In 1868, Étienne Léopold Trouvelot, a French scientist living in Medford, Massachusetts, introduced into the United States the gypsy moth (Lymantria dispar), which is now also termed the "spongy moth" within that nation. Because native silk-spinning caterpillars were susceptible to disease, Trouvelot imported the species in order to breed a more resistant hybrid species. Some of the moths escaped, found suitable habitat, and began breeding. The gypsy moth is now a major pest of hardwood trees in the Eastern United States. The first US outbreak occurred in 1889 in the New England states. In 1923 attempts were made to prevent the westward spread of the moth by maintaining a barrier zone extending from Canada to Long Island of nearly 27,300 km2. This barrier however broke down by 1939. By 1987, the gypsy moth had established itself throughout the Northeastern United States, southern Quebec, and Ontario. The insect has now spread into Michigan, Minnesota, Virginia, West Virginia, Illinois, and Wisconsin. Small, isolated infestations have sporadically occurred in Utah, Oregon, Washington, California, and British Columbia, but efforts have been taken to eradicate them. As of 2021 the Washington State Department of Agriculture is again trying to eradicate both L. d. d. and L. d. asiatica – using Bacillus thuringiensis kurstaki (Btk) toxin – to prevent their establishment in the state. Since 1980, the gypsy moth has defoliated over one million acres (4,000 km2) of forest each year. In 1981, 12.9 million acres (52,200 km2) were defoliated. In wooded suburban areas, during periods of infestation, gypsy moth larvae crawl over man-made obstacles and sometimes enter homes. When feeding, they leave behind a mixture of small pieces of leaves and frass, or excrement. During outbreaks, the sound of caterpillars chewing and dropping frass may be loud enough to sound like light to moderate rainfall. Gypsy moth populations usually remain low, but occasional increases to very high levels can result in partial or total defoliation of host trees. According to a 2011 report, the gypsy moth is now one of the most destructive insects in the Eastern United States. The moth and other foliage-eating pests cause an estimated $868 million in annual damages in the U.S. In June 2024, Scientific American reported the severity of the outbreak in the northeastern and midwestern regions of the United States. Some places in these regions were experiencing one of the most severe outbreaks ever recorded. Some areas reported densities exceeding 2,500,000 caterpillars per hectare (1,000,000 per acre). Some regions have grappled with this issue for five consecutive years.
Host species Gypsy moth larvae prefer oak trees, but may feed on many species of trees and shrubs, both hardwood and conifer. In the eastern US, the gypsy moth prefers oaks, aspen, apple, sweetgum, speckled alder, basswood, gray and paper birch, poplar, willow, and hawthorns, amongst other species. The gypsy moth avoids ash trees, tulip-tree, cucumber tree, American sycamore, butternut, black walnut, catalpa, flowering dogwood, balsam fir, cedar, American holly, and mountain laurel and rhododendron shrubs, but will feed on these in late instars when densities are extremely high. Older larvae feed on several species of softwood that younger larvae avoid, including cottonwood, hemlock, Atlantic white cypress, and pine and spruce species native to the east.
Moth rash The gypsy moth caterpillar has been reported to produce a poison ivy-like rash on some humans coming into contact with the hairs of the larval stage. The contact can be direct or even indirect, if the small hairs are carried by the wind and onto the skin or clothing of a person. Gypsy moth rashes were documented in the early 1980s, during a major infestation in the Northeastern United States. In coastal Maine and Cape Cod, Massachusetts, caterpillar-triggered rash is much more likely due to exposure to the browntail moth (Euproctis chrysorrhoea).
Effects of defoliation
The effects of defoliation depend on the species of tree, amount of foliage removed, health of the tree, number of consecutive periods of defoliation, and available soil moisture. If less than half of the crown is defoliated, most hardwood species will experience only a slight reduction in radial growth. If more than half of the crown is defoliated, most hardwoods will produce a second flush of foliage by midsummer. Healthy trees can usually withstand one or two consecutive major defoliation events. Trees weakened by previous defoliation or subjected to other stresses like droughts are frequently killed after a single half-defoliation. Trees use their energy reserves during re-foliation and may become weakened and exhibit symptoms such as the death of twigs and branches in the upper crown and sprouting of old buds on the trunk and larger branches. Weakened trees experience radial growth reduction of approximately 30 to 50 percent. Weakened trees are vulnerable to attack by disease organisms and other insects, or example, the Armillaria fungus may attack the roots, and the two-lined chestnut borer may attack the trunk and branches. Affected trees will eventually die two or three years after they are attacked by these pests. Although not preferred by the larvae, pines and hemlocks are subject to heavy defoliation during gypsy moth outbreaks and are more likely to be killed than hardwoods. A complete defoliation can kill approximately half of pine species and 90 percent of mature hemlocks because conifers do not store energy in their roots; an exception is larch.
Factors that affect gypsy moth populations Natural predators play an important role during periods of low population. Predators include wasps, flies, ground beetles, ants, many species of spider, several species of birds such as chickadees, blue jays, nuthatches, towhees, and robins, and approximately 15 species of common woodland mammals such as the white-footed mouse, various shrews, chipmunks, squirrels, and raccoons. Small mammals are the largest predators in low density gypsy moth populations and are apparently critical in preventing outbreaks. Calosoma (ground beetles of European origin), cuckoos, and flocking birds such as starlings, grackles, and red-winged blackbirds, are attracted to infested areas in high gypsy moth population years.
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