Vermont and the Northeast Hardest Hit by Imported Forest Pests

Introduced forest pests, both insects and disease-causing pathogens, are the most urgent ecological threat to the health of forests in the United States. Nonnative pests affect all states, but Vermont and other northeastern states are hit particularly hard. Trees in this region are threatened by Asian longhorned beetle, beech bark disease, emerald ash borer, hemlock woolly adelgid, and a long list of insects and diseases imported from other continents.

Introduced pests are the only forest threat that can nearly eliminate an entire tree species in a matter of decades. Chestnut blight, caused by a fungus introduced in the early 1900s, effectively removed a once-dominant tree species, American chestnut, from eastern forests. Hemlock woolly adelgid, a more recently imported insect pest that is killing our native hemlocks, is currently spreading northward through eastern forests, including Vermont.  

Over the past several decades, scientists have conducted substantial research on the biology and the ecological and economic impacts of these pests. But little attention has focused on strengthening national policies to prevent further introductions of nonnative pests.

Recently, a team of forest scientists, economists, and policy advisors, led by forest ecologist and senior scientist Gary Lovett at the Cary Institute of Ecosystem Studies in Millbrook, New York, conducted a comprehensive review of the ecological and economic impacts of introduced forest pests. Together with the Harvard Forest’s Science Policy Exchange, dedicated to using scientific information to develop policy, the team identified dominant pathways for introduction of these pests and reviewed policy options for preventing their establishment.

Pest Introduction and Establishment

Invasions by nonnative forest pests begin with arrival at a port of entry and subsequent introduction to the country. When a pest species reaches population sizes at which extinction is no longer probable, the new arrival becomes “established.” Over time, established pest populations grow and spread.  As they spread, eradication becomes difficult or impossible, so it makes much more sense to prevent them from entering the country in the first place.

The data reviewed by Lovett and his research team indicate that two to three new exotic forest pests are established in the United States each year. Most introduced pests make their way to the United States in wood packaging (crates and pallets) or on live plants imported for the nursery trade. Past introductions include the insect responsible for beech bark disease, which causes extensive damage to American beech —one of Vermont’s dominant hardwood trees — and the Dutch elm disease fungus, which removed the stately American elm from the landscape and many city streets.

Despite policies begun in the early 20^th century designed to reduce pest introductions, establishment of imported pests continues unabated. More than 400 different nonnative pests are damaging U.S. forests and currently more than 50 of them occur in Vermont, according to U.S. Forest Service data.

Introductions are most common in northeastern states because of the region’s high volume of commerce, the relatively high diversity of tree species in eastern forests, and the taxonomic similarity of eastern tree species to trees in forests of Asia and Europe, the United States’ most prevalent trade partners.

The impacts of nonnative pests on forests and landscapes typically occur in two phases. First, there is a physical disturbance phase in which targeted tree species, or host species, are damaged or killed by the pest. The second phase takes decades to centuries and involves changes in tree species and habitats that cause cascading effects throughout the ecosystem.

The death of hemlock by hemlock woolly adelgid opens up streamside forest canopies to shrub and hardwood species that do not provide adequate snow cover in winter or shade and cooling in summer. These changes affect survival of wildlife and fish populations and impact ecosystem functions, such as nutrient cycling.

Economic Costs and Policy

Economic impacts of pest damage to trees cost well over $4 billion per year nationally for timber producers, residential property owners, and local, state, and federal governments. Costs fall hardest on cities, towns, and homeowners. Tree removal, protection, and replacement associated with emerald ash borer invasions have cost property owners more than $1 billion a year in urban areas and closer to twice that if suburban trees are included.

Current polices to prevent new introductions are helpful but not sufficient in the face of growing global trade. Lovett and his team suggest that policies should focus on preventing importation of new pests, especially those entering the country in wood packaging and on live plants.

The authors indicate that improvements could be made in the practices of exporting countries to ensure clean shipments of plants and wood products. They also suggest improving inspections at ports of entry, and post entry measures such as quarantines, surveillance, and eradication programs. Stronger policies to reduce establishments of new forest pests would shift the major costs of control to the sources and lessen the economic burden to homeowners and municipalities.