Invasive Species Alert: Top 10 Threats to American Forests

The Top 10 Invasive Threats to American Forests

Invasive pests and diseases are among the greatest threats facing forests and landscape trees in the United States. Since the introduction of chestnut blight more than a century ago, non-native organisms have reshaped entire ecosystems, eliminated dominant tree species, and caused economic losses measured in the hundreds of billions of dollars.

Understanding the most dangerous current threats helps homeowners and communities take action before these invaders reach their areas. Here are the top 10 invasive threats to American trees today.

1. Emerald Ash Borer

The emerald ash borer (EAB) has killed hundreds of millions of ash trees since its discovery in Michigan in 2002. Now present in more than 35 states, this metallic green beetle from Asia has effectively eliminated untreated ash trees from vast areas of the eastern and central US. Larvae feed beneath the bark, girdling the tree's nutrient transport system.

2. Spotted Lanternfly

The spotted lanternfly (Lycorma delicatula) is a sap-feeding planthopper from Asia that threatens grapes, tree fruit, hardwoods, and ornamentals across the Northeast. Its rapid spread and broad host range make it a major agricultural and forestry concern. Quarantine zones now cover large portions of Pennsylvania, New Jersey, New York, and neighboring states.

3. Hemlock Woolly Adelgid

This tiny, aphid-like insect from East Asia feeds on the base of hemlock needles, gradually killing the tree over several years. Eastern hemlock and Carolina hemlock are both highly susceptible. The adelgid has devastated hemlocks from Georgia to New England, eliminating a keystone species from stream-side ecosystems.

4. Asian Longhorned Beetle

The Asian longhorned beetle (ALB) attacks maples, birch, elm, willow, and other hardwoods. Eradication efforts in several US cities have required the removal of tens of thousands of trees. Unlike many invasive pests that are established across large areas, ALB outbreaks have been contained through aggressive detection and removal — making early reporting critical.

5. Sudden Oak Death

Caused by the water mold Phytophthora ramorum, sudden oak death has killed millions of tanoak and coast live oak trees in California and Oregon. The pathogen has an unusually wide host range, with more than 130 susceptible plant species, and can spread through the nursery trade.

6. Thousand Cankers Disease

This disease results from the combined attack of the walnut twig beetle and the fungus Geosmithia morbida. It threatens black walnut — one of the most valuable hardwood species in North America. Originally a western US problem, it has been detected in several eastern states where black walnut is ecologically and economically important.

7. Laurel Wilt

Caused by the fungus Raffaelea lauricola and spread by the non-native redbay ambrosia beetle, laurel wilt has devastated redbay trees across the southeastern US and now threatens the avocado industry in Florida. The fungus clogs the tree's water-conducting vessels, causing rapid wilting and death.

8. Oak Wilt

While the oak wilt fungus (Bretziella fagacearum) may be native, its rapid spread through interconnected root systems and insect vectors makes it behave like an invasive threat. Oak wilt kills red oaks within weeks and can devastate entire neighborhoods. It is particularly severe in Texas, the Upper Midwest, and the Mid-Atlantic states.

9. Pine Bark Beetles (Multiple Species)

Several bark beetle species, including the mountain pine beetle, have caused unprecedented die-offs across western forests. While native, these beetles are behaving invasively due to climate-driven conditions — warmer winters, extended drought, and dense forests from fire suppression have fueled outbreaks killing billions of conifers.

10. Gypsy Moth (Spongy Moth)

Now officially renamed the spongy moth, this European import defoliates millions of acres of hardwood forest annually in the northeastern US. While most trees survive occasional defoliation, repeated attacks weaken them and make them susceptible to secondary killers. The moth continues to expand its range westward and southward.

How Invasive Species Arrive

Most invasive tree pests reach the US through:

• International trade — Packing materials, nursery stock, and solid wood products are common pathways
• Firewood movement — Domestic transport of firewood spreads pests to new areas
• Ornamental plant trade — Infested or infected nursery plants can introduce pathogens far from their origin
• Natural spread — Once established, invasive species expand their range through flight, wind, and water

What You Can Do

• Do not move firewood — Buy it where you burn it, and burn it all
• Report unusual insects or tree symptoms to your state agriculture department
• Use our diagnosis tool to identify potential invasive pest or disease symptoms
• Support quarantine programs and comply with movement restrictions
• Plant diverse species to reduce vulnerability to any single threat
• Find a qualified arborist who can help identify and manage invasive pest issues

Frequently Asked Questions

Q: What is the most destructive invasive tree pest in US history? A: By total trees killed, the emerald ash borer and chestnut blight are arguably tied. Chestnut blight eliminated approximately 3.5 billion American chestnut trees in the early 1900s. The emerald ash borer has killed hundreds of millions of ash trees and threatens approximately 8 billion more.

Q: Can invasive pests be eradicated once they are established? A: Rarely. The Asian longhorned beetle has been successfully eradicated from several localized infestations through aggressive detection and tree removal. However, once a pest becomes widely established (like EAB), eradication is generally not possible, and management shifts to containment and mitigation.

Q: How can I tell if my tree is affected by an invasive pest? A: Look for unusual symptoms like rapid decline, unexpected defoliation, boring dust or exit holes in bark, unusual insects on foliage, or canopy dieback. Compare your observations to known pest profiles on our ailment pages or use the diagnosis tool for guided identification.

Q: Are native trees more resistant to invasive pests than non-native trees? A: Generally no. Native trees often have no evolutionary defenses against non-native pests, which is why invasive organisms can be so devastating. Co-evolved pests and hosts from the invader's home range typically have a more balanced relationship.