Most scourges of our native trees were inadvertently introduced to North America. Think of chestnut blight; it wiped out the American chestnut throughout its entire range in about 25 years. And Dutch elm disease virtually eliminated the American elm and changed the look of American cities in a similar period of time. Both diseases are caused by fungi that were unintentionally imported on wood intended for furniture manufacture.

Gypsy moth (Lymantria dispar), which defoliates about 2 million acres of hardwood forest in any given year, is a different story. Gypsy moth is native to Europe, but in the late 1860s, artist, amateur entomologist, and French émigré E. Léopold Trouvelot imported gypsy moth to his home outside of Boston. Only a few years earlier (1862), Lincoln had established the Department of Agriculture (now USDA); so even then, in the midst of the Civil War, there was an awareness of the need for a controlling authority over, among other things, importation of exotic animals and plants.

However, Trouvelot was quite accomplished and no doubt could have obtained (if he did not and if one was required) a permit to import gypsy moth. He intended to breed them with native silkworms to develop a disease-resistant strain that he could use to launch a commercial silk industry. Trouvelot eventually had as many as 1 million caterpillars in cultivation. Unfortunately, some escaped.

Soon Trouvelot lost interest in entomology and concentrated on astronomical illustration. His work became well known for its detail. He produced as many as 7000 drawings and lithographs (See for example http://www.lib.umich.edu/divine-sky-artistry-astronomical-maps/drawings.html; verified 22 May 2010.) and published 50 scientific papers. A crater on Mars bears his name. Trouvelot returned to France in 1882. That same year the first gypsy moth outbreak occurred at Medford, MA, on the street where Trouvelot had lived.

Today, with no hope of eradication, we "manage" gypsy moth. Management strategies are based on the insect’s life cycle and biology. Its life cycle is relatively simple, involving four developmental stages—egg, larva (caterpillar), pupa, and moth (adult)—in one generation per year.

In late summer, female moths lay masses of eggs. Larvae overwinter in the eggs and hatch as caterpillars in spring. It is the caterpillar that is destructive. It prefers oak, aspen, and willow but will feed on almost 600 species of trees and shrubs. Some trees are resistant—dogwood, ash, locust tree, yellow poplar, and some maples, for example. Evergreens are generally resistant, but blue spruce and white pine are not. The caterpillar is thus "polyphagous", and caterpillars live to eat; smaller young ones feed during daylight, while older ones feed under cover of night. The gypsy moth expands its range primarily by "ballooning"; young caterpillars spin a silk thread that is caught by wind; they are usually carried no more than 300 feet. This stage lasts about 7 weeks. In early summer, caterpillars enter the pupal stage and develop into moths. Moths emerge from their cocoons in 10 to 14 days. The moth itself is short lived (about 14 days) and eats nothing; its biological function is to mate and reproduce.

Population dynamics of gypsy moth are unusual, going through boom and bust cycles. The pest may exist at low numbers for many years, but over one or two generations, its numbers can climb rapidly—an "outbreak". The population then collapses just as quickly. During these episodic population explosions, whole forests can be defoliated. Healthy trees usually survive single defoliations, but several years of defoliation in combination with other stresses such as drought or pathogens are lethal. These same population dynamics are seen in Europe, where the insect is native.

Small mammals (for example, mice) feed on gypsy moth eggs and are largely responsible for keeping gypsy moth numbers in check. Several insect parasites also prey on gypsy moth. But what tips the balance and causes an apparently stable population suddenly to explode is not known. Outbreaks tend to be regional, though, and because of this, it seems likely that weather events or some other broad, general influence may upset an otherwise stable biological balance and initiate an outbreak.

Crashes in gypsy moth populations are better understood and seem to be caused by the rapid spread of two gypsy moth-specific pathogens within the crowded, outbreak population. One of these is a virus and the other a soil-borne fungus that is highly virulent during wet, humid conditions. Under non-outbreak conditions, neither pathogen exists at a high enough level to check an outbreak.

Since it escaped from Trouvelot 140 years ago, gypsy moth has spread to only 30% of susceptible U.S. habitat. USDA Forest Service is charged with managing the pest and has concentrated on minimizing the rate at which the pest spreads—the Slow the Spread Program (STS), was funded by Congress in 2000. The overall strategy, conducted in cooperation with state DNRs and local municipalities, involves quarantine and suppression in infested areas, eradication in the pest-free areas where isolated gypsy moth are found, and suppressing the rate of spread in the transitional areas between infested and pest-free areas.

