Ash trees, belonging to the genus Fraxinus, are common deciduous trees found across various regions of North America. These trees typically feature an upright, oval to rounded silhouette with a dense canopy. Their leaves are compound, meaning each leaf is made up of several smaller leaflets, usually arranged oppositely along the stem. The bark of ash trees is generally light to dark gray, developing a distinctive diamond-shaped pattern as the tree matures.
The Primary Threat to Ash Trees
Ash tree decline is primarily caused by the emerald ash borer (EAB), Agrilus planipennis, an invasive wood-boring beetle. Native to northeastern Asia, including China, Korea, Russia, Mongolia, and Japan, the EAB was first detected in North America in 2002 near Detroit, Michigan, and Windsor, Ontario. It likely arrived in shipping materials, possibly as early as the late 1980s.
The EAB life cycle begins with metallic green, rice-sized adult beetles emerging from infested trees from late May to September. Females lay 40 to 200 eggs in bark crevices, hatching in about two weeks. Cream-colored, flattened larvae then bore into the tree, feeding extensively in the phloem and outer xylem, the tree’s water and nutrient transport layers. This feeding creates distinctive S-shaped, or serpentine, galleries beneath the bark, girdling the tree and disrupting its vascular system.
Larvae overwinter in sapwood chambers; the life cycle spans one to two years depending on climate and tree health. Signs of an EAB infestation include D-shaped exit holes (about 1/8 inch wide) left by emerging adults. Other indicators include S-shaped larval galleries under the bark, canopy thinning, crown dieback, epicormic shoots (sprouts from the trunk or base), vertical bark splitting, and increased woodpecker activity, often called “flecking” or “blonding,” also signal EAB presence.
Are Ash Trees Truly Endangered?
Ash trees face a severe threat. Many North American ash species, including green ash (Fraxinus pennsylvanica), white ash (Fraxinus americana), and black ash (Fraxinus nigra), are listed as critically endangered by the International Union for Conservation of Nature (IUCN) on their Red List.
Hundreds of millions of ash trees have been killed across North America. Experts anticipate the functional extirpation of the entire Fraxinus genus, meaning the species will largely disappear from native ecosystems, even if some individuals persist. In areas with severe EAB infestations, mortality rates can exceed 99%. Within about six years of an EAB invasion, an ash species can experience nearly 100% decline, with minimal natural regeneration. This rapid devastation occurs because North American ash species lack the natural defenses of their Asian counterparts, which co-evolved with EAB.
Why Ash Tree Decline Matters
Ash tree decline has significant ecological and economic ramifications. Ecologically, ash trees are a key component of North American forests, providing habitat and food for wildlife. Their nutrient-rich leaves decompose rapidly, enhancing soil quality and supporting diverse ground flora, fungi, and soil communities. They offer nesting and roosting sites for birds and bats, and their seeds feed small mammals and birds.
Ash loss creates canopy gaps, leading to increased light and invasive plant proliferation. Many arthropod species, including butterflies and moths, depend on ash trees, putting their populations at risk. Black ash, in particular, drives nutrient cycles with its high-quality leaf litter; its loss affects wetland hydrology and habitat.
Economically, the financial burden is significant. Urban ash tree removal and replacement costs in the U.S. range from $10.7 billion to $26 billion. Ohio alone projects losses between $1.8 billion and $7.6 billion, including landscape value, removal, and replacement.
Beyond direct costs, ash loss impacts wood-reliant industries. Ash wood is valued for its strength, flexibility, and shock resistance, making it a preferred material for furniture, tool handles, sports equipment, cabinetry, and flooring. Eastern U.S. ash timber is valued at about $25 billion annually. Pre-emptive harvesting initially depressed timber prices, but as supply diminishes, prices are expected to rise.
Protecting Remaining Ash Trees
Efforts are underway to mitigate emerald ash borer impact and protect surviving ash trees.
Chemical Treatments
Chemical treatments use systemic insecticides like emamectin benzoate, azadirachtin, imidacloprid, and dinotefuran. Emamectin benzoate is effective, offering two to three years of protection per application; trunk injections are preferred to minimize non-target effects. Treatments are reserved for healthy, high-value ash trees, as treating entire woodlands is impractical and trees with significant canopy thinning respond poorly.
Biological Control
Biological control introduces EAB’s natural predators from Asia, primarily tiny parasitic wasps. Species like Spathius agrili, Oobius agrili, and Tetrastichus planipennisi lay eggs on or inside EAB eggs or larvae, killing the pest. These agents undergo rigorous testing to ensure they do not harm native insects. While unlikely to eradicate EAB, biological control is a long-term strategy to reduce EAB populations to a manageable level.
Breeding Programs
Breeding programs develop EAB-resistant ash varieties. Scientists use two strategies: hybrid breeding (crossing resistant Asian ash with North American species) and traditional breeding (identifying naturally resistant native “lingering ash” trees). These rare “lingering ash” individuals, healthy despite widespread EAB mortality, are key to developing resilient genetic lines. Cloned material from these trees produces resistant offspring, a generational process that can take decades.
Quarantines and Regulations
Quarantines and regulations slow EAB spread by controlling ash material movement. A key public message is “Don’t move firewood,” as infested firewood is a primary means of long-distance EAB dispersal. Regulations cover ash logs, nursery stock, chips, mulch, and all hardwood firewood. Quarantined materials may require heat treatment, fumigation, or debarking before movement.
Public Involvement
Public involvement is important for ash conservation. Citizens can contribute by not moving firewood, reporting suspected infestations, and participating in citizen science programs like “Monitoring and Managing Ash” (MaMA). MaMA trains individuals to identify and report “lingering ash” to aid breeding programs. Community management plans involve public education and collaboration for responsible tree removal, targeted treatments, and planting diverse, non-host tree species to enhance urban forest resilience.