Global Gypsy - The Moth That Gets Around

W. E. Wallner - USDA Forest Service, Northeastern Center for Forest Health Research, Hamden, CT 06514.

From: Exotic Pests of Eastern Forests, Conference Proceedings - April 8-10, 1997, Nashville, TN, Edited by: Kerry O. Britton, USDA Forest Service & TN Exotic Pest Plant Council

Invasive pests are among the most serious threats to biological diversity in U.S. forest ecosystems. Additionally, they disrupt forest management and cause enormous financial loss. Efforts by the USDA Animal and Plant Health Inspection Service to detect and prevent new introductions cost an estimated $200 million annually. Despite efforts within the United States and with U.S. trading partners, additional pests are being introduced and some will become established. In this country, some 380 exotic insects and diseases attack native and exotic trees and shrubs (Hack and Byler 1993). The complete history of various invasive insects--from introduction via known pathways to establishment and spread-as well as their total economic and ecologic impacts can only be estimated. However, there is substantial documentation of the devastating effects of the gypsy moth, Lymantria dispar L., the dominant exotic insect pest of U.S. eastern forests. As a result, this "global gypsy" can serve as a template for appreciating the economic consequences of invasions by exotic forest pests.

Since its accidental introduction into Massachusetts from France in 1869, gypsy moth has spread southward and westward by larval dispersal and inadvertent movement of the insect in various life stages by humans (annual rate of 21 km) (Fig. 1) (Liebhold et al. 1992, 1995). Attempts to slow its spread into the highly vulnerable forests of the Southeast and Mid-South have been accelerated by estimates of $100 to 500 million in savings over the next 25 years (Leuschner 1991). Research and pest management programs have provided a basic understanding of the ecology of gypsy moth and its impact on forests, and biologically based technologies have been deployed to suppress the European strain of this insect (Doane and McManus 1981).

Figure 1. Establishment and spread of gypsy moth in the United States.

Figure 2. Transshipment locations of Department of Defense equipment through the Sunny Point military terminal, Wilmington, North Carolina, from Germany, 1993-95.

New introductions of the European strain of gypsy moth are controlled aggressively. Still, the Asian strain, with females capable of flight (Wallner et al. 1995) and larvae with a broad host range (Baranchikov 1989), would render efforts to constrain it technically difficult and more expensive. Following the introduction of the Asian strain of gypsy moth into the northwestern United States and Canada on Russian grain ships (Bogdanowicz et al. 1993) and into North Carolina on U.S. military equipment from Germany (Hofacker et al. 1993) eradication efforts during the 1990's exceeded $30 million. The military experience is instructive. During 1993-95, milvans and vehicles were inspected and presumed free of gypsy moth and transhipped from Wilmington, North Carolina, to 48 locations throughout the United States (Fig. 2). However, this activity could have founded widespread infestations if they were infested.

Gypsy moth is a polyphagous defoliator but prefers oak and poplar (Montgomery and Wallner 1988). Defoliation by this forest pest may increase seedling mortality, reduce tree growth and the production of mast for wildlife (Gottschalk 1990a), and cause occasional massive tree mortality (Allen and Bowersox 1989). The effect of several defoliation episodes on shifts in stand species composition is not well understood (cf. Campbell and Sloan 1977; Gansner et al. 1993), but the adverse impact of gypsy moth on aesthetic, recreation, and home values has been documented (Payne et al. 1973). During the last major outbreak when more than 16 million acres of mixed hardwood were defoliated, timber losses in the State of Pennsylvania alone exceeded $72 million. This does not include more than $9 million expended by that state on spray programs. From 1968 to 1985, Pennsylvania incurred $219 million in losses from gypsy moth defoliation (Gottschalk 1990b). Because gypsy moth is an episodic pest outbreaks do not occur annually, so variables such as the number of years of defoliation, tree vigor, and other environmental stressors influence its impacts. Trees weakened by defoliation are more susceptible to attack by secondary organisms like the two-lined chestnut borer and shoestring root rot fungus (Wargo 1977).

Average annual expenditures for gypsy moth eradication, suppression, and research in United States from 1980 to 1994 totaled $30 million (1995 dollars) (Fig. 3). This figure does not include $8 million for deploying 400,000 pheromone traps for monitoring ($20/trap). These yearly costs will increase as gypsy moth reaches the highly susceptible forests of the South, mid-South, and West, which contain high proportions of preferred host trees. Similar estimates of economic and environmental costs for other invasive organisms may be difficult (Wallner 1996), but accurate assessments will be critical in gaining political and economic support to establish programs to eradicate and/or control exotic insect pests (Wallner in

Figure 3. Yearly costs for gypsy moth eradication, suppression, and research programs in the United States, 1980-94.

