This is the first half of a two part series on gypsy moth. The following examines the insect’s distribution and range, its preferred hosts, and the four stages of its life cycle.
Lymantria dispar dispar, also known as gypsy moth, European gypsy moth, or North American gypsy moth, is a moth in the family Erebidae. Each year, Gypsy moth is responsible for defoliating millions of trees across Europe, Africa, Asia, and North America. Gypsy moth is of Eurasian origin. Carl Linnaeus, a Swedish botanist, physician, and zoologist, first described the species in 1758. Since its initial discovery, the species’ taxonomy has been disputed, with the insect being shifted between three families: Lymantriidae, Noctuidae, and Erebidae. Gypsy moth is classified as a pest. Its larvae consume the leaves of over five hundred species of trees and shrubs. It is considered one of the most destructive insects of hardwood trees in the eastern United States.
Distribution and Range
Gypsy moth is indigenous to Europe and Asia. It was introduced into the United States in 1869 by a French scientist named Etienne Leopold Trouvelot. Trouvelot intended on breeding gypsy moths with silk worms in an effort to establish a silkworm industry. In 1889, the moths were accidentally released from Trouvelot’s residence in Medford, Massachusetts. Subsequently, they flourished, and by 1987, the insect had become established throughout much of the Northeast. Gypsy moth has since spread south into Virginia and West Virginia, as well as west into Ohio, Michigan, Utah, Oregon, Washington, and California.
Gypsy moth larvae favor hardwoods. In the eastern United States, gypsy moth defoliates oaks, aspen, apple, sweetgum, speckled adler, basswood, gray birch, paper birch, white birch, poplar, willow, and hawthorn, among other species. Older larvae are attracted to several species of hardwood that younger larvae avoid, including hemlock, cottonwood, Atlantic white cypress, southern white cedar, pine, and spruce. Gypsy moth avoids ash trees, tulip-tree, American sycamore, butternut, black walnut, catalpa, flowering dogwood, balsam fir, cedar, American holly, mountain laurel, and rhododendrons.
Gypsy moth passes through four stages during its life cycle: an egg stage, a larval stage, a pupal stage, and an adult stage.
The Egg Stage
Adult females lay egg masses on the branches and trunks of trees. Females are flightless, so they often lay their eggs on a surface close to where they have pupated. Egg masses may also be found in other locations, such as on rocks, buildings, or vehicles. The egg masses are initially a buff yellow-brown color. During the winter months, the eggs may bleach out if exposed to direct sunlight. Egg masses are usually oval shaped. They grow one and a half inches long, and ¾ of an inch wide. Egg masses contain between one hundred and one thousand eggs. Eggs are covered in a coating of hairs, which protect them from predators and parasites. The hairs also help to seal in moisture, and insulate the eggs from cold temperatures. Once they have been laid, the eggs overwinter, a period that lasts for eight to nine months. The larvae contained within the eggs become fully developed within a month.
The Larval Stage
Eggs hatch from early to late spring, a period that coincides with the budding of most hardwoods. The larvae breach the chorion of the eggs, emerging onto nearby surfaces. Newly hatched larvae are black with long, hair-like setae. As the larvae mature, they develop five pairs of raised blue spots, and six pairs of raised red spots along their backs. This distinct coloring is one way to discern Gypsy moth from other caterpillars. Once the larvae have emerged, they begin developing into adults. During this period, the larvae rapidly consume foliage, increasing in size. Before they enter the pupal stage, male larvae go through five instars, while females go through six.
During the first three instars, larvae feed in the top branches of the crown. Feeding occurs primarily in the morning and late afternoon. Throughout the first instar, larvae feed on leaf hairs before moving on to the leaf epidermis. During the second and third instars, they begin feeding on the outer edge of the leaf, gradually advancing towards the center. When population numbers are sparse, the movement of the larvae is determined by the intensity of the sun. Larvae will navigate to sections of the tree that are partially or fully shaded.
Larvae in the fourth instar become nocturnal, feeding in the top branches of the crown at night. When the sun rises, larvae descend from the top of the crown, and make their way down the trunk of the tree to rest during the daylight hours. They conceal themselves in crevices, beneath flaps of bark, or under branches. If population numbers are dense, they may hide under leaf litter, where they become vulnerable to predators such as mice, shrews, and Calasoma beetles. Once the sun sets, larvae ascend to the top branches of the tree to feed. Dense populations will feed continuously until the foliage has been stripped from the tree. When this occurs, they migrate from the top branches, seeking out fresh sources of nourishment. Larvae reach maturity between mid-June and early July, upon which they enter the pupal stage.
Larvae are dispersed through natural or artificial means. Natural dispersal occurs when the newly hatched larvae construct silken threads that they cling to. They are then disseminated to various locations by the wind. Artificial dispersal takes place when eggs are transported from infested areas by animals or vehicles. They are also commonly distributed through the transportation of firewood.
The Pupal Stage
As larvae enter the pupal stage, they envelop themselves in a silken net. Larvae then proceed to pupate for seven to seventeen days. Once pupation is complete, moths emerge fully developed by splitting the pupal skin. When populations are sparse, pupation occurs in sheltered locations. If populations are dense, larvae may pupate in sheltered and non-sheltered locations.
The Adult Stage
Moths typically emerge in July. Male gypsy moths appear first, with females emerging a few days later. Males are brown, with feathery antennae, and forewings that are 20-24 mm long. Females are black and white. They have thin antennae, and possess forewings that measure between 31 and 35 mm long. Though their wings are fully formed, females are incapable of flight.
When egg-laden females emerge, they emit a pheremone from a small gland near the tip of their abdomen, a process referred to as calling. Males are drawn to the pheremone. They navigate through the air, flying in rapid zigzag patterns as they pursue the females. After mating, females lay around 500 eggs, depositing them close to where they pupated. They produce eggs throughout July and August. Males and females die shortly thereafter. Embryos develop into larvae four to six weeks later. Larvae overwinter, and hatch the following spring.