Of silken connections

Bill Meredith

Silk has had a peculiar hold on the imagination as well as the mercenary ambitions of the inhabitants of the Western World ever since it was brought from the Orient to Europe in the 5th century. It became the symbol of elegance, both in fact and in fancy; the wealthy classes were clothed in it, and it entered our language as the ultimate metaphor for smoothness, describing either the texture of a fabric, the palatability of whiskey, the quality of Nat Cole’s voice, the sheen of a maiden’s hair or the tempo of Payne Stewart’s golf swing. And inevitably, like money, the quest for it became the root of evil.

The problem was that silk came from worms… not real worms, but caterpillars, the larvae of a species of moth that was found only in such inaccessible places as China. At first this seemed to present no difficulty; enterprising merchants bought, borrowed and stole the moths, brought them to Europe via caravan routes through Persia and Syria, supplied them with leaves from local trees, and sat back to wait for the funds to start rolling in. The silkworms, alas, did not cooperate; like finicky preschoolers, they resolved to die before they would eat such alien stuff, and they did, in droves. The resourceful entrepreneurs then imported mulberry trees from China to provide a source of fodder, and eventually a silk industry began to grow, first in Italy and, by the 18th century, in France. But problems abounded. The Mediterranean climate was not as congenial as that of their native land, and the silkworms succumbed to one disease after another. Viruses and bacteria had not been discovered yet, but that didn’t stop them from killing the silkworms. Eventually, as science advanced, better methods of feeding and tending the worms were developed, but it was a labor-intensive process at best. Hopeful growers were constantly looking for better food sources and trying to breed more productive stock. The industry became modestly successful in France, and the British naturally tried to get into the act. The English climate frustrated efforts to establish it there, but the dream stayed alive, and British entrepreneurs took their hopes to the America; as early as the 18th century, hopeful developers attempted to start a silk industry in the colonies. The results were disastrous; we are still paying for it, and the price is going up.

The first disaster originated from the food supply. In their native China, silkworms feed on several species of trees; the mulberry is the best known, but the list also includes Ailanthus altissima, known in polite circles as the Tree of Heaven, and to the less tactful public as the Stink Tree. Both species of trees were planted in New England and points south in the 1700’s as a source of food for silkworms; some silk was produced, but the industry was, by and large, a failure. Not so, the trees; they flourished and spread, invading lawns, parks, pastures, and vacant lots with unbounded vigor. Like other introduced species such as starlings, English sparrows, kudzu vines and multiflora roses, they became pests. They spread rapidly; mulberry seeds are sown by birds, while those of Ailanthus travel in a winged fruit similar to that of maples, and they crowd out native trees in their habitats. They are neither attractive as ornamentals nor useful for timber; they have no redeeming features. The most positive thing that can be said is that they are nuisances. Just check around the edges of my yard if you want evidence.

The second disaster has proved more serious. It began with a young French astronomer named Leopold Trouvelot, who came to Harvard University to study in 1869. He spent his nights dutifully peering into the telescope, but in his spare daytime hours he dreamed of the riches and renown that would be his if he could find a way to make silkworms resistant to disease. Using my wife’s line of reasoning (if you’ve seen one moth you’ve seen them all), he set out to cross silkworms with gypsy moths, with the expectation that he could create a new variety of moth that would produce good silk and be disease-resistant. Although theoretical genetics was still 30 years in the future, practical biologists knew that gypsy moths and silkworms belonged to different families, and hence crossing them was impossible. But Trouvelot, unencumbered by such knowledge, ordered a shipment of gypsy moth pupae from France, put them in a box with some silkworms in his bedroom, and waited for nature to take its course.

Nature did take its course, but not as Trouvelot expected. As any freshman biology student could have predicted, the two species of moth ignored each other. Whether Trouvelot got tired of waiting and threw them out in disgust, or some escaped by accident is not known for certain; but the following summer, gypsy moths defoliated some trees along the street where Trouvelot lived. He completed his studies and went back to obscurity in France. The moths also disappeared into the surrounding woods and were forgotten… for a while. Twenty years later their population exploded. They defoliated local forests, and then spread to orchards and city parks; residents in the area compared their effect to Biblical plagues. Local farmers and other authorities responded by spraying trees with solutions of arsenic, with an intensity that would have appalled even the Bush administration; the moths were killed in impressive numbers, along with birds, wildlife, and some livestock and pets. But it was too late. The moths initially spread northeast into New England, as the young larvae were carried on the prevailing winds each spring, and inevitably they were also borne southward by nor’easters. By the 1905 they were in New York and parts of Pennsylvania. Arsenic-based sprays were used until 1943, and DDT until the mid-‘70’s, but the march continued.

The gypsy moths reached Maryland in 1980, and found it to their liking. Climate favored them, and the predominant forest trees were oaks, which they love beyond all other trees. In 1980 they defoliated three acres of forest in the state; by 1990, that increased to nearly 190,000 acres. They are now present all over the state, and have extended their range through most of the Appalachian region and as far west as Minnesota. The damage they have done to forests, to say nothing of orchards and ornamental trees, is beyond measure.

Gypsy moth populations are cyclic; combinations of weather, predators, disease and food scarcity in areas where the trees have been killed cause them to crash periodically, as they did around here in the past few years. Spraying with Bt (a bacterium that kills the larvae) can control them, but the spray is too expensive to be used universally. So even when they seem to be on the decline, their populations are always ready to explode. Last summer when I took my grandson on our annual hike to Indian Lookout, the air was full of male moths that were homing in on the sedentary females, each of which produced over 400 eggs. On a walk this spring I saw hundreds of egg masses on the trees on College Mountain. Their effect is most severe on trees that are under stress, and as I write this we haven’t had measurable rain for over a month. For the oak trees on the mountain, it’s going to be a long summer.