Horticulture/Lymantria dispar

The gypsy moth, Lymantria dispar, is a moth in the family Lymantriidae of Eurasian origin. Originally ranging from Europe to Asia, it was introduced to North America in the late 1860s, where it has been expanding its range ever since.

Description
The hatching of gypsy moth eggs coincides with budding of most hardwood trees. Larvae emerge from egg masses from early spring through mid-May.

Gypsy moths are dispersed in two ways. Natural dispersal occurs when newly hatched larvae hanging from host trees on silken threads are carried by the wind for a distance of up to about 1 mile, although most go less than 50 meters. Eggs can be carried for longer distances. Artificial dispersal occurs when people transport gypsy moth eggs thousands of miles from infested areas on cars and recreational vehicles, firewood, household goods, and other personal possessions. Females are flightless in most varieties, so these are the only means of spread.

Symptoms and Signs
The effects of defoliation depend primarily on the amount of foliage that is removed, the condition of the tree at the time it is defoliated, the number of consecutive defoliations, available soil moisture, and the species of host. If less than 50 percent of their crown is defoliated, most hardwoods will experience only a slight reduction (or loss) in radial growth.

If more than 50 percent of their crown is defoliated, most hardwoods will refoliate or produce a second flush of foliage by midsummer. Healthy trees can usually withstand one or two consecutive defoliations of greater than 50 percent. Trees that have been weakened by previous defoliation or been subjected to other stresses such as droughts are frequently killed after a single defoliation of more than 50 percent.

Trees use energy reserves during refoliation and are eventually weakened. Weakened trees exhibit symptoms such as dying back 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.

Trees weakened by consecutive defoliations are also vulnerable to attack by disease organisms and other insects. For example, the Armillaria fungus attacks the roots, and the two-lined chestnut borer attacks the trunk and branches. Affected trees will eventually die 2 or 3 years after they are attacked.

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 single, complete defoliation can kill approximately 50 percent of the pines and 90 percent of the mature hemlocks. This is because conifers do not store energy in their roots; an exception is larch.

Development and reproduction
Larvae develop into adults by going through a series of progressive moults through which they increase in size. Instars are the stages between each molt. Male larvae normally go through five instars (females, go through six) before entering the pupal stage. 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.

During the first three instars, larvae remain in the top branches or crowns of host trees. The first stage or instar chews small holes in the leaves. The second and third instars feed from the outer edge of the leaf toward the center.

When population numbers are sparse, the movement of the larvae up and down the tree coincides with light intensity. Larvae in the fourth instar feed in the top branches or crown at night. When the sun comes up, larvae crawl down the trunk of the tree to rest during daylight hours. Larvae hide under flaps of bark, in crevices, or under branches - any place that provides protection. When larvae hide underneath leaf litter, mice, shrews, and Calosoma beetles can prey on them. At dusk, when the sun sets, larvae climb back up to the top branches of the host tree to feed. When population numbers are dense, however, larvae feed continuously day and night until the foliage of the host tree is stripped. Then they crawl in search of new sources of food.

The larvae reach maturity between mid-June and early July. They enter the pupal stage. This is the stage during which larvae change into adults or moths. Pupation lasts from 7 to 14 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 locations where larvae rested. Pupation will take place in sheltered and non-sheltered locations, even exposed on the trunks of trees or on foliage of nonhost trees. Sometimes the caterpillars create flimsy cocoons made of silk strands holding the leaf together, while others do not cover their pupae in cocoons, but rather hang from a twig or tree bark, like butterfly pupae do.

The brown male gypsy moth emerges first, flying in rapid zigzag patterns searching for females. The male gypsy moths are diurnal unlike most moths, which are nocturnal. When heavy, black-and-white egg-laden females emerge, they emit a chemical substance called a pheromone that attracts the males. The female lays her eggs in July and August close to the spot where she pupated. Then, both adult gypsy moths die. The European and most Russian forms of the gypsy moth have flightless females. Although they have large wings, the musculature is not developed. However, the Japanese gypsy moth females do fly and are attracted to lights. During outbreaks they have been known to fly to ships in port and lay their eggs on the ships.

The egg is the overwintering stage. After an acclimation stage, eggs can withstand freezing temperatures. The longer they are chilled in winter, the less heating is required for their hatch in spring. Gypsy moth egg masses are typically laid on branches and trunks of trees, but egg masses may be found in any sheltered location. Egg masses are buff colored when first laid but may bleach out over the winter months when exposed to direct sunlight and weathering. As the female lays them, she covers them with hair-like setae from her abdomen. Many individuals find these hairs irritating, and they may offer the eggs some protection. Egg masses contain from a couple of hundred to about 1200 eggs.

North American Introduction
The gypsy moth was introduced into the United States in 1868 by a French scientist, Leopold Trouvelot, living in Medford, Massachusetts, who enjoyed raising many types of caterpillars including silkworms. It is now one of the most notorious pests of hardwood trees in the Eastern United States. The first outbreak there occurred in 1889. By 1987, the gypsy moth had established itself throughout the Northeast USA and southern Quebec and Ontario. The insect has spread south into Virginia and West Virginia, and west into Michigan and Wisconsin. Infestations have also occurred sporadically in Utah, Oregon, Washington, California, and British Columbia.

