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Invasive Species in Eastern Forests

by Faith Campbell, Ph.D.

In 1751, John Bartram travelled through Pennsylvania and reported the tree species he encountered. Since 1900, nearly every one of these species has been attacked by exotic insects, pathogens, or combination of these. Some have been virtually eliminated, e.g., American chestnut; others are now being extirpated, e.g., butternut. Newly introduced pests threaten others species like the sugar maple.  More than 24 harmful exotic pathogens & 400 harmful exotic insects that attack trees and shrubs have been introduced to the US.  Eastern forests have been more heavily impacted than western forests because they were colonized earlier by Europeans, are along or near long-established trade routes, and the diversity of plants provides opportunities for exotic organisms to find suitable hosts.

Dr. David Pimentel of Cornell University estimates that exotic forest pests cost the country $4 billion per year. Introductions of exotic pests are likely to increase in the future due to increasing trade. U.S. imports rose more than 50% between 1990 and 1997. Each container, ship, or plane provides an opportunity for a pest to hitchhike to this country. The crates, pallets, and other forms of wood packaging that contain these imports are particularly likely to transport ("vector") forest pests. The US is importing larger quantities of particularly dangerous articles that can carry forest pests or "vectors" such as logs, wood chips and living plants. The US imports of logs increased 30 times from 1990 to 1999. We now import more of these goods from new trading partners - countries such as China, Chile, and Mexico - with similar ecosystems that harbor pests which could damage our forests. The insects and fungal pathogens in these countries' forests - even in Mexico - cannot reach the U.S. on their own because of ecological barriers. But they can hitchhike here on wood or other articles that we import - or even on the planes, ships, trains, and trucks themselves.

Examples of Pests Introduced in the Past

Chestnut Blight

American chestnut was once the most important hardwood species. From Maine to Georgia, chestnut comprised one-quarter of all standing timber in the eastern forests. Its nuts were extremely important in wildlife food chains. Chestnut Blight was introduced to North America in 1904 on imported nursery stock. It spread rapidly throughout the tree's range. It has changed the species composition of eastern North American forests. Several scientists call it the worst ecological disaster ever recorded.

Dutch Elm Disease

Dutch Elm Disease is the most devastating shade tree disease in the U.S. The American elm was the primary ornamental tree in Eastern & Midwestern cities. Although American officials were aware of the disease in Europe, in the 1930s they allowed importation of diseased logs imported from Europe for processing. Dutch Elm Disease entered the country on these logs and then spread rapidly throughout eastern North America. A more aggressive species of the causal fungus, apparently imported accidentally in recent years, is now killing additional elms in the U.S. and Europe. A third, even more damaging fungus has been discovered in the Himalaya Mountains.

European Gypsy Moth

The European gypsy moth defoliates a wide variety of woody plants, especially hardwood trees; oaks are particularly vulnerable. The European gypsy moth was imported deliberately in 1869 by an individual experimenting with developing a U.S. silk industry. The gypsy moth is widespread in the East as far south as North Carolina, Tennessee, and Arkansas; there are smaller infestations in the West and the Rocky Mountain states. Gypsy moth defoliation induces trees to drain energy reserves growing new leaves. Trees might not die, but diameter and volume growth will decline. Mortality from defoliation can be as high as 90 percent where gypsy moth populations are at an epidemic level. The result is altered species composition and wildlife food chains; decreased water storage capacity; and lower warter quality due to nitrogen and other nutrients from the decaying leaves.

White Pine Blister Rust

White pine blister rust is a fungal disease that infests five needle pines throughout North America. Although the disease was known to be present in Europe, nursery stock was imported that introduced blister rust in Kansas (1892), eastern Canada (1906), and western Canada (1921). White pine populations have dropped dramatically in many areas. In Minnesota, they went from 3.5 million acres in 1837 to 67,000 acres in 1990. In Quebec, white pine areas dropped by 60% from 1917 to 1970. Pines in the West are even more vulnerable. Western white pine now surives in only 5% of its former range. Ninety percent or more of whitebark pines in Idaho and Montana have died; this tree's seeds are eaten by 110 species of animals in the northern Rockies, especially the grizzly bear, black bear, red squirrel, and Clark's nutcracker. Efforts to combat white pine blister rust "were the most extensive in time, labor, and money in the history of forestry in the United States" - but the fungus continues to spread.

Balsam Woolly Adelgid

The balsam woolly adelgid (Adelges piceae) was introduced to New England in 1908 on nursery stock; it then spread down the Appalachians. The greatest damage has been to northern bracted balsam fir (Abies balsamea var. phanerolepsis) on the highest peaks in Virginia and West Virginia and Fraser fir (Abies fraseri) on the peaks along the North Carolina-Tennessee border. Following infestation, Fraser fir declined from 80% to 2.5% of living crown trees. The number of breeding birds dropped 35% and the species changed. A spider and a liverwort were listed under the Endangered Species Act because the changes in the forest had dried out their habitat.

Butternut Canker

Butternut (Juglans cinera) grows as scattered trees throughout the eastern deciduous forest; it is similar to black walnut (Juglans nigra L.), but grows on poorer, drier sites. Wildlife eat the nuts. Butternut canker, caused by the fungus Sirococcus clavigignenti-juglandacearum, girdles and kills the tree. The fungus was discovered 1967 in Wisconsin, but it is believed to have struck earlier along the east coast. Butternut canker has spread throughout much of the tree's range, virtually eliminating the tree from the southern portions of its range.

