Yes, the emerald ash borer is still very much a threat. Since its discovery in Michigan in 2002, it has killed tens of millions of ash trees across North America, and it continues to spread into new territory. The federal government actually stopped trying to quarantine it in 2021, not because the problem was solved, but because the beetle had become too widespread to contain.
Why the Federal Quarantine Was Dropped
USDA’s Animal and Plant Health Inspection Service removed its domestic quarantine regulations for emerald ash borer in a final rule effective January 14, 2021. That decision reflected a shift in strategy, not a reduction in risk. The beetle had spread to so many states that restricting the movement of ash wood and nursery stock was no longer a realistic containment tool. Management now falls largely to individual states, municipalities, and property owners.
The practical effect is that there are fewer federal restrictions on moving firewood and ash products across state lines, which could actually accelerate spread into the remaining unaffected areas. Several states maintain their own quarantine zones, but enforcement varies widely.
How the Beetle Kills Ash Trees
Emerald ash borer larvae feed in the cambial region just beneath the bark, which is the thin layer of tissue responsible for moving water, nutrients, and sugars through the tree. As larvae chew S-shaped tunnels through this layer, they progressively cut off the tree’s internal transport system. Once enough of the cambium is destroyed, the canopy thins, branches die back, and the tree starves. Nearly all untreated ash trees in an infested area eventually die.
Every native North American ash species tested so far is susceptible. Green ash, white ash, black ash, and blue ash all fall victim, though some species decline faster than others. The beetle has even been confirmed completing its full life cycle in white fringetree, a non-ash species native to the southeastern U.S. that belongs to the same plant family. In one survey near Dayton, Ohio, four of 20 mature ornamental white fringetrees showed signs of active emerald ash borer attack, including exit holes, canopy dieback, and live larvae beneath the bark with evidence of at least three generations of use.
Cold Weather Won’t Stop It
One common hope is that harsh winters will kill off the beetle, especially in northern states and Canada. The reality is more complicated. Emerald ash borer larvae are freeze-avoidant, meaning they die if ice forms inside their bodies, but they can survive extraordinarily low temperatures as long as they don’t actually freeze. And their cold tolerance adjusts to their local climate.
Larvae overwintered in southern Ontario had supercooling points (the temperature at which they would freeze) as low as roughly minus 30°C (minus 22°F). But populations exposed to Manitoba winters developed far greater cold hardiness. Researchers measured supercooling points as low as minus 52°C (minus 62°F) in prepupae from the Winnipeg region. Individual larvae survived one hour at minus 45°C, and prepupae survived minus 50°C, as long as they avoided freezing. This plasticity means the beetle can adapt to progressively colder climates, making even Canada’s harshest provinces potentially vulnerable.
Biological Control: Helpful but Not a Cure
The USDA has released three species of tiny parasitoid wasps from the beetle’s native range in Asia. These stingless wasps target different life stages: one species can parasitize up to 90% of larvae it encounters, another kills up to 50% of larvae, and a third destroys up to 60% of eggs. Release sites are monitored for at least three years to assess whether the wasps establish self-sustaining populations.
The goal of this biological control program is not to eliminate the beetle. It’s to moderate population growth enough to slow ash tree losses. In areas where the wasps have established, they add a layer of pressure that, combined with other factors, may help keep some ash trees alive. But on their own, these wasps cannot stop an active infestation from killing mature trees.
Chemical Treatment Protects Individual Trees
If you have an ash tree you want to save, trunk injections of a systemic insecticide can be effective. Research from the U.S. Forest Service found that treated trees across a range of sizes, from about 3 inches to 11 inches in trunk diameter, showed similar protective benefits. The key requirement is consistency: trees need to be re-treated every two years for the entire duration of the outbreak in your area. Skip a cycle and the tree becomes vulnerable again.
Treatment works best when started before the tree has lost significant canopy. Once a tree is heavily infested and visibly declining, injections become far less effective because the damaged cambium can no longer distribute the insecticide through the trunk and branches. The practical takeaway is that if emerald ash borer has been detected anywhere near your area, early treatment of valued trees is far more effective than waiting for symptoms to appear.
Some Trees Survive, but Nobody Knows Why Yet
In areas where emerald ash borer has caused near-complete mortality of mature ash, researchers have found a small number of healthy trees still standing among the dead, sometimes called “lingering ash.” These trees are scattered and rare, but they’ve generated significant scientific interest. The open question is whether they carry genuine genetic resistance to the beetle or are simply the last trees to be attacked due to chance or location.
Breeding programs are exploring whether traits from these survivors could be used to develop resistant ash varieties for replanting. But this work is in early stages, and it will take decades before resistant trees, if they can be produced, could meaningfully replace what has been lost. For now, the North American ash population remains in steep decline with no large-scale solution in place.
The Ongoing Scale of the Problem
North America has an estimated 8 billion ash trees, and the beetle is now confirmed in at least 36 states and several Canadian provinces. Tens of millions of trees have already been killed. The economic toll includes not just the loss of timber and urban shade trees but the cost of removing dead trees that become safety hazards along streets, in parks, and near power lines. Municipalities have spent hundreds of millions of dollars on removal alone.
Ecologically, ash trees play outsized roles in certain habitats. Black ash dominates many wetland forests in the upper Midwest and Northeast, where no other tree species readily fills the same niche. Losing ash from these ecosystems changes water dynamics, soil chemistry, and habitat for dozens of other species. Green ash is one of the most common trees in urban landscapes because of its tolerance for poor soils and pollution, making its loss particularly visible in cities and suburbs.
The emerald ash borer is not a past crisis. It is an ongoing one, still expanding its range, still killing trees in newly infested areas, and still requiring active decisions from anyone who owns or manages ash trees.