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Nonnative Insects of Hardwoods

Gypsy moth

The gypsy moth, Lymantria dispar, is native to Europe and Asia. In 1869, Leopold Trouvelot introduced the European strain of the gypsy moth into the United States. Since then, it has spread across the landscape of the Eastern United States, defoliating vast acreages of forest. The insect spread into northeastern Virginia in the early 1980s. By the middle 1990s, it had reached the eastern seaboard of North Carolina, and had infested much of Virginia. At the insect’s current rate of spread, specialists predict that a significant portion of the Southeast will be infested in the next 30 years.

The gypsy moth causes its damage by feeding on and defoliating forest and shade trees during the caterpillar stage (Doane and McManus 1981, U.S. Department of Agriculture Forest Service and APHIS 1995). Caterpillars feed on a wide range of trees and shrubs (Liebhold and others 1995, Zhu 1994) but prefer oaks.

Natural enemies, including small mammals and parasitic insects, often keep gypsy moth populations low (Elkinton and Liebhold 1990). Occasionally, however, populations increase above the capacity of these natural enemies to control. Then an outbreak occurs that can last for several years. Outbreaks culminate when populations collapse, either as the result of disease or starvation. The most important disease agents are the gypsy moth nucleopolyhedrosis virus and the gypsy moth fungus, Entomophaga maimaiga (Andreadis and Weseloh 1990, Hajek and others 1990).

Management of gypsy moth utilizes three strategies: eradication, suppression, and slowing the spread (Gottschalk 1993, U.S. Department of Agriculture Forest Service and APHIS 1995). Eradication concentrates on the elimination of gypsy moth populations outside the quarantined area. Suppression concentrates on managing gypsy moth populations in the quarantine area to limit defoliation. Slowing the spread concentrates on limiting population spread along the leading edge of the quarantine area.

The gypsy moth is spreading into the South along a wide arc from the eastern shore of Virginia and North Carolina to the Appalachian Mountains in western Virginia. At this time, the impact of gypsy moth defoliation in the South is limited to Virginia and the northeastern shore of North Carolina (fig. 17.16).

The impact of repeated gypsy moth defoliation on the health of oak forests is significant (Campbell and Sloan 1977). Repeated severe defoliation of oaks weakens trees to such an extent that they may be attacked and killed by secondary pest organisms, such as the two-lined chestnut borer and Armillaria root rot (caused by Armillaria mellea). Extended drought intensifies the rate of death.

Species are attacked preferentially without respect to forest type. Highly favored species include northern red oak, basswood, and sweetgum. Species of limited suitability include maples, ash, beech, pine, and cherry. Species that are not favored or are avoided include yellow-poplar, blackgum, black locust, cypress, magnolia, and tupelo.

Increased intensity of management of forest stands may improve forest health, reduce susceptibility to defoliation by gypsy moth once stands are colonized, or remove individual trees and species that are vulnerable to damage. Overmature stands of red oaks, particularly scarlet and black oak, are highly vulnerable to loss after defoliation. Young, vigorously growing stands are thought to be less vulnerable to damage from gypsy moths. Alternatively, actively managed stands may be vulnerable to damage if they are defoliated soon after thinning. However, most silvicultural recommendations have not been experimentally verified at this time.

In a general sense, ownership does not influence impact. However, management objectives may limit treatment options for reducing outbreak populations of gypsy moth or they may limit opportunities to manage stand and species composition to favor nonpreferred species of trees.

Damaging populations of gypsy moths are managed by applying chemical or biological insecticides from the air and on the ground. Unfortunately, some treatments may adversely impact a nontarget species of crustaceans and insects, particularly rare species of moths and butterflies. Biological insecticides, including Bacillus thuringiensis var. kurstaki, a naturally occurring soil-borne bacterium, and Gypchek, a nucleopolyhedrosis virus, are believed to have fewer negative environmental effects than other available treatments.

Very low-density populations of gypsy moths, particularly isolated populations, may be eliminated using a formulation of the sex pheromone of the female moth, or by mass trapping using the pheromone for bait.

Insecticides are most often applied to residential areas where the caterpillar is considered to be a serious pest (U.S. Department of Agriculture Forest Service and APHIS 1995). Treatment of uninhabited forests is generally only done to slow the spread of gypsy moths. Impact of this pest on the South’s forests will increase as it continues to spread.

Outbreaks and their damage will be most conspicuous in the upland hardwood type, where oaks reach their greatest abundance. Bottomland hardwood and oak/pine forests will also sustain serious outbreaks.

How far south it will spread and how effective natural controls will prove to be are unknown.