Issue 9

Gypsy Moths: Balloonists and Hitchhikers

Gypsy moth (Lymantria dispar L.) is a nonnative insect introduced in 1869 into the Boston area, where some of the larvae escaped and spread into New England. The moth has since spread west and south to Ohio, Michigan, and Wisconsin, into the Mid-Atlantic States and south into Virginia and North Carolina.

Female gypsy moths are flightless, so the insects spread slowly. Newly hatched gypsy moth larvae crawl to the tops of trees, where they spin strands of silk they can hang from until the wind "balloons" them to other trees. If this were the only way gypsy moths spread, advancing infestation would be glacial; unfortunately, the insects are also great hitchhikers. Females often lay their eggs on portable objects- vehicles, plant stock, campers, or lawn furniture-which carry them to new locations, sometimes hundreds of miles away.

Gypsy moth caterpillars feed on the leaves of many woody plants; a large buildup of the insects can defoliate whole stands, especially hardwood stands with many of the species the moth prefers. White oaks, followed by other oaks, are generally the species most preferred. Older stands suffer more damage; there is a strong relationship between age, drought, oak decline, and gypsy moth defoliation.

One defoliation will not usually kill a tree, but even healthy trees may become stressed and die if defoliation is repeated for several years. Gypsy moth outbreaks come in cycles which can last 1 to 5 years in oak-dominated stands. After that, gypsy moth populations decline due to buildup of disease, natural enemies, and starvation; populations can remain low for 4 to 12 years before building up again.

The Cumberland Plateau and the Southern Appalachians are considered favorable for supporting large gypsy moth populations; the forests contain many of the species the insect favors. Damage from defoliation could be significant; a majority of the oaks grow on ridgetops and steep southand west-facing slopes with the infertile soils and low moisture that further stress trees, making them more susceptible to damage from defoliation.-ZH

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