STS suppression efforts focused along the 1000-mile leading edge of the gypsy moth infestation zone include the states of Minnesota, Wisconsin, Iowa, Illinois, Indiana, Kentucky, Ohio, Virginia, West Virginia, and North Carolina. The primary tactic is aerial spraying with an insecticide composed of Bacillus thuringiensis var. kurstaki (Btk), applied in May through June when young caterpillars are present. In 2009, 440,000 acres were treated. Btk is a naturally occurring soil bacterium that kills caterpillars when they ingest it. It has been used as an aerially applied insecticide for at least 30 years and is considered safe to humans, pets, live stock, and property. But it is controversial because it is not specific to gypsy moth; it also kills any other moth or butterfly caterpillar that eats it. Gypcheck, another insecticide that’s used, is specific to gypsy moth caterpillars. Gypcheck is based on the gypsy moth-specific virus that is so virulent in gypsy moth outbreak populations. However, Gypcheck has been used only where endangered or threatened moths or butterflies are present, since, it is difficult and expensive to make.

Mating disruption is a strategy that is particularly well suited for low-density gypsy moth populations. It utilizes pheromones, chemicals that insects produce to communicate with each other. Gypsy moth females attract males to them by releasing a sex pheromone when they are ready to mate. Aerial applications of a synthetic gypsy moth sex pheromone mask the female’s scent and confuse males so that they cannot find females. Moths are at the end of their life and die without mating. About 100,000 acres in Virginia, Ohio, Illinois, and Wisconsin were treated with synthetic pheromone in 2009. The pheromone has no effect on humans or other animals and is not long lasting in nature.

Despite these efforts, gardeners and homeowners still encounter gypsy moths, and there are simple ways to reduce their impact. Look for egg masses in late summer through spring; they usually contain 100 to 600 eggs, or more. They are tan, roughly tear-drop shaped, and about 1 to 1.5 inches across. They are laid in protected places such as under tree branches and loose bark. In residential lots, they are also found on manufactured objects (lawn furniture, fences, grills, sides of houses, boat trailers). Egg masses found after early May have probably hatched. Gypsy moth can cause skin irritation; use gloves when handling egg masses, caterpillars, or pupae. Egg masses should be scraped into soapy water infiltrated with an emulsion of water and vegetable oil, or microwaved for a minute or two. Simply crushing them will not kill all eggs.

Gypsy moth caterpillars appear in midspring and are easy to distinguish from other common leaf-feeding caterpillars (See the following link: http://www.dnr.state.mn.us/invasives/terrestrialanimals/gypsymoth/id.html; verified 22 May 2010). They’re initially quite small but grow to 1.5 to 2.0 inches long. Older caterpillars have coarse, black hairs and 11 sections with colored spots; the first five pairs of spots are blue and the last six are red. They do not produce webs or tents as do webworms and tent caterpillars.

Insecticides, both biological and synthetic, can be sprayed in early spring if caterpillar numbers warrant it. The most common one, the Forest Service’s choice, is Btk, which is sold under various labels (Foray, for example). Check with county agricultural agents or garden stores for recommendations and spraying schedules. Certified arborists will spray for gypsy moth also.

Mechanical barriers work well in small suburban lots but are not practical for larger areas. "Sticky" barrier bands target young caterpillars that have fallen from trees or that have been transported by wind. These should be installed in early spring when the young caterpillars are hatching. One method is to wrap duct tape around trees at 5 to 6 feet above the ground and cover it with a sticky compound such as "Tanglefoot", a commercial product, or petroleum jelly. Caterpillars trying to climb up trees to feed will be caught and can be scrapped off into soapy water or a sealable bag. This type of barrier also works for other caterpillars such as tent caterpillars.

Older caterpillars are more mobile than young ones and crawl from tree to tree. They hide from predators such as birds during the day and are attracted to bands of fabric such as burlap placed around trees. These bands can replace or supplement the sticky barrier bands. They should be installed in early to midsummer. The cloth strip should be 12 to 18 inches wide and long enough to wrap around a tree completely without a gap. The cloth is secured to the tree trunk with a piece of twine that is tied around the cloth’s center; the cloth is then folded down over the twine to cover the cloth’s bottom half. These bands should be placed above sticky bands at 5 to 6 feet above the ground. Caterpillars that hide under the flap of the band can be captured and killed in soapy water. When an infestation is high, bands should be checked every day in the afternoon.

Look for pupae as well as moths in late summer. Pupae are dark brown, about 2 inches long, covered with hairs, and are found hanging in sheltered spots such as branches or in leaf litter. Female moths have white to cream-colored wings, a tan body, and a 2-inch wingspan; they do not fly. Males are smaller, dark-brown, and have feathery antennae. Pupae can be crushed. Male moths cruising for females will be attracted to and caught by ordinary commercial pheromone traps.

Over time, barring some technical or natural breakthrough, the pernicious gypsy moth will infest all of the hardwood forests in the USA and Canada. It may well change the composition of these forests. The best we can do is reduce its spread and moderate its effect. The Forest Service STS program has been quite successful. It has reduced by 70% the Gypsy moth rate of spread from its historical average of 13 to 3 miles per year. Gardeners can do their part too by recognizing this pest, knowing its habits, and killing it whenever they find it.