Gypsy Moth Invasion

Though they seem harmless, gypsy moths are one of the most invasive insect species to date.

The caterpillars were everywhere—in the pool, on the side of the house, in the eaves, in your hair. Where there were large infestations, you could hear them at night. Their droppings, or frass, sounded like a light rain. Perfectly sane people went about protected beneath umbrellas on sunny days.

It was spring in New England, and gypsy moth caterpillars had normally stolid Yankees in a tizzy. The ravenous larvae had chewed their way through 5.1 million acres of land in the northeast. In 1981, the situation was worse, with nearly 11 million acres devastated before its peak. The caterpillar doesn't eat whole trees, but it does devour leaves. A single caterpillar can eat a square foot of foliage in a single night. And for most deciduous trees, three or four massive attacks spell death.

Gypsy moths were first brought to this country in 1868 by French naturalist Léopold Trouvelot, who settled in Medford, Massachusetts, hoping to develop a cheap domestic silk. Somehow two gypsy moths escaped, and the first serious infestation occurred in the summer of 1889. The gypsy moth caterpillars ate Medford, and they've been eating and moving on ever since.

According to Dr. Charles Schwalbe, of the US Agriculture Department's Cape Cod bureau, the siege of the gypsy moth reached an all time high in the 80s. The reason is that these tiny monsters were swept across the country by wind currents that carry them farther away from Medford with each successive generation. "Right now," said Dr. Schwalbe, "the front of defoliation is moving rapidly over the mountain ridges of Pennsylvania at the rate of at least 20 miles a year."

A map in Schwalbe's office showed the main corridor of gypsy moth movement. Where the Appalachians and the Allegheny Mountains come together ink marks and arrows flew in all directions. The range of devastation in the past has extended from Maine to Maryland and included all of Pennsylvania and parts of Virginia and West Virginia.

Photo via WSDA

Stages of Invasion

The onslaught of the gypsy moth begins in late April or early May, when the first eggs hatch. The larvae go through several molting stages as they eat. They prefer oak leaves, but some 500 other plant species also provide a hearty repast. Flowering dogwood, ash, balsam fir, and mountain laurel are among the few plants they avoid.

A caterpillar will munch its way to the highest branches of a tree, where it spins a silken thread. And, like a gypsy, it will go whichever way the wind is blowing. By mid-July the feeding stops and homeowners begin to relax, but the insect is only in its pupal stage, having spun itself a cocoon. A couple of weeks after pupation, adult moths emerge. They don't eat, and the females don't even fly, but they do mate and lay a single egg mass before they die. One egg mass contains between 500 and 700 eggs.

The eggs remain dormant during the winter months, but they can get around nonetheless. If you have to move or ship firewood, Christmas trees, or woody shrubs from one place to another, you may be spreading gypsy moths. Egg masses could be stowaways on your deck chairs, picnic table, or wheelbarrow.

Potentially, the gypsy moth is everywhere. "Gypsy moths seem to like it here as much as Americans do," Schwalbe says. "It is probable that the gypsy moth can live anywhere in the continental United States where suitable host trees exist in sufficient abundance."

Avoiding Destruction

In the past, Schwalbe says scientists tried to control the gypsy moth with chemical pesticides. But that proved to be a poor idea. DDT, for example, was so toxic that it killed or altered just about everything that moved, and it had to be taken off the market. Sevin, another extensively used pesticide, killed gypsy moths, but it also killed honeybees. With public fear of chemical pesticides on the rise, researchers decided to try other tactics.

Ladies and gentlemen, the pest war is on, and sex is a weapon. The adult female gypsy moth emits a sex attractant, called a pheromone, a chemical plume that no male gypsy moth can resist. Scientists have been able to synthesize this alluring substance and are using it in two ways. Milk-carton like traps, containing the pheromone, are placed where the moth is suspected. Any male that catches a whiff will follow the scent into the trap; Instead of mating with a female, it will be killed by an insecticide. The USDA used 100,000 traps last year to locate new infestations. Homeowners can also buy these traps. If the traps are to be effective, they must be in place well before the adult moths emerge from their cocoons.

Pheromones are also used to confuse male moths. Synthetic pheromones in slow-release capsules are sprayed on a population so that the entire area smells like a single passionate female. The males don't know which way to turn, and the females are ignored. Sounds good in theory, but how well it works is still not known.

Another experiment involves what is called the sterile-male technique. Male pupae are zapped with cobalt 60, not enough to make them radioactive, but enough to make them sterile. If you know the population density of an area, you can release enough sterile males to compete with fertile males. Since females usually mate once, if the partner is sterile, the eggs are infertile.

In nature, disease controls the size of a population that has grown too large. With this in mind, some scientists are trying to replace chemical pesticides with bacteria or viruses that are lethal to the gypsy moth but harmless to people and other forms of life. BT (Bacillus thuringiensis), for example, is a bacterium that kills only butterfly and moth larvae. Gypchek is a virus that affects only the gypsy moth caterpillars. It is not yet commonly used, but experiments are under way to make it more virulent and to distribute it by spraying it from airplanes.

Then, of course, there are parasites. All kinds of flies were imported from Europe and Asia early this century because it was known that some parasites lay their eggs inside the gypsy moth caterpillars. Others lay their eggs on leaves that are eaten by the caterpillars, which then become the host for the insect parasite. The parasite lives off the caterpillar, eventually killing it. Scientists want to mass-produce the flies and unleash them at just the right time to thwart the foraging caterpillars.

The goal is to integrate all of these techniques and produce an arsenal of controls, suitable for any situation. The techniques now being applied to the gypsy moth have been used successfully against other insect pests. Screwworms were virtually wiped out in Florida through the sterile male technique. Mexican bean beetles, which are harmful to soybeans, are being attacked by parasitic wasps in Virginia. Maryland, Delaware, and New Jersey. It's all part of a nationwide program called Integrated Pest Management (IPM).

Photo via PEI Invasives

Integrated Pest Management

IPM attempts to harness every available weapon, but it comes about largely as a response to the outcry against the use of chemical pesticides. For example, in Texas, where 75 percent of the cotton is grown under IPM, the use of pesticides has fallen from 20 million pounds a year in 1965 to about 2 million pounds now. Pheromones are used in the Texas cotton fields to lure the boll weevil into traps and to disrupt the mating of the pink bollworm.

Biological controls are not a new idea. The ancient Chinese planted nests of predatory ants in citrus trees to control leaf-eating insects. Pesticides were not the main weapons used in the pest wars in the United States until after World War II. The number of registered pesticides increased from about 30 in 1936 to more than 900 in 1971. Today 1 billion pounds of pesticides are used every year.

IPM is a more subtle, sophisticated approach to pest management than a killing spree with toxic chemicals. As an applied ecology IPM has the advantage of targeting its deathblow to a specific insect population, ensuring not only a strong agriculture but a healthier environment.

When you deal with gypsy moths, or any other pest, you have to consider the economic threshold. An evergreen farmer in Maine sprayed his trees six times this year to harvest his Christmas crop. Yet no one notices when an Oak in a remote forest—too far away to be worth its weight in firewood—falls victim to the gypsy moth. But, if that oak is in your front yard and the caterpillars are so heavy that you can watch the grass move, act quickly.

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Gypsy Moth Invasion