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SERAMBO - Southern Appalachian Biological Control Initiative Workshop - Biological Control of the Gypsy Moth
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Biological Control of The Gypsy Moth:

AN OVERVIEW

Richard C. Reardon
USDA Forest Service
Forest Health Technology Enterprise Team
Morgantown, WV 26505

Abstract

Biological control is the regulation by natural enemies (pathogens, parasites and predators) of an organism's population at a lower density than would otherwise occur. Biological control can occur spontaneously due to native natural enemies, can be applied by people, or can be fortuitous, that is, due to accidental introduction of natural enemies.

Most biological control of the introduced gypsy moth, Lymantria dispar (L.), in the United States has been applied, following a classical approach using parasites. This approach involves searching for parasites in native habitats of the gypsy moth, importing them, and releasing them in the hope that they will become established in this country and exert biological control on gypsy moth populations. Use of augmentation, which is the manipulation of natural enemies by people for more immediate control, has been minimal against the gypsy moth.

This paper give background material on the gypsy moth, and describes the effect of pathogens, parasites, and predators on gypsy moth populations.

About the Gypsy Moth

The gypsy moth caterpillar is a serious defoliator of broadleaved forests in eastern North America. In addition, this pest defoliates trees and shrubs in residential areas causing economic and aesthetic impacts and, when infestations are heavy, creates a nuisance to residents. Caterpillars prefer hardwoods, but may feed on several hundred different species of trees and shrubs. During periods when gypsy moth populations are dense, larvae feed on almost all vegetation. Trees weakened by consecutive defoliation are vulnerable to attack by disease organisms and other insects. For example, the Armillaria fungus attacks the roots of weakened trees, and the two-lined chestnut borer attacks the trunk and branches.

Life Cycle

The gypsy moth has one generation per year passing through four life stages: egg, caterpillar, pupa, and adult (moth stage) (Figure 1). Only the caterpillar which reach maturity between mid-June and early July defoliate trees and shrubs. After 6 to 8 weeks, the caterpillar enters the pupal stage for 7 to 14 days, which changes into adults (moths). Flightless female moths mate and lay their eggs in masses in July and August. Four to six weeks later, embryos develop into caterpillars. The caterpillars remain in the eggs during the winter. The caterpillars emerge from the eggs the following spring, coinciding with budding of most broadleaved trees (McManus et al 1989).

Distribution

The gypsy moth is not native to North America but was introduced from Europe in 1869 near Boston, Massachusetts. Historically, populations of the gypsy moth have undergone periodic outbreaks to extremely high densities that resulted in widespread defoliation to an average of 3.0 million forested acres per year. More recently (1992 through 1996), populations have been declining to an average of 1.0 million forested acres per year partly due to the rapid spread of an introduced fungus (Figure 2).

Since the introduction of the European or North American strain of gypsy moth, it has spread south and west, and continues to spread along the leading edge of infestation at the rate of approximately 12 miles per year (Figure 3). The Asian strain of gypsy moth was recently introduced on the East and West coasts of North America and eradicated.

Biological Control of the Gypsy Moth

Pathogens -- In eastern North America, the gypsy moth is subject to a variety of naturally occurring infectious diseases caused by several kinds of pathogens -- bacteria, fungi, and a nucleopolyhedrosis virus (NPV). The NPV, which was inadvertently introduced with the gypsy moth or its parasites, and an introduced fungus Entomophaga maimaiga cause widespread mortality and are described here. The other pathogens cause only limited mortality.

Virus

The naturally occurring disease caused by the NPV is often referred to as "wilt" due to the soft, limp appearance of the diseased larvae (Figure 4). The disease can reach outbreak (epizootic) proportions as gypsy moth population densities increase. These outbreaks result from increased transmission rates, within and between generations of the gypsy moth, as small caterpillars become infected and die on leaves in the crowns of trees. These caterpillar cadavers disintegrate and serve as inocula for healthy feeding caterpillars. Also, virus transmission occurs when adult females deposit their egg masses on NPV-contaminated surfaces. Caterpillars hatching from these contaminated eggs in the following spring have a high risk of contracting the disease. Birds and mammals have the ability to pass and disperse active gypsy moth NPV, and parasites and invertebrate predators may play a role in the transmission of gypsy moth NPV within natural populations. In many dense gypsy moth populations, the virus kills up to 95% of the larvae and reduces populations to levels where they cause only minimal defoliation and tree damage in the following year (Reardon and Podgwaite 1992).

In the late 1950's, the USDA Forest Service began to explore the feasibility of developing this pathogen as an alternative to chemical insecticides for suppressing gypsy moth populations. In April 1978, the gypsy moth nucleopolyhedrosis virus product Gypchek was registered for use by the Environmental Protection Agency (US EPA). Today, Gypchek is produced in live gypsy moth caterpillars in the laboratory by the USDA Forest Service and Animal and Plant Health Inspection Service (APHIS), processed, and made available for aerial and ground application as part of the Federal and State Cooperative Suppression Program.

