The emerald ash borer (EAB), Agrilus planipennis, is a small, invasive beetle native to northeastern Asia. Accidentally introduced to North America, likely via wood packaging materials, it was first identified near Detroit, Michigan, in 2002. The EAB has since spread rapidly, becoming one of the most destructive invasive forest insects encountered. Its presence has led to the death of millions of ash trees (Fraxinus spp.), initiating cascading ecological effects that fundamentally alter forest and urban ecosystems. The long-term consequence is the functional extirpation of native ash species across large regions.
Mechanism of Ash Mortality and Canopy Loss
The widespread death of ash trees begins when the adult female beetle lays her eggs in the crevices of the bark. Once hatched, the larval stage is responsible for the destruction, burrowing into the layers directly beneath the outer bark. They feed specifically on the phloem and the cambium, creating characteristic S-shaped galleries.
The phloem transports sugars produced during photosynthesis from the leaves to the roots. The larvae’s extensive feeding cuts off this flow of nutrients, effectively girdling the tree internally. While they do not typically feed on the xylem, the disruption of the phloem and cambium quickly starves the roots.
The tree’s ability to transport resources is compromised, causing the canopy to thin and branches to die back. North American ash species have not evolved defenses against this pest, leading to near-complete mortality (often exceeding 99%) within six to ten years of the beetle’s arrival. This mass die-off results in the sudden loss of a major canopy component, creating standing dead trees (snags) and eventually, a large pulse of coarse woody debris on the forest floor.
Alterations to Forest Understory and Plant Succession
The rapid death of mature ash trees creates significant openings in the forest canopy, fundamentally changing light and moisture conditions on the forest floor. This gap formation allows a massive influx of sunlight to reach the understory, previously a shaded environment. This change creates an opportunity for certain plant species to proliferate.
This disturbance often favors the growth of light-loving, non-native invasive shrubs and plants, such as Amur honeysuckle (Lonicera maackii), buckthorn, and multiflora rose. These invasive species thrive in the high-light environment, forming dense thickets that outcompete native tree seedlings and understory flora. The resulting invasive growth suppresses the regeneration of native tree species, hindering forest succession.
The loss of ash removes an entire generation of future trees, as ash seed production and seedling survival decline dramatically once the EAB is established. The forest’s long-term composition shifts away from ash toward other species, or toward a degraded state dominated by invasive shrubs. In black ash wetlands, canopy loss can lead to increased water levels and longer periods of standing water, potentially converting forested wetlands into marsh-like conditions.
Consequences for Dependent Wildlife
The disappearance of a dominant tree species initiates a ripple effect through the food web, impacting numerous wildlife species. Nearly 300 species of arthropods, including insects and spiders, are associated with ash trees in North America. A significant number of these are dependent on ash for at least one life stage, placing them at high risk of local extinction.
Birds that rely on ash seeds (such as chickadees and cardinals) and certain mammals (like squirrels and chipmunks) experience a sudden loss of forage. The ash canopy also provides nesting and roosting habitat for many bird species, which is lost once the trees die and fall. The initial phase of the EAB invasion does, however, temporarily benefit some insect predators.
Woodpeckers experience a temporary surge in food availability as they feast on the EAB larvae beneath the bark. Their increased foraging activity is often one of the first visible signs of an infestation. However, as the ash population collapses and the EAB food source dwindles, the long-term outlook for these specialist predators involves a corresponding population decline. Dead snags also provide temporary habitat for cavity-nesting birds and mammals, but these snags eventually fall and decay, leading to a loss of this resource.