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The Health of Southern Forests, USDA Forest Service, Forest Health Protection, Southern Region

Stressors of Hardwood Forests


Gypsy Moth

The gypsy moth was first introduced from Europe into the United States in Massachusetts in 1869. Since then, it has steadily expanded its range north, west and south. Since the adult female does not fly, dispersal is accomplished via wind dissemination of the small caterpillars. Before 1965, the pest expanded its range by 3 to 9 km (2 to 6 miles) per year. Since then, it has averaged 21 km (13 miles) per year.

Human beings have undoubtedly increased the rate of spread of the gypsy moth. An example is the Blue Ridge Parkway, an area where traps have produced an inordinately high number of catches. In this case, newly hatched larvae have landed on automobiles and trailers and have been transported southwest by motorists.

The gypsy moth entered the Southern Region in the early 1980's. In Virginia, 452,745 acres were defoliated in 1994. The moth has moved steadily along the eastern seaboard into northeastern North Carolina and southwest along the Appalachian mountain chain in Virginia.

The gypsy moth produces one generation per year. Larvae begin emerging from egg masses in April or early May, usually completing their development by late June. They seek sheltered areas in which they pupate for about two weeks. Figure 7 shows a late-stage larva.

Figure 7
Figure 7. A late-stage larva of the gypsy moth.

Adult male moths are dark brown with many dark bands across the forewings. The nearly white females are flightless because they are so heavily laden with eggs. They crawl short distances from the place where they emerge to release a potent pheromone to attract males. After mating, the female deposits her eggs in a single mass that contains from 75 to 1,000 eggs and then dies. Larvae hatch the following spring.

Gypsy moths feed on a wide variety of trees, shrubs, and vines. Preferred hosts include all oak species, sweetgum, apple, beech, birch, basswood, and willow. Hosts intermediately favored include maple, hickory, black cherry, elm, sassafras, all pines, eastern hemlock, and black gum. Least preferred are ash, yellow-poplar, redcedar, sycamore, dogwood, American holly, and black locust.

Defoliation by the gypsy moth can result in growth loss, crown dieback, and tree mortality. Trees that have been defoliated begin the next year with much lowered food reserves and are far more vulnerable to other insect and disease pests. Tolerance for defoliation varies greatly, with dominant trees on good sites able to tolerate defoliation. Weaker trees often succumb the second year.

Survey information on the gypsy moth is compiled in two very different ways. The first is through trap catches, and the second is by defoliation mapping.

In the first method, male moths are lured to and trapped in boxes baited with synthetic sex pheromone. Pheromone trap grids can yield valuable information on isolated infestations beyond generally infested areas. In 1994, such a system revealed heretofore unknown infestations in Arkansas, Tennessee, Georgia, and North Carolina.

The fungal pathogen Entomophaga maimaiga caused widespread collapse of gypsy moth populations over generally infested portions of Virginia from 1996 through 1999. But in 2000, aerial surveys detected over 70,000 acres of defoliation by gypsy moth in Virginia. Due in part to drought, the effects of the fungal outbreak had subsided, and gypsy moth populations rebounded throughout its eastern range. In 2001, aerial surveys detected over 440,000 acres of defoliation by gypsy moth in Virginia. Infestations were highly variable due in part to continued drought and the gypsy moth nucleopolyhedrosis virus. Gypsy moth populations noticeably increased in the central and eastern portions of the State in 2001. In 2002, total defoliation by gypsy moth caterpillars declined substantially to just over 55,000 acres. The decrease resulted from a combination of adverse weather during egg hatch, larval disease and an effective cooperative suppression program.

More specific information on the fluctuation of gypsy moth populations in the South can be found in archived Southern Region Forest Insect and Disease Conditions Reports: http://www.fs.fed.us/r8/foresthealth/pubs.html#conditions.

Scientists estimate that the entire area of the gypsy moth host type in the East, through eastern Texas and northern Florida, will be infested by the year 2160. Estimated spread of the gypsy moth through 2025 is shown here: http://www.ento.vt.edu/STS/maps/action99q.gif