Silk
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 simile for smoothness;
it described with equal accuracy 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 die
they did, in droves. The resourceful
entrepreneurs then imported mulberry trees from
China to provide familiar 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 the
1700’s in New England and points south 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 of the
time 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 foretold, 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 st. 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 thousands of
birds, wild animals, 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 1905 they were in
New York and parts of Pennsylvania. In 1943
arsenic-based sprays were replaced by DDT, which
was used 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 had 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 decline 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,
and as soon as the young leaves came out the air
was filled with tiny larvae, ballooning on
silken threads. Their effect is most severe on
trees that are under stress, and this year
promises to be a tough one; 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.