The Unstoppable Gypsy Moth
“THEY are everywhere,” complained the young woman to her husband as she desperately flailed her arms to ward off a spiny, dark-brown caterpillar swinging toward her in the wind. She was right. The caterpillars were everywhere—crawling on tree trunks and branches or dangling on invisible silken threads. The two of them, tired of picking caterpillars off each other, cut short their walk in the woods. The caterpillars were a nuisance.
A nuisance? Authorities say they are a plague. In 1980 these caterpillars defoliated five million acres of northeastern forests in the United States. Worse, in 1981 they stripped a record 12.8 million acres. “They have also defied every assault mounted against them,” says Smithsonian, “until their invasion front now stretches along a wavy and indistinct line from Virginia and Maryland, through West Virginia and into western Pennsylvania and New York, with numerous isolated pockets scattered beyond.” Even the West Coast states of California, Oregon, and Washington are threatened.
The enemy, it seems, is unstoppable. Emerging from their eggs in late April or May, the one-eighth-inch-long (0.3 cm) larvae will feast on any of 500 types of trees and shrubs if necessary, though oak leaves remain their favorite. Carried by winds that catch their silken threads, they float from tree to tree or hitchhike on cars, campers, or lawn furniture into new territories—hence the name gypsy. By the time they reach a full 2 1/2 inches (6.5 cm) in length, each caterpillar can put away 12 square inches (77 sq cm) of leaf in 24 hours. When on a rampage, they are truly pests—crawling over driveways, dropping onto plates at backyard barbecues, creating a steady rain of excrement and half-eaten leaves. At their worst, they kill trees that are too weak to withstand successive years of leaf loss.
The moth was accidentally introduced to America in 1869 when a windstorm upset a cage of gypsy moth caterpillars imported by a French naturalist working in Medford, Massachusetts. The caterpillars crawled out the window. Twenty years later Medford was swarming with them. For the next ten years pest fighters in Massachusetts fought the insect with torches, creosote paint, scrapers, sticky paper, and sprays of lead arsenate and other poisons, and they almost succeeded in eradicating them. But as the number of moths dwindled, so did the interest in fighting them.
Yet their territory kept expanding. By 1900 they had nibbled through Massachusetts. They were in Rhode Island by 1901. They moved to New Hampshire in 1905, Connecticut in 1906, Vermont in 1912, and New York in 1922. By 1934 they were in Pennsylvania. But even as their territory expanded, the amount of forests they defoliated varied—a disconcerting fact for pest fighters trying to predict the enemy’s next move.
A faint hope of stopping them came in the 1950’s with the aerial spraying of DDT. But pest fighters were forced to turn to less toxic chemicals when DDT was banned due to its lethal effects on other wildlife. In the meantime, more selective controls were developed. One popular bacterial agent in use, Bacillus thuringiensis, is fatal only to moths and butterflies. Another viral agent kills only gypsy moths but is very expensive and not available in large quantities. Additionally, of the some 45 predators and parasites of the gypsy moth that have been introduced to North America, about a dozen have gained a foothold.
But despite millions of dollars spent on poisons and biological weapons, the moths still push west and south at 5 to 15 miles (8 to 25 km) a year. Some authorities maintain that the widespread use of pesticides stimulates the spread of the moth. How? Moths that spread rather slowly or that have no immunity to poisons are quickly targeted and killed off. Therefore, surviving moths tend to be more mobile and poison resistant. Furthermore, these hardy strains reproduce at astounding rates, since they have no competition for food and because the most common poisons have eliminated their natural enemies.
Biologist Jack Schultz of Dartmouth College suggests that we rely on nature’s own defenses. He has demonstrated that defoliated trees grow replacement leaves high in tannin—making them less palatable to the caterpillars. “Just let the trees and plants alone,” he urges. “Variability is a more potent control than uniform spraying, and tannin doesn’t introduce toxic substances into the environment.” Interestingly, a variety of natural causes—viral disease, predators, and stress from a lack of food—begin to operate when moth populations grow too large. For these reasons, since 1981 forest defoliation by the gypsy moth has dropped drastically.
The sight of a naked, dead-looking forest of oaks ravaged in midsummer by hungry caterpillars is shocking. However, the forest may not be as devastated as it appears. It has been suggested that such defoliation in eastern forests simply accelerates the growth of American beech, sugar maple, and eastern hemlock—climax species that are not favored as food by gypsy moths. Perhaps nature is demonstrating another natural control that someday will lessen the wild rampages of the gypsy moth. In any event, Conservationist concludes: “Long-range biological consequences of forest invasion by the gypsy moth now seem predictable and may actually be beneficial.” That’s taking a positive view. We might as well. After all, the little fellows are here to stay.
[Pictures on page 22, 23]
Life of an adult moth lasts only a week, long enough for mating. The female lays egg clusters that may contain as many as a thousand eggs
USDA Forest Service
USDA Forest Service
USDA Forest Service