Global Gypsy - The Moth That Gets Around
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
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 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.
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. 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
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
||Cumulative Costs (Millions of 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
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