Ash trees (Fraxinus genus) are valued across North America for their hardiness, shade canopy, and strong wood. They are a common feature of urban landscapes and forests, but they face an unprecedented crisis. A severe, widespread mortality event is decimating ash populations, threatening to eliminate the species from many ecosystems. This decline is the result of a single, highly destructive invasive insect, not natural disease or climate change.
The Emerald Ash Borer Invasion
The primary driver of this ecological disaster is the Emerald Ash Borer (EAB), Agrilus planipennis, a small, metallic-green beetle native to northeastern Asia, including parts of China, Korea, and Russia. This invasive pest was first discovered in North America near Detroit, Michigan, in 2002, likely arriving years earlier in solid wood packing materials from its native range. The beetle has since spread rapidly, largely due to the human transport of infested firewood, and is now established in dozens of states and provinces across the continent.
The adult EAB is relatively small, measuring only about one-half inch long, and its metallic green color often allows it to go unnoticed in the canopy. After emerging in late spring and early summer, females lay numerous tiny eggs in the crevices of ash bark. The larvae that hatch from these eggs bore through the bark to begin feeding on the tree’s internal tissues.
The EAB life cycle typically takes one to two years to complete. In its native Asia, the beetle generally attacks only ash trees that are already weakened or stressed, but North American ash species lack natural defenses against this new threat. As a result, EAB larvae successfully infest and kill both healthy and stressed ash trees, leading to nearly complete mortality in infested areas.
How EAB Causes Tree Death
The mechanism of death centers on the larval feeding activity beneath the bark. The larvae tunnel and feed on the cambium and phloem layers, which are the tree’s vascular systems responsible for transporting essential resources. The phloem moves sugars, produced during photosynthesis in the leaves, down to the roots for storage and growth.
As the larvae grow, they create winding, S-shaped tunnels, known as galleries, that effectively sever the connection between the leaves and the roots. This widespread tunneling disrupts the flow of nutrients and water throughout the tree, a process known as girdling. Once the tree is girdled, the roots are starved of sugars, leading to death within three to five years of initial infestation.
The feeding also compromises the tree’s ability to transport water from the roots to the canopy through the outer sapwood, or xylem, further stressing the tree. This physiological impact, rather than the adult beetle’s feeding on leaves, is what makes the EAB so devastating to ash populations.
Secondary Threats to Ash Trees
While the Emerald Ash Borer is the dominant cause of ash mortality, other factors can weaken trees and contribute to their decline. One disease is Ash Yellows, which is caused by a bacteria-like organism called a phytoplasma. This organism infects the phloem, leading to stunted growth, yellowing leaves, and premature canopy dieback.
Other common afflictions include Ash Anthracnose, a fungal disease that causes dark lesions on leaves and premature defoliation, particularly in cool, wet spring weather. Though rarely fatal alone, chronic Anthracnose can significantly stress a tree. These diseases, along with environmental stressors like drought, make ash trees more susceptible to EAB attack or accelerate the death of an already infested tree.
Recognizing Infestation and Management Strategies
Early detection is crucial for managing the EAB threat. Homeowners should look for several telltale signs that indicate an infestation:
- The small, D-shaped exit hole (about one-eighth inch in diameter) left when the adult beetle emerges from the bark.
- Canopy dieback, often starting at the top of the tree.
- The appearance of epicormic sprouts, which are small, weak shoots growing rapidly from the trunk or base as the tree attempts to compensate for lost nutrient flow.
- Increased woodpecker activity, resulting in “blonding” (the removal of outer bark as they forage for larvae).
Management options primarily involve insecticide treatment or tree removal. Systemic insecticide treatments, typically applied as trunk injections or soil drenches, can protect ash trees from EAB. These treatments are most effective as a preventative measure or when the infestation is caught early, requiring application by a certified professional every one to two years.
For trees that are heavily infested or significantly damaged, removal is often the only practical option. They pose a safety risk due to loss of structural integrity. Following removal, it is recommended to replace the ash tree with a different species to enhance the diversity and resilience of the local tree population.