continue applying silk strands all around themselves, and a solid cocoon takes shape. As they continue extruding their thread of silk, they line the cocoon cavity with more silk, which is then cemented together with fluid from the mouth to produce a tough case.
Caterpillar silk is both tough and flexible, and for centuries it has been the raw material for luxury clothing (although from a different species, Bombyx mori). A cocoon is made from one very long continuous strand of silk. In Calosamia and most other silk moths innumerable strands of silk are cemented together to form a stiff armor. It can be dented, but it is almost impossible to tear into except with scissors. One might assume that once the caterpillar is encased inside the cocoon, nothing else can enter, and that the cocoon would also trap the eventual moth inside, preventing its exit. However, without exception, in all of the many hundreds of Calosamia cocoons I have handled in the last seventeen years, every one had a built-in escape hatch. You can be sure that if it had been up to me to build such a hatch every time, I would have missed several. But the caterpillar does not think ahead—if it did, it too would have failures. Rather, it is behaviorally programmed, and it cannot do otherwise than leave an outward-flanging sleeve at one end of the cocoon. In contrast, the common silk moth, Bombyx mori, and some of the luna and polyphemus moths leave no escape hatch; instead, after molting from the pupa the adult secretes a saliva that contains a silk-digesting enzyme (cocoonase) that dissolves an exit hole out of the cocoon prison.
The Calosamia caterpillars have a curious behavior that is not found in any of the other local silk moth species. They spin an extension of their cocoon along the one-inch-to ten-inch-long stem (petiole) of the leaf and—more important—onto the twig beyond. This beltlike strap of silk, on reaching the twig, is wrapped in a tight ring around it. By fall the leaf wrap dries and shrivels around the cocoon, and since it is “silked” to the twig it remains on the tree even after the leaf loosens its hold on the twig. Calosamia cocoons may hang from trees for several years, long after their contents have emerged and long after the camouflaging leaf wrap has disintegrated. However, despite all the protection that the cocoon provides from birds, the enclosed pupa is not safe from parasitoids. They have honed their attack strategy as much as the caterpillars have honed their defense.
There are a number of different ichneumonid parasitoids (predators that kill their hosts by eating them from the inside). One of them, Gambrus nuncius, is tiny and prettily red-colored, with white-ringed antennae. More than forty may emerge from a single cocoon. Another parasitoid, the large yellow-orange ichneumon Enicospilus americanus, is relatively rare, and only one individual ever develops in any one moth pupa. Others, mainly fly parasitoids, attack the caterpillar and emerge from it before it would normally spin its cocoon.
Fig. 23. Enicospilus americanus injecting an egg into a cocoon-spinning caterpillar, and the ichneumon’s development into the pupa.
On my hill in Maine, there have usually been enough Calosamia promethea cocoons for me to find ten to a dozen in an hour. However, I have never seen a female moth flying free in the woods; nor have I picked up a dead one. The moth’s life is restricted to about a week, and the whole population is active at the same time, in early June only. To see male moths is easier, but one needs to resort to a trick. Using a thread tied around her ample waist, I tether to a branch a female that has just emerged from her cocoon. By late afternoon males fly in, following the female’s odor plumes. I collected eggs from the mated females to raise the caterpillars.
The best time to find Calosamia, so as to raise them from eggs in the summer, is in the winter. After the deciduous trees have shed, the few individual leaves that remain are conspicuous and may be curled around and contain a cocoon. I keep an eye out for them on every winter walk in the woods. It has not been merely an idle pastime, because I justify it by hatching females from cocoons to tether onto twigs the next summer, and by trying to find out what parasitoids the cocoon contained or still contains. I collect about 100 or 200 cocoons each winter. In the winter of 2007 I made a thorough search of the same 300-acre patch of woods that I had perused in previous years, and managed to collect 359 cocoons.
For about ten years starting in the mid-1980s, I could always find cocoons containing live Calosamia pupae, although those infected with the Gambrus parasitoid were common, too. For a long time it didn’t make any difference to me how rare or common they were, but later it appeared that a pattern might be emerging, so I became more methodical and tried to find out if there was one. I found that through the years 1993 to 2006 the number of live moth pupae in cocoons decreased steeply from about 50 percent to less than 1 percent. Of the 359 cocoons collected in the winter of 2007 only 1 contained a live pupa! During the next winter (early 2008) I recruited my ten eager Winter Ecology students as Calosamia cocoon hunters. One student in particular developed an impressive record as a searcher, and all of us together retrieved 242 cocoons. Many were two-year-old cocoons I had missed previously, but in any case we found not a single current live Calosamia pupa, and only 1 contained a live parasite (Gambrus) pupa. In other words, the moths had now become at least locally extinct. We had collected the artifacts of a past population. We have no idea what caused the collapse, or why other giant silk moth populations have exhibited similar spectacular recent declines.
In western Vermont near Burlington, Calosamia cocoons have, in contrast, been very difficult to find. All the students who hunted Calosamia in Maine are ardent field naturalists. One of their academic requirements—one that they find easy to fulfill—is their “Friday field walk” in the surrounding Vermont woods. For the last ten years they have been assigned to find Calosamia cocoons. I had found none; they found four. Three of the cocoons had successfully eclosed moths, and the fourth contained viable Gambrus pupae. Calosamia still live, but at very low densities. Perhaps the density of Calosamia is so low in Vermont that disease organisms and parasitoids have a hard time finding them, but the moths can still find each other to mate.
