Ash trees (genus Fraxinus) are not an invasive species in North America, but they are currently facing an immense threat that has led to widespread decline and death. The confusion arises because the primary cause of this devastation is an invasive insect. North American Ash species, such as White Ash and Green Ash, are native trees that have historically been valuable parts of the continent’s forests and urban landscapes. The issue is not that the tree itself is a problem; rather, it is the target of a non-native pest that has no natural predators here.
Defining Native Status
A native species occurs naturally in a particular ecosystem without human intervention, having developed specific relationships with other organisms. Native Ash trees are integral to North American ecology, providing habitat and food for wildlife and playing a role in the forest structure. An invasive species, by contrast, is an organism introduced from a different region that causes economic or environmental harm.
Common native species like White Ash (Fraxinus americana) and Green Ash (Fraxinus pennsylvanica) are foundational to many eastern and central North American forest types. White Ash is found across a wide range, from Nova Scotia to Texas. These trees are highly valued for their strong wood, which is used in furniture and tools, including baseball bats.
While North American Ash species are native, some non-native species, like the European Ash (Fraxinus excelsior), have been introduced for landscaping. These introduced trees can sometimes become problematic locally, but they are not the primary ecological concern. The crisis facing North American Ash trees centers on protecting the native species from a foreign threat.
The Emerald Ash Borer Crisis
The true invasive threat is the Emerald Ash Borer (EAB), a small, metallic-green beetle scientifically known as Agrilus planipennis. EAB is native to northeastern Asia, including China, Japan, and Russia, where it is a non-destructive insect. It was first detected in North America near Detroit, Michigan, in 2002, likely arriving years earlier inside solid wood packaging materials.
The adult beetle lays its eggs in the crevices of ash tree bark. Once hatched, the larvae bore into the tree and feed on the phloem and cambium layers just beneath the bark. This larval feeding creates characteristic S-shaped, or serpentine, galleries that disrupt the tree’s vascular system. This damage effectively girdles the tree, cutting off the transport of water and nutrients, which rapidly starves and kills the tree.
The mortality rate in native North American Ash populations is extremely high because the trees have no natural defenses against this pest. Unlike Asian ash species, which have evolved resistance, North American Ash trees lack the chemical defenses needed to stop the larvae. Unprotected trees often show severe canopy dieback within two years of infestation and typically die within three to four years. Since its introduction, EAB has killed tens of millions of ash trees across over 35 states and several Canadian provinces, establishing itself as the most destructive invasive wood-boring insect in North American history.
Key Characteristics for Identification
Identifying an Ash tree is important for determining if a property is at risk from the Emerald Ash Borer. A primary way to identify a Fraxinus species is by its opposite branching pattern, where the main branches and buds grow directly across from one another on the twig. This pattern is uncommon and helps distinguish ash from most other deciduous trees.
The leaves of an Ash tree are pinnately compound, consisting of a central stalk with multiple smaller leaflets attached. Most Ash species have between five and eleven leaflets per compound leaf. The only common tree with a similar opposite and compound leaf structure is the Boxelder, which typically has only three to five leaflets.
Mature Ash trees, such as White Ash and Green Ash, develop a distinctive bark pattern that looks like interweaving ridges forming a diamond shape. Younger trees have smoother bark, so the combination of opposite branching and compound leaves is the most reliable identification method. The tree also produces single, winged seeds called samaras that hang in clusters.
Management and Response
Homeowners who have identified an Ash tree must decide between protective treatment and eventual removal. For healthy, high-value trees, preventative chemical treatment is the recommended long-term option. These treatments involve systemic insecticides that are either injected directly into the trunk or applied to the soil at the base of the tree.
A highly effective insecticide is emamectin benzoate, which is injected into the trunk and can protect the tree for up to three years with a single application. Other options, such as soil-applied imidacloprid or dinotefuran, are applied annually and are effective only for smaller trees. Treatments must be applied before an infestation becomes severe, as the tree cannot be saved once significant damage is visible.
If a tree is already heavily infested or is not considered a high-priority landscape tree, removal is the safest course of action. Dead Ash trees become brittle and pose a significant falling hazard, making professional removal necessary and often costly. After removal, it is recommended to replace the Ash tree with a non-host species to promote biodiversity and prevent future pest issues.