Fungus

In 1908, pest managers in the Boston area introduced the fungus Entomophaga maimaiga via infected gypsy moth larvae collected in Japan. Releases continued until 1911, when the local gypsy moth populations collapsed and there was no way to continue propagating the fungus. In June and July 1989, E. maimaiga was first recovered in North American gypsy moth and was found causing extensive epizootics in populations of gypsy moth in seven contiguous northeastern States (Connecticut, Massachusetts, Vermont, New Hampshire, New Jersey, New York and Pennsylvania). By 1990, the fungus was also recovered in three more northeastern states (Maine, Delaware, Maryland) and in southern Ontario. Today E. maimaiga occurs in most areas where the gypsy moth occurs and is prevalent in low-to-high density gypsy moth populations, causing up to 95% mortality of large caterpillar (Figure 5). The fungus is highly variable, and as yet unpredictable, in reducing gypsy moth populations. It is not applied as a direct control. Fungal resting spores in soil and infected gypsy moth cadavers are collected and dispersed by hand to spread the fungus to new locations although natural spread has been fairly rapid (Reardon and Hajek 1993).

Parasites -- Using parasites against the gypsy moth has been one of the most massive programs in biological control history (Reardon 1981).

1905 to 1980

From 1905 to 1980, approximately 78 species of parasites (over 200,000 individuals) were sent to the USDA Agricultural Research Service (ARS) quarantine facilities in the United States. Of these, approximately 53 species were shipped to cooperating agencies for initiation of laboratory colonies or release. Between 1905 and 1914, gypsy moth caterpillars and pupae containing parasites were collected in Europe, Japan, and Russia and shipped to the United States. Six of the parasite species imported and introduced became established (Table 1). Parasite importation was reinstated in 1922 to 1933 with searching for gypsy moth infestations in France, Spain, Italy, Germany and Japan. These efforts led to the establishment of two flies and the possible establishment of one wasp (Table 1).

During both periods of foreign exploration, 1905-1914 and 1922-1933, hosts and parasites were collected from high-density gypsy moth populations. Limited foreign exploration was resumed in the 1960's, and in the 1970's ARS established gypsy moth projects at their European Parasite Laboratory in France and Asian Parasite Laboratory in Japan. Only one exotic species of parasite was established during this period probably due to numerous problems associated with rearing and releasing parasites. Problems with rearing include inadequate taxonomic identification and poor and variable host quality and quantity. Problems with releasing parasites include inadequate numbers, "laboratory" strains that were not adaptable to field conditions, lack of alternate or overwintering hosts, and lack of host density and habitat requirements. One exotic species of parasite was released in the late 1960's and 1970's but was not recovered until 1996. Several parasites native to the United States have became opportunistic parasites of the gypsy moth, that is, when gypsy moths are available. The augmentation approach either as inundative releases (released individuals regulate) or inoculative releases (progeny of released parasites regulate generations of the gypsy moth) has been attempted with numerous species against artificial and natural gypsy moth populations. In general the incidence of parasitism by the released species increased, but no impact on gypsy moth populations was detected. Also, combinations of natural enemies (e.g. aerial application of the bacterial insecticide Bt and releases of Cotesia melanoscela, to transmit NPV) have been used with limited success.

1980 to 1992

Prospects for using classical, and augmentation approaches to improve biological control of the gypsy moth were explored again during the 1980's and early 1990's. Foreign exploration for parasites shifted to Asia, and 17 parasite species were received at ARS quarantine in the United States. Most of these were from Korea, Japan, and India (parasites of Indian gypsy moth, Lymantria obfuscata Walker), whereas little material was obtained from the other promising regions, China and the Russian Far East. Releases of 15 species were made, but establishment of only one species, the pupal parasite Coccygomimus disparis (Viereck) was confirmed. This species appears to be dispersing well over the generally infested area, but with limited effectiveness against the gypsy moth because it parasitizes numerous species.

1993 to 1997

Recent interest in the classical approach to biological control has been provided through the National Biological Control Institute (USDA APHIS) and "New Directions in Biological Control of the Gypsy Moth" with efforts focusing in non-outbreak sites on promising species that have not been previously introduced. Dominant species from southern Europe that failed to become established in New England or the Middle Atlantic States (e.g., Glyptapanteles porthetriae (Muesebeck)) are being imported and reared for release in the southern states with different forest habitat types, climate, and availability of alternate host species (Fuester 1993). Manipulative experiments conducted in New England suggest that artificial elevation of gypsy moth populations might be useful for maintaining populations of insects that parasitize caterpillars, such as Compsilura concinnata (Meigen), Parasetigena silvestris (Robineau-Desvoidy), and Cotesia melanoscela (Ratzeburg).

