Lymantria dispar dispar, commonly known as the gypsy moth, European gypsy moth, LDD moth, or spongy moth in North America, is a moth species belonging to the family Erebidae. The larvae of this moth are polyphagous, meaning they consume the leaves of over 500 species of trees, shrubs, and plants. This broad diet contributes to its classification as a pest in its invasive range, particularly as one of the most destructive pests of hardwood trees in the Eastern United States.
Taxonomy and Nomenclature
Carl Linnaeus first described the species as Phalaena [Bombyx] dispar in 1758. Over the years, the classification has changed, resulting in taxonomic confusion. The family has shifted between Lymantriidae, Noctuidae, and Erebidae. L. d. dispar is the nominate subspecies of Lymantria dispar. The species L. dispar has been divided into several subspecies, including L. d. asiatica and L. d. japonica. The current family classification is Erebidae, subfamily Lymantriinae. The name Lymantria dispar is derived from Latin.
Confusion exists over the species and subspecies. For regulatory purposes, the U. S. Department of Agriculture defines Asian Gypsy Moth as "any biotype of Lymantria dispar (sensu lato) possessing female flight capability," even though Lymantria dispar asiatica is not the only subspecies capable of flight; even females of L. d. dispar can fly, albeit weakly. Traditionally, all L. dispar populations were considered a single species.
Origin and Invasive Spread
L. d. dispar is indigenous to Europe. It was widespread in southeastern England in the 19th century, but became extinct except for occasional migrants soon after 1900. Lymantria dispar dispar was first introduced to North America in 1869 and quickly became an invasive species. Étienne Léopold Trouvelot imported the moths to interbreed them with silk moths to establish a new silkworm industry in the West. The moths were accidentally released from his residence in Medford, Massachusetts.
Life Cycle
Egg Stage
Egg masses are typically laid on branches and trunks of trees, but can be found in any sheltered location, including rocks, foliage, and vehicles. Females are flightless, so they lay eggs on a surface near their pupal emergence site. The eggs are covered in a coating of hairs and contain a fully developed larva after about one month, which then enters diapause to overwinter. The egg remains in this overwintering stage for eight or nine months. Development ceases in preparation for winter, and the larva reduces its water content to withstand freezing temperatures. In the spring, the larva resumes activity and reabsorbs water. Egg clusters are usually an oval, about 19 millimetres (3⁄4 in) wide and 38 millimetres (1+1⁄2 in) long, with a buff yellow-brown color, likened to a manila folder, which may bleach out over the winter. As the female lays the eggs, she covers them with hair-like setae from her abdomen. Egg clusters contain from 100 to 1000 eggs.
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Larval Stage
Larvae (caterpillars) emerge from egg masses in the spring. Most larvae hatch within a week, but can take as long as a month. The new larvae remain on or near the egg cluster in rainy weather or if the temperature is below 7 °C (45 °F). The larva will first feed on the leaf hairs and then move onto the leaf epidermis. Feeding occurs in the daytime, primarily in the morning and late afternoon, becoming a nocturnal activity as the larva grows. To grow, the larva must molt. Larvae are characterized by the term instar, which refers to the number of times a larva has molted. Males typically have five instars, and females have six. Newly hatched larvae are black with long, hair-like setae. Older larvae have five pairs of raised blue spots and six pairs of raised brick-red spots along their backs, and a sprinkling of setae. The larvae reach maturity between mid-June and early July, then enter the pupal stage, during which larvae change into adult moths.
Pupal Stage
Pupation lasts from 14-17 days. When the population is spread out and running low, pupation can take place under flaps of bark, in crevices, under branches, on the ground, and in other places where larvae rested. During periods when population numbers are dense, pupation is not restricted to these locations, but can take place in sheltered and open locations, even exposed on the trunks of trees or on foliage of nonhost trees.
Adult Stage
The males have feathery antennae versus the thin antennae of females. Size differences are also noted, with the forewing of the male moth being 20-24 millimetres (13⁄16-15⁄16 in) long, and that of the female 31-35 millimetres (1+1⁄4-1+3⁄8 in). The females are bigger than the males. Another important difference between the sexes is that females possess fully formed wings but do not fly. Moths usually emerge from pupae in July, but it can vary with population density and climate. The brown male gypsy moth emerges first, usually one or two days before the females do. The males fly in rapid zigzag patterns, but are capable of direct flight. Like most moths, the males are typically nocturnal, but can sometimes be seen flying during the day as well. The males fly up and down tree trunks, or other vertical objects in search of females. When heavy, black-and-white egg-laden females emerge, they emit a pheromone that attracts the males. The female has a small gland near the tip of the abdomen that releases the pheromone with a pumping motion, termed "calling". It can attract males from long distances, tracking the scent through its erratic flight pattern. The sex pheromone of female gypsy moths is (7R,8S)-7,8-epoxy-2-methyloctadecane. Courtship is not elaborate: the female must raise her wing to allow the male to couple with her. The moths remain in copula for up to an hour, but the transfer of the spermatophore is usually accomplished within 10 minutes. A male moth can inseminate more than one female. Multiple mating in females is possible but uncommon, as the female stops releasing the attracting pheromone after mating. Adult moths live about one week. They do not possess an active digestive system and cannot feed, but they can drink in moisture. The reproductive chance for females lasts about two days, with the pheromone for attracting males being diminished by the third day. Females lay their eggs on trees, shrubs, rocks, vehicles, and plants of many types. Each of them typically lays about 500 eggs.