Since 1980, the gypsy moth has defoliated over 1,000,000 acres (4,000 km²) of forest each year. In 1981, a record 12,900,000 acres (52,200 km²) were defoliated. This is an area larger than Rhode Island, Massachusetts, and Connecticut combined.

In wooded suburban areas, during periods of infestation when trees are visibly defoliated, gypsy moth larvae crawl up and down walls, across roads, over outdoor furniture, and even inside homes. During periods of feeding they leave behind a mixture of small pieces of leaves and frass, or excrement. During outbreaks, the sound of chewing and frass dropping is a continual annoyance.

Gypsy moth populations usually remain at very low levels but occasionally populations increase to very high levels which can result in partial to total defoliation of host trees for 1-3 years.

Host plants
Gypsy moth larvae prefer oaks, but may feed on several hundred different species of trees and shrubs, both hardwood and conifer. In the East the gypsy moth prefers oaks, aspen, apple, sweetgum, speckled alder, basswood, gray and paper birch, poplar, willow, and hawthorn, although other species are also affected. The list of hosts will undoubtedly expand as the insect spreads south and west. The gypsy moth avoids ash, tulip-tree, American sycamore, butternut, black walnut, catalpa, flowering dogwood, balsam fir, arborvitae, American holly, and shrubs such as mountain laurel and rhododendron, but will feed on these in late instars when densities are extremely high. Older larvae feed on several species of hardwood that younger larvae avoid, including cottonwood, hemlock, Atlantic white cypress, and the pines and spruces native to the East.


 * ../Abies/ (Fir)
 * ../Acer/ (Maple)
 * ../Alnus/ (Alder)
 * ../Betula/ (Birch)
 * ../Carya/ (Hickory)
 * ../Catalpa/
 * /Chamaecyparis/ (False cypress)
 * ../Cornus/ (Dogwood)
 * ../Crataegus/ (Hawthorn)
 * ../Fagus/ (Beech)
 * ../Fraxinus/ (Ash)
 * ../Juglans/ (Walnut)
 * /Liquidumbar/ (Sweetgum)
 * ../Liriodendron/ (Tuliptree)
 * ../Malus/ (Apple)
 * /Picea/ (Spruce)
 * ../Pinus/ (Pine)
 * ../Platinus/ (Sycamore)
 * ../Populus/ (Poplar)
 * ../Prunus/ (Stone Fruits)
 * ../Quercus/ (Oak)
 * ../Sassafras/
 * ../Salix/ (Willow)
 * /Thuja/ (Arborvitae)
 * ../Tilia/ (Linden, Basswood)
 * ../Tsuga/ (Hemlock)
 * ../Ulmus/ (Elm)

Further information

 * Andreadis TG, Weseloh RM. 1990. Discovery of Entomophaga maimaiga in North American gypsy moth, Lymantria dispar. Proceedings of the National Academy of Sciences of the United States of America 87 (7): 2461-2465.
 * Barbosa P, Greenblatt J. 1979. Suitability, digestibility and assimilation of various host plants of the gypsy moth Lymantria dispar L (Lepidoptera, Lymantriidae). Oecologia 43 (1): 111-119 1979
 * Barbosa P, Waldvogel M, Martinat P, et al. 1983. Developmental and reproductive performance of the gypsy moth, Lymantria dispar (L) (Lepidoptera, Lymantriidae), on selected hosts common to mid-atlantic and southern forests. Environmental Entomology 12 (6): 1858-1862.
 * Bogdanowicz SM, Wallner WE, Bell J, et al. 1993. Asian gypsy moths (Lepidoptera, Lymantriidae) in North America - evidence from molecular data. Annals of the Entomological Society of America 86 (6): 710-715.
 * Dwyer G, Elkinton JS. 1993. Using simple-models to predict virus epizootics in gypsy-moth populations. Journal Of Animal Ecology 62 (1): 1-11.
 * Elkinton JS, Healy WM, Buonaccorsi JP, Boettner GH, Hazzard AM, Smith HR, Liebhold AM. 1996. Interactions among gypsy moths, white-footed mice, and acorns. Ecology 77 (8): 2332-2342.
 * Gould JR, Elkinton JS, Wallner WE. 1990. Density-dependent suppression of experimentally created gypsy moth, Lymantria dispar (Lepidoptera, Lymantriidae), populations by natural enemies. Journal of Animal Ecology 59 (1): 213-233.
 * Liebhold AM, Halverson JA, Elmes GA. 1992. Gypsy moth invasion in North America - a quantitative analysis. Journal of Biogeography 19 (5): 513-520.
 * Myers, Judith H. 1993. Population Outbreaks in Forest Lepidoptera.  American Scientist 81, 240-251.
 * Rossiter MC. 1991. Maternal effects generate variation in life-history - consequences of egg weight plasticity in the gypsy moth. Functional Ecology 5 (3): 386-393.
 * Weseloh RM, Andreadis TG. 1992. Epizootiology of the fungus Entomophaga maimaiga, and its impact on gypsy moth populations. Journal of Invertebrate Pathology 59 (2): 133-141.