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 and has been expanding its range ever since.
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.
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.
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 spreading.
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 (and females, 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 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. After mating, 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.
Gypsy moths (at least of the introduced American population) fly all day and night, with the possible exception of the late morning. They are most active soon after dusk and in the latter hours of the night (Fullard & Napoleone 2001).
The gypsy moth was introduced into the United States in 1868 by a French scientist, Leopold Trouvelot, living in Medford, Massachusetts. The native silk spinning caterpillars were proving to be susceptible to disease. So Trouvelot brought over gypsy moth eggs to try to make a caterpillar hybrid, that could resist diseases. When some of the moths escaped from his lab, they found suitable habitat and started to multiply. Gypsy moth 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, Wisconsin and Minnesota. Small, isolated infestations have also occurred sporadically in Utah, Oregon, Washington, California and British Columbia, but these have all been successfully eradicated.
Since 1980, the gypsy moth has defoliated over 1 million acres (4,000 km²) of forest each year. In 1981, a record 12.9 million 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. This phenomenon sounds eerily like a light to moderate rain. Gypsy moth populations usually remain at very low levels, but occasionally increase to very high levels which can result in partial to total defoliation of host trees for 1–3 years.
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.
There is not any evidence that releasing or enhancing gypsy moth predators or parasites can reduce gypsy moth populations. Manual removal of gypsy moths may be a viable method for reducing damage on small, open-grown trees and shrubs.
During periods when numbers of gypsy moth larvae are dense, pesticides may be the most effective method of reducing the number of larvae and protecting the foliage of host trees. Application of pesticides should be done by a certified applicator, because special equipment is required. Large areas, such as wooded residential areas and forests, should be treated by aircraft.
Available pesticides fall into two broad groups: microbial or biological and chemical (table 1).
Microbial and biological pesticides contain living organisms that must be consumed by the pest. Microbials include bacteria, viruses, and other naturally occurring organisms; biologicals include manmade synthetics of naturally occurring organisms. These pesticides should be applied before the larvae reach the third stage or instar of development. As they mature, larvae become more resistant to microbial pesticides and are, therefore, more difficult to kill.
Low dose pheromone systems are being employed in some areas (Jersey, Channel Islands, UK) to flood areas with synthetic pheromone and effectively 'blind' males so they are unable to locate females.
Nucleopolyhedrosis virus (NPV), a naturally occurring organism, has been developed as a microbial pesticide. It is presently registered under the name "Gypchek" and is available for use in USDA Forest Service sponsored suppression programs. NPV and Gypcheck are specific to the gypsy moth.
Bacillus thuringiensis (Bt) is microbial and biological. It is the most commonly used pesticide. In addition to being used against the gypsy moth, Bt is used against a number of other pests, including the western spruce budworm and other Choristoneura, and tent caterpillar. When Bt is taken internally, the insect becomes paralyzed, stops feeding, and dies of starvation or disease.
Chemical pesticides are contact poisons in addition to being stomach poisons. The timing of the chemical application is less critical to the successful population reduction of the pest than the timing of the application of the microbials and biologicals. Chemical pesticides can affect non-target organisms and may be hazardous to human health.
Table 1 - Microbial and chemical pesticides commonly used for gypsy moth control
|Active ingredient||Representative trade names||Remarks|
|Bacillus thuringiensis||Foray||Registered for aerial and ground application. Available under a variety of trade names. Toxic to other moth and butterfly larvae. Can be used safely near water.|
|Acephate||Orthene||Registered for aerial and ground application. Available under a variety of trade names. Toxic to bees and some gypsy moth parasites. Commonly used from the ground to treat individual trees.|
|Carbaryl||Sevin||Registered for aerial and ground application. Available under a variety of trade names. Toxic to bees and gypsy moth parasites. At one time, the most widely used chemical in gypsy moth control programs.|
|Diflubenzuron||Dimilin||A restricted-use pesticide that can be applied only by certified applicators.|
The most commonly used chemical pesticides currently registered by the U.S. Environmental Protection Agency (EPA) for use against the gypsy moth contain carbaryl, diflubenzuron, or acephate. Malathion, methoxychlor, phosmet, trichlorfon, and synthetic pyrethroids (permethrin) have also been registered by EPA for control of gypsy moth, but are used infrequently.
Several studies done by Peter G. Kevan and associates of the University of Guelph, between 1975 and 1995 in eastern Canada have shown serious reduction in blueberry and other crop pollination due to forest aerial applications of insecticides that killed non-target wild bees.
Diflubenzuron represents a new class of pesticides called insect growth regulators. It kills gypsy moth larvae by interfering with the normal molting process. Diflubenzuron has no effect on adult insects. Aquatic crustaceans and other immature insects that go through a series of molting stages are often sensitive to this pesticide.
Another very effective way to remove these pests is by fire. In most southern states, people use gasoline and fire to kill the pupae stage of the creature.
Stands of trees that are predominantly oak and grow on poor, dry sites (such as sand flats or rock ridges) are frequently stressed and often incur repeated, severe defoliations. Trees growing under these conditions frequently possess an abundance of structural features such as holes, wounds, and deep bark fissures that provide shelter and habitats for gypsy moth larvae and aid their survival.
Stands of trees that are predominantly oak but grow on protected slopes or on sites with adequate moisture and organic matter are more resistant to defoliation by the gypsy moth.
Slow-growing trees on poor sites frequently survive a single, severe defoliation better than fast-growing trees typically found on well-stocked better sites.
More trees are killed in stands that contain mainly oak species than in oak-pine or mixed hardwood stands. Subdominant trees are killed more rapidly and more often than dominant trees.
Predefollation treatments: When gypsy moth defoliation is anticipated, but not within the next 5 years, predefoliation thinning to selectively remove preferred-host trees can reduce the severity of defoliation, increase the vigor of residual trees, and encourage seed production and stump sprouting. Thinnings should not be conducted in fully stocked stands that will reach maturity within the next 6 to 15 years. Thinning results in a short-term "shock effect" to residual trees. This shock effect, coupled with defoliation-caused stress, renders trees vulnerable to attack by disease organisms such as Armillaria.
In fully stocked stands that will reach maturity within the next 16 or more years, two kinds of thinning can be applied. The method of thinning should depend on the proportion of preferred host species present.
If more than 50 percent of the basal area in a stand is preferred host species (mainly oaks), presalvage thinning should be applied. Presalvage thinning is designed to remove the trees most likely to die (trees with poor crown condition) from stress caused by gypsy moth defoliation.
If less than 50 percent of the basal area in a stand is in preferred host species, sanitation thinning can be applied to reduce further the number of preferred host trees. This will result in fewer refuges for gypsy moth larvae and in improved habitats for the natural enemies of the gypsy moth.
Treatment during outbreaks: If defoliation is current or is expected within the next 5 years, thinnings should be delayed because of potential "shock effect." High-value stands can be protected by applying pesticides. In low-value stands or those that are at low risk (less than 50 percent basal area in preferred host species), protective treatments are optional.
Post-outbreak treatments: After a defoliation episode, the land manager or woodlot owner should pursue efficient salvage of dead trees, but should delay decisions about additional salvage, regeneration, or other treatments for up to 3 years. At the end of 3 years, most defoliation-caused mortality will be complete and the need for treatments can be assessed on the basis of damage level, current stocking conditions, and stand maturity.