The males’ female detectors are evolved to what looks like extraordinary perfection. In early June 2004, 2005, and 2006, my screen cage in Vermont containing moths from cocoons that I had collected in Maine was aflutter with freshly emerged moths. I did not release them, because I was hoping to get eggs for a crop of caterpillars. The day after they emerged, at around six PM in bright sunshine, the big magenta males “with a bolt of lightning crackling down their wings” as one moth aficionado described them, were flying all around our house. It seemed that a swarm of them had gathered. Males were again flying around the house the next day, in the late afternoon, but by the day after that I saw no more of them.
I next hoped to raise caterpillars from the eggs laid by the mated females.
Using a thread looped around their “waist” I had tied females onto both ash and chokecherry bushes, and each of these mated females deposited about 100 to 200 eggs onto the leaves and twigs there. Freshly laid eggs are sticky and become glued onto the substrate. I checked later, expecting the bushes to be crawling with caterpillars, but found not a single one. In the third summer I took the mated females into my study and let them deposit their eggs in screened cages so that I could keep track of them. All the eggs hatched, and hundreds of young caterpillars started feeding on the ash leaves I had provided for them. Then, soon after their first or second molt, they all died—every single one. Not since I raised caterpillars as a small child, when a fellow student zapped my screened caterpillar cage with spray from a Flit insecticide dispenser, had I seen such caterpillar mortality. The signs this time pointed to death by a virus. I noticed also that the few gypsy moth caterpillars that I had found near the house in Vermont died as well; they turned into a liquid broth.
Fig. 24. Various contents of Calosamia cocoons. Left: An empty cocoon chewed into by a rodent. Top left: Mummified caterpillar killed by a mold. Top right: Cocoon of the Enicospilus wasp, showing a hole at top where it emerged. Lower left: Cocoon packed with numerous Gambrus wasp cocoons. Lower right: Cocoon showing the pupal skin of a successfully emerged moth.
The gypsy moth, Lymantria dispar, is a forest defoliator in North America. It evolved in Europe and Asia and was introduced near Boston in 1868 by E. Leopold Trouvelot; and, as they say, “The rest is history.” It became such a serious forest pest in all states east of the Mississippi that state and federal governments instigated eradication attempts, spraying millions of acres of forest with pesticides and also releasing biocontrol agents that have included viruses, bacteria, fungi, a beetle, and fly and wasp parasitoids. Some of these biocontrol agents affect the gypsy moth specifically. Unfortunately, others have a broad spectrum, attacking so-called nontarget species of native moths and butterflies.
Recent studies by George H. Boettner and Joseph S. Elkinton, entomologists at the University of Massachusetts-Amherst, have demonstrated a deadly effect of one of those gypsy moth biocontrol agents on the caterpillars of these moths. It is a parasitoid fly, Compsilura concinnata. This fly alone “controls” at least 200 species of moths. “It is,” Boettner told me, “an evil beast that should never have been released.” Are there also evil beasts among the viruses, bacteria, fungi, and introduced wasp parasitoids?
About 150 species of insects alone have been released deliberately for biological control. To test for the presence of parasitoids, Boettner regularly puts thousands of hand-reared silk moth caterpillars into the wild. Sometime later he retrieves them to study their parasite load. He has found that in some localities 100 percent of the silk moth caterpillars are infected (killed) by the one introduced fly alone, in less than one week. That’s a disastrous first inning for the caterpillar in its game with just one of its many enemies, as it tries to reach adulthood. The caterpillars are normally exposed to these flies not for one week but for a month, and in order to turn into adults they have to survive not only this exposure but the pupal stage as well. Add to this mortality caused by flies, also that of caterpillar-foraging birds, and by the fungal, bacterial, and viral diseases that also affect the larvae, and it is a wonder that any ever remain to spin a cocoon.
I had, perhaps by chance, observed one very prominent moth’s precipitous decline on my little plot of land on the hill in Maine. However, there are thousands of species, all invisible links in the fabric of any ecosystem. It is hard for anyone to care about any of them, because so little is known about them. And probably not much ever will be known—because there is no apparent proximal reason to know about them except curiosity for its own sake. “Every country boy” was familiar with one very conspicuous moth—the Calosamia—in the past, as Holland wrote in 1903. Is anyone familiar with it now? There are thousands of other more or less invisible species. Yet we don’t have people able to identify even most of the big brightly colored “canaries,” never mind important players like Gambrus wasps that are “biocontrol agents” and play a vital role. There is even debate about how many and what kinds of Gambrus wasps there are, and we can’t afford to lose any of them.
I think the lesson of the moths is the value and power of even the rarest players in the maintenance of a natural ecosystem, and the danger that even a few members of an alien species can disrupt it. High summer temperatures allow for rapid growth, and the long growing season then permits several generations in small animals. Generation times that are speeded up to a year or less, and mortality due to predators and parasites that is then ramped up to at least 99 percent, may appear alien to us. But in such a system any small difference in an inherited trait has an extremely high chance of being “noticed” in terms of evolution. The parasitoids (and bacteria and viruses) should therefore always be “ahead” of the slow reproducers. Of course the parasite’s raison d’être is not to kill. It is to live. But if its host’s population is dense, then virulence is adaptive because the parasite can grow without restraint within its host, eating every bit to proximally put out the maximum number of offspring. Killing the hosts in the process does little harm to the parasite, which simply jumps onto the next host. However, once the population of hosts becomes very rare, then any parasites that kill the host quickly become extinct, and the benign parasites are selected instead. It’s a case of “overexploitation of the resource.” It is wildly successful—for a while. But once the resource—in this case, live hosts—become rare, then the parasite, or the bacterium, or any other infective agent will be committing suicide if it kills, because it will then die with its host. At low population the tables turn, and then only the benign parasites will live, a situation analogous to what happens when a society produces an invention that allows it to tap new resources or invade virgin territory. The exploitive strategy is favored when resources are plentiful; but when they run out or there is no place else to go, the frugal strategies then persist preferentially.