The ecological "ripple effect" of exotic pests is nearly impossible to predict. For example, at least two significant changes occurred in the aftermath of the chestnut blight, which eliminated more than 8 million American chestnut trees, one of the most important tree species of eastern U.S. forests (Kuhlman 1978). Oak replaced chestnut which created more extensive forests susceptible to gypsy moth. Also, oak cohorts did not adapt well on sites previously occupied by chestnut and now are senescing due to environmental stress (Starkey et al. 1989). Thus, Appalachian forests are experiencing delayed consequences of two exotic agents introduced more than a century ago.

Two organisms, gypsy moth and zebra mussel, were responsible for a congressionally mandated report on harmful nonindigenous species in the United States (Office of Technology Assessment 1993). As mentioned previously, the available literature and this congressional report make it clear that there are few data on the economic impacts of specific exotic forest pests. Niemela and Mattson (1996) acknowledge this problem bluntly: "When the outrageous economic and ecological costs of the wanton spread of existing exotics and continued entry of new ones becomes common knowledge, there will be a public outcry to mitigate the potentially dire consequences." Lacking precise economic loss estimates, land managers and regulators will be hard pressed to provide justification for what if any action should be taken and the priorities in selecting among several exotic pest programs. As an entire ecosystem is devastated by an exotic insect, we can comprehend how insidious and sometimes overwhelming the effects can be. However, predicting which ones may survive and have a negative economic impact is not easy.

About 40 percent of the major insect pests in the United States are exotic. The use of pest risk assessment (PRA) procedures, common in evaluating the potential hazard associated with international commodity trade (Orr et al. 1993), has proven useful in identifying insect pests that could be imported into this country on unprocessed wood from several foreign countries. For example, the potential cumulative economic impact from the introduction of insects from Siberia and New Zealand could be as high as $60 billion (Table 1). While these estimates may seem excessive, they are consistent with those given in the Office of Technology Assessment report, which estimates losses to introduced insect pests from 1906 to 1991 at $92 billion.

Table 1. Estimated cumulative economic impacts to U.S. forest resources from selected introduced insect pests from Siberia and New Zealand (1990 dollars).

^a Source: USDA For. Serv. Misc. Publ. 1495 (1991).
^b Source: USDA Misc. Publ. 1508 (1992).

Of all introduced insects, those with parthenogenetic capabilities have the best chance of becoming established (Neimela and Mattson 1996). Examples include the adelgids, about 50 species of which attack conifers in North America. Two introduced species that gained entry into this country on nursery stock devastated mature trees in fragile forests. The balsam woolly adelgid (European origin) threatens to eliminate relic stands of Fraser fir in the southern Appalachian Mountains (Dull et al. 1988), while the hemlock woolly adelgid (Asian origin) is decimating eastern hemlock forests from New England to North Carolina (Souto et al. 1996). Neither species have been subjected to the PRA process nor have their economic effects been assessed despite direct and indirect impacts on the host resource. For eample, nursery sales of eastern hemlock have plummeted due to the hemlock woolly adelgid. Comparable impacts by other exotic pests include those on the dogwood nursery trade from dogwood anthracnose.

A recently completed PRA for importing unprocessed wood from Mexico acknowledged the severe potential impact of adelgids on conifers. Adelgids are known to damage pines in Mexico, but their distribution and economic impact are not well known. Should a Mexican Pineus species become established in the U.S. southern pine region, some 8 1/2 million acres could be at risk. Assuming losses totaling $243 per acre for mortality, growth loss, and replanting, annual costs could approach $20 million. Using a discount rate of 4 percent, losses in net value over the next 30 years would amount to $258 million. Comparable estimates of loss in other forest ecosystems from invasions by exotic insects would be invaluable to the PRA process and aid in establishing priorities for allocating scarce resources.

The genesis of this conference was to create an atmosphere of sharing information on exotic insect pests, increase our understanding of their complex roles, and underscore the critical importance of cooperation among various agencies. The continuing challenge to land managers will be to prevent loss of forest productivity while deterring further erosion of eastern U.S. forests. Research must be able to anticipate questions concerning potential impacts of exotic pests. For example:

• Is eradication the first and only consideration?
• Should we modify our concepts of ecosystems that already have been altered by exotic pests?
• Can we manage, much less restore, ecosystems altered by invasive pests?
• As resources become limiting, who will decide which exotic pest receives priority attention?

Literature Cited

Allen, D., and Bowersox, T.W. 1989. Regeneration in oak stands following gypsy moth defoliation, pp. 67-73. In Rink, G. and Budelsky, C.A. (eds.), Proceedings of the 7th Central Hardwood Forest Conference. USDA Forest Service General Technical Report NC-132.

Baranchikov, Y.N. 1989. Ecological basis of the evolution of host relationships in European gypsy moth populations, pp. 319-338. In: Wallner, W.E. and McManus, K.A. (tech. coords.), Proceedings, Lymantriidae, a Comparison of Features of New and Old World Tussock Moths, USDA Forest Service General Technical Report NE-123.

