FEATURE ARTICLE

The History of Gypsy Moth Control in the United States:
A Reflection of Changing Attitudes and Technology

The economic and nuisance factors associated with the presence of the gypsy moth (Lymantria dispar) have led to numerous and varied attempts to control this pest or forest and landscape trees. The history of gypsy moth control efforts reflects changes in pest management technology and public attitudes.

For 20 years after its introduction, the gypsy moth was considered little more than a curiosity. Not until the population reached outbreak levels for the first time in 1889 did Massachusetts initiate a program for gypsy moth control. Massachusetts established a commission to oversee gypsy moth control. After consultation with a number of prominent scientists, particularly C. V. Riley, head of the USDA's Division of Entomology, the commission chose to attempt total extermination of the gypsy moth. Such strong action was reasonable, as the pest was still confined to a limited area and the costs of long-term treatment would certainly be greater than the costs of eradication. Riley recommended poisoning the caterpillars and using imported natural enemies to eliminate any survivors of the sprays; he thought this approach would eradicate gypsy moth in one year at a cost of approximately $100,000.

This program was not successful, because repeated application of high doses of paris green insecticide damaged foliage, and the machines used for application, designed for use in orchards, could not maneuver in the rougher forest areas. The commission was forced to adopt a program which utilized egg mass removal in the winter and sprays, traps, and sticky bands on tree trunks in the summer. This program required more time and funding than initially estimated, but the commission still felt that eradication was possible. By the mid-1890's the state legislature had begun to cut funding for the project, which they felt was only benefiting a small portion of the state, at the expense of all taxpayers. In 1900, the legislature eliminated all funding for the gypsy moth project, as the population size had been reduced and gypsy moth was only considered a minor threat. Similarly, the federal government refused to assist Massachusetts in its campaign against the gypsy moth, because it was considered a local problem.

The involvement of the federal government in the gypsy moth campaign began with the second eruption of the gypsy moth population in 1905. By this time, the gypsy moth had spread into several states, so attempts at eradication were no longer reasonable. The government chose to pursue a biological control program based on the importation of natural enemies from Europe. L. O. Howard, Riley's successor, believed that natural enemies would be able to control gypsy moth in 2-3 years. However, this belief was based less on a knowledge of gypsy moth ecology than on the spectacular success of the biological control of cottony-cushion scale in California by the Vedalia beetle. The promise of a quick and inexpensive means of control was received enthusiastically by the public.

This biological control campaign did not produce the results that were predicted. While a lot of valuable scientific information was gathered during this exploration, by 1911 it became obvious that biological control was not going to be a quick and inexpensive solution to the gypsy moth problem. With this revelation, public enthusiasm for the program disappeared and funding was again cut. The program ultimately was successful, as natural enemies were having a significant effect on the gypsy moth population by the 1920's.

Loss of support for biological control led to a return to pesticide use in an attempt to reduce damage in high value areas. When the gypsy moth reached New York in 1922, a barrier zone was established along the Hudson River until a new control method could be found. Any infestations found outside this zone were eradicated.

During the 1920's another natural control method was proposed. A variety of individuals and commissions recommended the use of silvicultural control against the gypsy moth in New England. This method of replacing preferred food plants, such as oak, with less preferred plants, like maple, was rejected because of its relatively high cost.

In the early 1940's, scientists began testing a new insecticide, DDT, against the gypsy moth. They quickly discovered that DDT was very effective. By 1946 speculation that the gypsy moth could be eradicated with DDT began. The federal government planned to treat the outer population in Michigan, Pennsylvania, and New York first and then, if this first step toward eradication was successful, treat the population in New England. Despite concern about the pesticide's effects on the environment, the government proceeded with its plan and treated 3 million acres with DDT in 1957. This program met with violent opposition from both the general populace and scientists. The federal government was again forced to abandon its plan for gypsy moth eradication.

With increasing demands for the reduction of pesticide use since the early 1960's, a wide variety of biological controls have been employed against the gypsy moth. These controls range from imported natural enemies to aerial applications of the gypsy moth nuclearpolyhedrosis virus (NPV).

The most common control used against the gypsy moth is the microbial pesticide Bacillus thuringiensis (Bt). Bt has been used for both gypsy moth control and eradication of isolated populations. Bt does not affect humans and is relatively host specific. Furthermore, Bt does not appear to impair parasitoid performance when used in conjunction with most parasitoids. Bt has even been shown to improve percent parasitism when used with the larval parasitoid Cotesia melanoscela.

Research toward gypsy moth control continues as the gypsy moth spreads. New control methods, such as the fungal pathogen Entomophaga maimaiga, continue to be identified and our understanding of commonly used predators and parasitoids grows. As the public demands more environmentally-friendly pest control, biological control methods may be our best hope against the gypsy moth in the future.

Biological Controls Used Against the Gypsy Moth

Gypsy Moth Basics

Gypsy moth is the most important defoliator of deciduous trees in North America and can also affect conifers. This pest was first introduced from Europe in Massachusetts in 1869, when businessman Leopold Trouvelot hoped to breed the gypsy moth with the silkworm to produce a hardier silkworm. Larvae escaped from Trouvelot and the gypsy moth began its spread. Gypsy moth's spread across the United States has been slow because female gypsy moths are unable to fly. Gypsy moth is mainly dispersed by larvae crawling, 'ballooning' larvae being carried by the wind, or a variety of life stages being accidentally transported by humans.

Gypsy moth goes through one generation per year. Egg mass hatch typically coincides with budbreak. On the east coast, where the majority of gypsy moth research has been done, gypsy moths hatch in early May. Male gypsy moths typically have five instars and females typically have six. Gypsy moth spends four to six weeks in the larval stage. The pupal stage of the life cycle lasts approximately two weeks. In the east, adults generally emerge in July. Adults mate soon after emergence. Egg masses, which contain 100-1000 eggs, are usually laid almost immediately after mating. The egg mass is the overwintering stage.

Early instar gypsy moth damage is characterized by small holes on the interior of leaves. Larger larvae begin feeding at the leaf edges and will ultimately consume the entire leaf except for large veins and the midrib. When the number of larvae on a given tree is high, feeding can result in total defoliation of the tree. Two to three successive years of defoliation can kill hardwoods, but only a single year of defoliation can kill conifers. While it may not always directly kill trees, gypsy moth defoliation adds to the complex of stresses that trees face and may lead to increased mortality. This mortality can alter forest composition and may result in the loss of high value trees, particularly oak. In urban and residential areas, gypsy moth is mainly a nuisance factor, particularly when large numbers of larvae and their droppings are present. Gypsy moth can also kill trees, which add to property values, in these areas.

- Amy Chenot, University of Wisconsin-Madison

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