The Moth that Eats a Million Acres

Contact: Bob Tichenor, USDA APHIS |

The Moth that Eats a Million Acres
Karla Salp, Washington State Department of Agriculture,
The Moth that Eats a Million Acres
Gypsy moth caterpillars defoliating an oak tree. Photo: Bill McNee, Wisconsin Dept of Natural Resources,

ANNAPOLIS, MD (June 2, 2008) – The gypsy moth story begins in the late 1860’s. There is a shortage of cotton in the aftermath of the Civil War, and a persistent disease is plaguing the silk worm industry. Arriving on the scene is Leopold Trouvelot, an amateur naturalist from France, with an enterprising scheme involving breeding silk moths with a robust species of moth from his home country that eats most common trees and many other plants. Trouvelot brought gypsy moth eggs to his home in Medford Massachusetts. In 1869, some of his caterpillars took advantage of an open window, walked out to the yard and into entomological history.

Soon after this, the gypsy moth began to show itself as the kind of invader it would be in its new environment. Records mention that by 1880 some 400 square miles around Medford were already infested. In another 10 years, 360 square miles of trees were defoliated, and marauding caterpillars had become a notable public nuisance. Virtually every year since, this scene has repeated itself somewhere in the expanding arena of infestation. In Maryland, annual defoliation totals have averaged over 40,000 acres in the years since the first defoliation was recorded in 1980, reaching a total of 1.13 million. Over 89 million acres of defoliation have been recorded nationally since 1924, when the first records were begun. Only five times in that 85-year span has the defoliation total been less than 10,000 acres; in 23 of those years it was greater than 1 million.

Beginning over 100 years ago and continuing to this day, gypsy moth has received a great deal of attention from the public and has been the subject of considerable effort to study and control it. The Maryland Invasive Species Council has selected the gypsy moth as Invader of the Month for June 2008.

Gypsy moth, Lymantria dispar, has 5-6 caterpillar size stages but usually only the last two or three stages, or instars, are noticed. They have dark grey segmented bodies with several sparse tufts of stiff hairs and a pattern of paired dots on the top side of each segment. The first five segments have pairs of blue dots, the next six, pairs of red ones. This unique pattern allows the late stage caterpillars, which may be 1” to 3.5” long, to be positively identified, differentiated from any of the thousands of other American caterpillars. The large caterpillars, which do the vast majority of feeding, appear in June. They are followed by the brown teardrop shaped pupal stage, located in “resting areas” and secured by sparse strands of silk.

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Photo: Karla Salp, Washington State Department of Agriculture,
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Photo: Bill McNee, Wisconsin Dept of Natural Resources,

The adult moth emerges about a week later. Males are light brown day-flying moths with feathery antennae. In contrast, the female moth has white wings with small black markings and a light brown body and is much larger. The female moth does not fly and remains near the resting area where she emerged from the pupal stage. Mating typically takes place within 2 days and the eggs are laid the same day, after which the female dies. Males live only about a week. Neither gender has functional mouth parts. Eggs are laid in a rounded oval 1”x2” mass of 1000 or more eggs. The female covers the mass with the light brown-tan hairs and scales from her body. The eggs are laid in the characteristic resting sites – the undersides of tree limbs, rocks, and objects on the ground, behind bark flaps or signs, inside crevices, and similar places on homes or any man-made object. In Maryland, the egg stage lasts from July until April or early May when they hatch.

The gypsy moth possesses a unique combination of five traits that result in significant social and economic impacts in its new environment. First, it is a relatively large insect (with a correspondingly large appetite). Second, the female is flightless, resulting in poor dispersal. Third, reproductive rate is high. These three work together so that large numbers of hungry caterpillars build up rapidly in an area, with defoliation of their favorite trees, the oaks, the inevitable result. A fourth character, the ability to consume some 400 kinds of plants, enables the caterpillars to continue feeding on other trees after the oaks are all denuded. In times of severe defoliation, entire hillsides or mountains are virtually stripped bare. No native caterpillar species so completely strips such large areas.

However it is a fifth characteristic that is the killer – literally. Gypsy moths take about eight weeks to grow from the time they hatch in mid-April. The last instar eats more foliage than all the others combined, resulting in the most leaf damage in early- to mid-June. By that time of the season, tree leaves are fully developed, unlike younger leaves eaten by other spring feeding caterpillars, which are easily replaced. In destroying mature leaves, gypsy moths consume significant amounts of a tree’s nutrients. June is early enough in the season, however, that trees have reaped very little return from their photosynthetic processes. The loss of the majority of their leaves stimulates many trees to refoliate, further depleting their stored nutrients. One or two such defoliation-refoliation cycles leave the trees severely weakened, if in fact they are not killed. Weak trees are more susceptible to diseases and other insects (especially, for oaks, two-lined chestnut borers).These trees often die 1-3 years after the last defoliation.

On the more hopeful side, a naturally occurring viral disease will normally decimate gypsy moth populations one to three years into an outbreak, and the gypsy moth seems to disappear for a few years. In 1989, a new fungal disease of gypsy moths showed up in the US. This fungus, called Entomophaga maimaiga, probably arrived from Japan, which is near the ancestral home of the gypsy moth species. This disease will effectively depress gypsy moth populations in most years. This means that the average amount of damage is reduced, although outbreaks do still occur, like the current one that has plagued Maryland and other mid-Atlantic states since 2005. In 2007, Maryland, New Jersey and Pennsylvania suffered the worst defoliation in many years. Consequently each of these states has mounted large spray programs to protect weakened and threatened trees – more than has been needed in 17 years in Maryland. These programs will conclude by June 1, before the period of heaviest caterpillar feeding, with the desired result — much reduced forest damage in 2008.

For more information about other Invasive Species of Concern, visit or call the Maryland Department of Agriculture at 410-841-5920.

For more information on the Internet:

Gypsy Moth Program. Maryland Department of Agriculture

Gypsy Moth Digest. USDA Forest Service

Gypsy Moth Pest Alert. USDA Forest Service