GLOBAL GYPSY-THE MOTH THAT GETS AROUND W. E. WALLNER USDA Forest Service Northeastern Center for Forest Health Research Hamden, CT 06514 Abstract. It is difficult to document the total economic impacts of exotic insect pests on eastern U.S. forests. Annual losses to a single introduced pest, the gypsy moth, Lymantria dispar L., have exceeded $30 million from 1980 to 1996. The complicated behavior and actions of humans in accelerating the spread of this "global gypsy" are discussed. Examples of predicted economic impacts derived from pest risk assessments are given that demonstrate potential losses to other exotic insect pests. 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 of21 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 ). 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 northweste~ 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 63 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. 19.65 1990...... ,--:_·~..../ I f'~~~~ ,.., ._.• Figure 1. Establishment and spread of gypsy moth in the United States. 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 1s 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). 64 Figure 2. Transshipment locations of Department of Defense equipment through the Sunny Point military terminal, Wilmington, North Carolina, from Germany, 1993-95. Average annual expenditures for gypsy moth eradication, suppression, and research in United State; 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 Wes~ which contain high proportions of preferred host trees. Similar estimates of economic arrl environmental costs for other invasive organisms may be difficult (Wallner 1996), but accura1e assessments will be critical in gaining political and economic support to establish programs 1o eradicate and/or control exotic insect pests (Wallner in preparation) and sustain current programs. And competition for resources to confront new introductions will only increase in the future. 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. fores1s (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. 65 35r-------------------------------------------~ GYPSY MOTH COSTS 30 0 SUPPRESSION • rJ ERADICATION ~ 25 :5_. • RESEARCH 8 20 u. 0 (/) 15 z 0 :3 10 ~ 5 0 Figure 3. Yearly costs for gypsy moth eradication, suppression, and research programs in the United States, 1980-94. Two organisms, gypsy moth and zebra mussel, were responsible for a congressionally mandatoo report on harmful nonindigenous species in the United States (Office of Technology Assessmett 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 Mattsm (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 commm 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 provi~ 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 compreherrl how insidious and sometimes overwhelming the effects can be. However, predicting which one:;' 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. Fer example, the potential cumulative economic impact from the introduction of insects from Sib ern and New Zealand could be as high as $60 billion (Table 1). While these estimates may sean 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. 66 TABLE 1. Estimated economic impacts to U.S. forest resources from selected introduced insect pests from Siberia and New Zealand (1990 dollars). Cumulative costs Best case Worst case Insects Diseases Insects Diseases Millions of dollars Siberiaa 35,210 295 60,000 2,254 New Zealandb 45 7 295 69 ~Source: USDA For. Serv. Misc. Publ. 1495 (1991). Source: USDA Misc. Publ. 1508 (1992). Of all introduced insects, those with parthenogenetic capabilities have the best chance ofbecoming 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