Beech Bark Disease

American beech (Fagus grandifolia) provides nuts for wildlife species. In the Northeast, it is infested with a disease complex comprised of an exotic insect (introduced on nursery stock) and a native fungus. In the Northeast, more than 50 per cent of the trees in some areas have died. The disease has reached as far south as Great Smoky Mountains National Park.

Hemlock Woolly Adelgid

The hemlock woolly adelgid (Adelges tsugae) attacks eastern hemlock (Tsuga canadensis) and Carolina hemlock (T. caroliniana). The adelgid -- a type of aphid -- was probably introduced from Asia. First introduced into the West, where it apparently causes little harm, the adelgid was observed in Virginia in the early 1950s. By 1999, it had spread to 25% of the eastern hemlocks' ranges from North Carolina to Massachusetts. The entire range is at risk over the next 20 - 30 years. There has been extensive decline and mortality in Virginia, Pennsylvania, New Jersey, and Connecticut -- all within 10 years of first detection. Hemlocks' are ecologically very important; their dense, heavily shaded stands along creeks create distinctive microclimates that are home to a variety of wildlife. In northern New Jersey, 96 bird and 47 mammal species are associated with hemlock forests. Stream fauna differ significantly between hemlock and hardwood streams; for example, hemlock streams are three times more likely to have native brook trout than hardwood streams.

Asian longhorned beetle

The Asian longhorned beetle was discovered in Summer 1996 feeding on trees in two locations in New York City. A second cluster of beetles was detected in 1998 in Chicago. The Asian longhorned beetle came to America in crates and pallets from China. It was probably established 10 years before it was detected. The ALB feeds on many hardwood tree species, especially maples, but including poplars, black locust, willow, and elm. As of March 2000, federal, state, and local officials had destroyed more than 5,500 trees in New York and Chicago - at a cost of $25 million -- in an effort to eradicate this insect. If the eradication campaign fails, the Asian longhorned beetle would probably alter dominant species composition and age structure in hardwood forests across North America. At particular risk are the forests composed largely of maple and poplars which occupy more than 48 million acres from New England to Minnesota. The USDA has estimated that establishment of 4 genera of Chinese wood-boring insects [Anoplophora, Ceresium, Hesperophanes, and Monochamus] could cause $41 billion in losses to the forest products, commercial fruit, maple syrup, nursery, and tourist industries. If all the vulnerable trees planted in urban/suburban areas were killed, the replacement cost would be $522 billion.

Possible New Introductions:

Woodwasp-Amylostereum complex - The woodwasp Sirex noctilio and associated fungus Amylostereum areolatum are native to Eurasia and North Africa; they have already been introduced to New Zealand, Australia, and several countries in South America. The wasp-fungus combination would threaten any pine in the "lower 48", especially the loblolly pine (Pinus taeda) that is the foundation of timber industry in the Southeast.

Asian gypsy moth - the Asian gypsy moth belongs to the same species as the European one already established in the country. However, the AGM more readily feeds on conifers and could spread faster because the female flies. The Asian gypsy moth is now also found in Europe because of human transport of infested material. It has reached North America several times as egg masses on ships or containers and heavy equipment. Each time, emergency control programs have apparently succeeded in eradicating the moth. New introductions of Asian gypsy moth appear inevitable; and infestations would probably be extremely difficult to control.

What is Being Done to Minimize the Damage Caused by Exotic Forest Pests?

Responsibility for combatting established pests rests with the USDA Forest Service. It lacks sufficient resources to do an adequate job. The Forest Service' research program -- the foundation for determining best control or mitigation strategies has lost half its experts since 1985. Funding has remained steady at $8.6 million over the past 5 years. "Full" funding would be almost four times that much -- $30 million.

Total federal expenditures on all invasive exotic species was $631.5 million in FY 2000. This was less than 0.5% of David Pimentel's estimate that these invaders cost America $137 billion per year. In January 2001, the National Invasive Species Council released its plan, titled Meeting the Invasive Species Challenge. This report calls for numerous useful actions - but unless we provide additional funds, we will be unable to reach its goals.

Preventing Introduction of New Pests Responsibility for preventing the introduction of new damaging forest pests falls to another USDA agency, the Animal and Plant Health Inspection Service (APHIS). To be effective, APHIS must change how it operates. It should stop relying on inspection of incoming goods in hopes of finding pests. Instead, it should rely on "integrated vector management". Under this system, APHIS would apply all available control techniques at the appropriate times before, during, and after shipment to reduce the numbers and variety of organisms hitchhiking into the country. The goal should be to sterilize shipments - to prevent any unwanted organism from arriving alive. APHIS should focus its efforts on those "pathways" that carry the greatest risk of introducing pests. That means tightening regulations governing imports of crates and pallets; logs, lumber, and wood chips; and living plants (nursery stock). APHIS should depend on its inspectors to verify compliance with the regulations and to monitor incoming shipments to evaluate the effectiveness of the mandated safeguards.

Species observed by John Bartram in a 1751 trip through Pennsylvania: