How do Exotic Pests Impact Forested Ecosystems and Landscape Trees in North America?

Gaston Laflamme and Thomas Hofacker

Invasions of exotic forest pests into North America (NA) likely began with the arrival of settlers from Europe. These settlers had a direct and enormous impact in the East and South as forests were cleared for cities, towns, and agricultural purposes. So much so, that by the late 1800s, the eastern and southern forests of NA bore little resemblance to the native forests that the European settlers first encountered. This is the backdrop against which invasions of exotic forest insects and pathogens began to be recognized as the sciences of entomology and plant pathology emerged on the continent.

Then came gypsy moth (Lymantria dispar) and chestnut blight (Chryphonectria parasitica). In the early 1900s, a massive (and unsuccessful) gypsy moth eradication and suppression program began. Efforts to deal with these introductions continue today, with millions of dollars spent annually. While the gypsy moth was intentionally imported into North America, chestnut blight was unintentionally introduced on chestnut seedlings brought to the New York Botanical Garden from Asia (Schumann 1991). The disease spread rapidly, invading 80% of the host range in 10 years. American chestnuts had virtually no resistance to this fungus, and over the next 50 years, an estimated four billion trees were killed. Today, only stump sprouts are produced and they are also killed by the pathogen. The entire species, which once comprised one-quarter of the eastern U.S. forest, became functionally extinct; that is, the species still exists, but not in sufficient numbers to provide a useful ecological structure or function in a significant manner.

The disease caused considerable economic loss in wood and nut production. In many areas, oaks have replaced the chestnut. These oak trees were later severely defoliated by gypsy moth. Even a century later, the impact of these exotics continues. To learn more about chestnut blight, click on to the nicely illustrated article at http://www.apsnet.org/online/feature/chestnut/top.html.

Less severe effects are more common. For example, butternut canker has basically eliminated butternut from the southern parts of its range, but still exists in some northern areas where it may have some natural resistance. It may have been introduced through infected seeds (Innes 1998). Dutch elm disease has wiped out American elm in our urban forests, but elms are still present in significant numbers in many native forests. The disease arrived on elm logs imported for veneer from Europe in 1930. The fungus was also isolated from wood boxes made of European elm and used for shipping goods (Boyce J.C. 1961; Stipes and Campana 1981). Dogwood anthracnose has restricted American dogwood populations to forest edges in many higher elevation sites in the eastern United States, but it still exists in forests at lower elevations. Balsam woolly adelgid has killed most mature Fraser fir in native forests, but it is widely grown as a holiday tree. Mattson, (1997), estimated that only about 5% of the exotic insects that attack woody plants in NA have produced " well recognized, severe ecological impacts on the trees and ecosystems that they occupy." This 5% figure may be a result of a lack studies as well as a lack of impact. In fact, we can show that hemlock woolly adelgid has killed many streamside eastern hemlocks, Port-Orford cedar root disease has killed many streamside Port-Orford cedars, and beech bark disease/beech scale has killed many American beech, but the effects of these trees deaths is largely unknown. Scleroderris canker, caused by the European race, has affected nursery growers in localized areas because of the probation on the movement of pine stock. Several of the diseases, such as beech bark disease, Dutch elm disease, and European larch canker continue to spread in NA. For more information, see Liebhold et al. (1995).

Likewise, the more subtle effects caused by exotic insects and pathogens are even more unclear. More than 400 species of exotic insects alone are known to attack woody plants in NA. We can speculate that these exotics add additional stress to trees and forests that must already cope with unnatural stresses such as global warming, acid deposition, stratospheric ozone depletion, soil compaction, fire control, human development, and a host of other factors. Manion (1991) has postulated that this additional stress may result in a decline death spiral from which it is difficult for affected hosts to recover. We can also speculate that these effects will often play out over relatively long time frames compared to human life spans. This, indeed, seems to be the case, but studies to determine and document the direct and indirect effects of exotic invasions over time are urgently needed in order to make a better case for adequate funding to help stem the invasive tide.

Plant quarantine and international trade
Canada, Mexico, and the United States have tried to develop a common NA approach to deal with potential exotic pests by participating in the North American Forestry Commission and the North American Plant Protection Organization. These groups discuss the distribution, regulatory and mitigation measures, and research on exotics of common concern.

Stipes, R.J. and R.J Campana, eds. 1981.Compendium of elm diseases. APS Press. 96pp.