Bogdanowicz, S.M., Wallner, W.E., Bell, J., ODell, T.M., and Harrison, R.G. 1993. Asian gypsy moth (Lepidoptera: Lymantriidae) in North America: evidence from molecular data. Annals of the Entomological Society of America 86: 710-715.

Campbell, R.W., and Sloan, R.J. 1977. Forest stand responses to defoliation by the gypsy Forest Science Monograph 19.

Doane, C.C., and McManus, M.L. 1981. Gypsy Moth: research toward integrated pest management. USDA Technical Bulletin 1584.

Dull, C.W., Ward, J.D., Brown, H.D., Ryan, G.W., Clerke, W.H., and Uhler, R.J. 1988. Evaluation of spruce and fir mortality in the Southern Appalachian Mountains. USDA Forest Service Protection Report R8-PR 13.

Gansner, D.A., Arner, S.L., Widman, R.H., and Alerich, C.L. 1993. After two decades of gypsy moth is there any oak left? Northern Journal of Applied Forestry 10: 184-186.

Gottschalk, K.W. 1990a. Gypsy moth effects on mast production, pp. 42-50. In: Charles, E. (ed.), Proceedings of Workshop on Southern Appalachian Mast Management, University of Tennessee, Knoxville.

Gottschalk, K.W. 1990b. Economic evaluation of gypsy moth damage in the United States of America, pp. 236-246. In Proceedings, Division 4, IUFRO 19th World Congress, Canadian IUFRO World Congress Organizing Committee, Montreal.

Hack, R.A., and Byler, J.W. 1993. Insects and pathogens: regulators of forest ecosystems. Journal of Forestry 91: 32-37.

Hofacker, T.H., South, M.D., and Meilke, M.E. 1993. Asian gypsy moths enter North Carolina by way of Europe: a trip report. Michigan Entomological Society Newsletter 38: 1, 4.

Kuhlman, H.G. 1978. The devastation of American chestnut by blight, pp. 1-3. In: McDonald, W.L., Cech, F.C., Luchok J., and Smith, H.C. (eds.), Proceedings, American Chestnut Symposium. West Virginia University Books, Morgantown.

Leuschner, W.A. 1991. Gypsy moth containment program economic assessment. Final report. USDA Forest Service, Washington, DC.

Liebhold, A.M., Halverson, J.A., and Elmes, G.A. 1992. Gypsy moth invasion in North America: a quantitative analysis. Journal of Biogeography 19: 513-520.

Liebhold, A.M., MacDonald, W.L., Bergdahl, D., and Mastro, V.C. 1995. Invasion of exotic forest pests: a threat to forest ecosystems. Forest Science Monograph 30.

Montgomery, M.E., and Wallner, W.E. 1988. The gypsy moth: a westward migrant, pp. 253-275. In: Berryman, A.A. (ed.), Dynamics of Forest Insect Populations: Patterns, Causes, Implications. Plenum Press, New York.

Niemela, P., and Mattson, W.J. 1996. Invasion of North American forests by European phytophagous insects. BioScience 46: 741-753.

Office of Technology Assessment. 1993. Harmful nonindigenous insect species in the United States. OTA-F-565. Office of Technology Assessment, Washington, DC.

Orr, L., Cohen, S.D., and Griffen, R.L. 1993. Generic nonindigenous pest risk assessment process. USDA Animal and Plant Health Inspection Service, Planning and Risk Analysis Systems, Policy and Program Development, Washington, DC.

Payne, B.R., White, W.B., McCay, R.E., and McNichols, R.R. 1973. Economic analysis of the gypsy moth problem in the Northeast. II. Applied to residential property. USDA Forest Service Research Paper NE-285.

Starkey, D.A., Oak, S.W., Ryan G.W., Tainter, F.M., Redmond, C., and Brown, M.D. 1989. Evaluating oak decline areas in the South. USDA Forest Service Protection Report R8-PR 17.

Souto, D., Luther, T., and Chianese, R. 1996. Past and current status of hemlock woolly adelgid in eastern and Carolina hemlock stands, pp. 9-15. In: Salom, S.M., Tigner, T., and Reardon R.C. (eds.), Proceedings, 1st Conference on Hemlock Woolly Adelgid. USDA Forest Service Forest Health Technology Enterprise Team FHTET-96-10.

Wallner, W.E., Humble, L.M., Levin, R.E., Baranchikov, Y.N., and Carde, R.T. 1995. Response of adult Lymantriid moths to illumination devices in the Russian Far East. Journal of Economic Entomology 88: 337-342.

Wallner, W.E. 1996. Invasive pests (biological pollutants) and U.S. forests: whose problem, who pays? EPPO Bulletin 26: 167-180.

Wargo, P.M. 1977. Armillaria mellea and Agrilus bilineatus and mortality of defoliated oak trees. Forest Science 23: 485-492.