Larval Diet and Host Plants
The larvae of Lymantria dispar are polyphagous, feeding on a wide range of host plants. The gypsy moth brings one of the largest impacts in defoliation of deciduous trees in the Northern Hemisphere. 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 leaves of oaks, aspen, apple, sweetgum, speckled alder, basswood, gray birch, paper birch, poplar, willow, and hawthorns, among other species. 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.
Natural Enemies and Control
Many species have been identified as preying on L. dispar. Some species, such as the white-footed mouse or Anastatus disparis, have a significant impact on the population dynamics of the moth. The white-footed mouse (Peromyscus leucopus), is considered important for regulating sparse moth populations. Rodents consume larvae and pupae that seek resting sites near or on the ground. Insectivorous birds prey upon the larva of the gypsy moth, but the egg clusters are protected by their hair coverings. The effects of bird predation have not been fully studied in North America, but it has been well documented in Japan and Eurasia. When outbreaks of gypsy moths occur, bird predation has no significant effect on the population. Birds that consume gypsy moth larvae, pupae and adults include the blue jay, red-eyed vireo, eastern towhee, northern oriole, catbird and the European robin. Many bird species feed on gypsy moth larvae, but they are not a major food source for any common bird species. Calosoma sycophanta is a beetle that preys upon the gypsy moth larvae and pupae. Four species of parasitic flies prey on gypsy moth larva. Parasetigana silvestris and Exorista larvarum lay an egg on the gypsy moth larva. If that egg hatches before the gypsy moth larva molts, the fly larva will penetrate the host. Compsilura concinnata pierces the gypsy moth larva and deposits its own larva inside. Blepharipa pratensis lays its eggs on leaves. Eight species of parasitic wasps attack the gypsy moth. Ooencyrtus kuvanae and Anastatus disparis attack the eggs. O. kuvanae attacks the eggs, but the effectiveness is limited by the ovipositor which can only penetrate the surface layer of the egg cluster. A. disparis has limited success as a predator because it can only attack unembroyonated eggs and the female wasps do not have wings. Even so, A. disparis is the only species that is known to occasionally affect the population dynamics of the gypsy moth. Apanteles melanoscelus and Phobocampe disparis parasitize the early larva stages. Glyptapanteles portheriae and G. liparidis are both wasps that lay eggs on the gypsy moth larva. Study of G. liparidis showed almost 90% success when the host is parasitized during premolt to the third instar. The population in Austria suffered high parasitism by the tachinids Parasetigena silvestris, which may have contributed in prevention of further increase in L.
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Viral and Bacterial Pathogens
Killed by a virus. Gypsy moth populations in different locations show vulnerability to different viral species. Nuclear polyhedrosis virus caused significant mortality in some cases. The most important pathogen is the Lymantria dispar multicapsid nuclear polyhedrosis virus (LdMNPV), sometimes referred to as NPV or Borralinivirus reprimens. Viral particles consumed by the larva when eating through the egg chorion will kill them during the first instar. The bodies disintegrate, spreading the virus on the foliage, which will then be consumed by other larvae. Streptococcus faecalis is another pathogen that is notable for its mortality.
Environmental Factors
Population density of larvae plays a major role in their behavior. Temperature is important to the gypsy moth. Low temperatures are fatal. Temperatures of −9 °C (16 °F) can be withstood during the winter, an extended period will kill the larvae inside and −23 °C (−9 °F) even for a short time is lethal. Egg deposits that are low or on the ground can be insulated against temperatures, including snow, and survive the lethal temperatures. Temperatures above 32 °C (90 °F) increase growth and development. Rainfall can drown larvae before they are established; low populations are correlated to heavy rainfall during the larval stage. Wind is also critical to the dispersal of the larvae.
Control Methods
Disparlure has been synthesized artificially and has been used to confuse mating patterns or lead male moths into traps.
Ecological and Economic Impact
L. d. dispar causes widespread defoliation and costs the economy millions of dollars in damages. Forest defoliation by the gypsy moths each year affects the populations and reproductive success of forest-dwelling birds. Nests placed in defoliated sites suffered a higher predation rate than those in non-defoliated sites. The caterpillar has been reported to produce a poison ivy-like rash when some people come into contact with the hairs of the larvae (caterpillar) stage. The contact can be direct or even if the small hairs are carried by the wind and onto the skin or clothing of a person.
Management Strategies
Remove egg masses off of trees or structures before April, when egg hatching begins. Scrape them into bags for disposal, or into a jar filled with soapy water. Wear gloves and be careful not to inhale the hairs: some people are allergic to the hairs on the egg masses. Use sticky barrier bands to trap young caterpillars before they enter treetops. Use hiding bands to provide a refuge for migrating caterpillars that can then be removed and destroyed. Insecticide using the active ingredient Bt (Bacillus thuringiensis, strain aizawai or kurstaki) products will control young caterpillars early in the spongy moth life cycle.
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