Predators -- Many species of animals in the United States eat the gypsy moth and other defoliating insects. The gypsy moth predator community is complex and includes about 50 species of birds and 20 species of mammals, along with some amphibians, reptiles, fish, insects, and spiders. Only a few of these predators are known to affect gypsy moth population dynamics. The predators are all opportunistic feeders, which means that their taste for the gypsy moth depends upon the scarcity of other preferred foods. Vertebrate predators, especially the white-footed mouse (Peromyscus leucopus), are major sources of large caterpillar and pupal mortality in low density gypsy moth populations. Recent studies of bird predation tend to show that gypsy moth is not a major food item of most species.

Insect predators especially ants and the imported carabid beetle Calosoma sycophanta (Figure 6) have a limited impact on gypsy moth populations. Calosoma sycophanta was imported from Europe in 1905-1910 and became established easily. It is common throughout most of New England and extends into New York, New Jersey, central Pennsylvania, and northeast Maryland. The beetle is a specific predator of gypsy moth and usually associated with high density gypsy moth populations.

Conclusions

In general, parasites together with other natural enemies (predators and pathogens) help regulate populations of the gypsy moth by reducing their numbers. Whether these introduced parasites have reduced the average population density of the pest or lengthened the period between outbreaks is difficult to determine. The rate of parasitism from a particular parasite species varies from site to site and from year to year, depending on such factors as the number of gypsy moth larvae, the number of alternate hosts, and the weather. Eleven exotic species of parasites have been established and continue to disperse along with the gypsy moth. Natural enemies are thought to help maintain low density populations, but not to prevent the buildup of already increasing populations. Foreign exploration for natural enemies has occurred throughout most of the native range of the gypsy moth. In the continued search for biological control agents, selection of candidates focuses on species that are (1) from low density gypsy moth populations, (2) limited to one generation per year, (3) new or not previously released, and (4) found to preferentially attack female gypsy moth caterpillars or pupae.

References

Fuester, R. 1993. Evaluating biological control potential of established and exotic parasites. pp. 49-58. In: Fosbroke, S. and K. Gottschalk, eds. Proceedings, U.S. Department of Agriculture interagency gypsy moth research forum 1993; 1993 January 19-22; Annapolis, MD. Gen. Tech. Rep. NE-179. Radnor, PA: Northeastern Forest Experiment Station, Forest Service; 127 p.

McManus, M., N. Schneeberger, R. Reardon, and G. Mason. 1989. Gypsy moth USDA Forest Service Forest Insect and Disease Leaflet 162. Radnor, PA: Northeastern Forest Experiment Station, Forest Service; 13 p.

Reardon, R. 1981. Chapter 6.1 Parasites. pp. 299-421. In: Doane, C. and M. McManus editors. The Gypsy Moth: Research Toward Integrated Pest Management. USDA Forest Service Science and Education Agency Tech. Bull. 1584. Washington, DC.

Reardon, R. and A. Hajek. 1993. Entomophaga maimaiga in North America: a review. USDA Forest Service NA-TP-15-93. Radnor, PA: Northeastern Area State and Private Forestry; 22 p.

Reardon, R. and J. Podgwaite. 1992. The gypsy moth nucleopolyhedrosis virus product. USDA Forest Service. NA-TP-02-92. Radnor, PA: Northeastern Area State and Private Forestry; 9 p.

Figures


Figure 1 - Stages of gypsy moth development:
egg, caterpillar (larva), pupa, adult.
Figure 2 - Acres of gypsy moth defoliation
by year in the United States.

Figure 3 - Distribution of the gypsy moth
in North America in 1996.
Figure 4 - Gypsy moth caterpillar killed
by the nucleopolyhedrosis virus.

Figure 5 - Gypsy moth caterpillar killed
by the fungus Entomophaga maimaiga.
Figure 6 - Larval and adult stages
of the carabid beetle Calosoma sycophanta.

Table 1. Gypsy Moth Parasites Established in the United States
Gypsy moth life stage parasitized Parasite species Type of parasite Imported and Introduced
Egg Ooencyrtus kuvanae Wasp 1905-1914
Anastatus disparis Wasp 1905-1914
Caterpillar Cotesia melanoscela Wasp 1905-1914
Phobocampe unicinta Wasp 1905-1914
Rogas indiscretus Wasp 1966-1979
Compsilura concinnata Fly 1905-1914
Parasetigena silvestris Fly 1922-1933
Blepharipa pratensis Fly 1905-1914
Exorista larvarum Fly 1922-1933
Pupa Brachymeria intermedia Wasp 1922-1933
Coccygomimus disparis Wasp 1980-1992
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