Emerald Ash Borer: What Works, What Doesn’t, and Why It Matters
The arrival of the Emerald Ash Borer (EAB) has changed the future of ash trees across North America. This small metallic-green beetle attacks all native ash species and has already killed tens of millions of trees. In many communities, untreated ash trees experience mortality rates approaching 99% once infestations become established. The good news is that modern arboriculture has developed several treatments that are highly effective at protecting valuable ash trees. The challenge is separating proven science from wishful thinking.
Proven Treatments That Work
Trunk Injection with Emamectin Benzoate
The gold standard for Emerald Ash Borer management is trunk injection using emamectin benzoate. Numerous university studies have demonstrated greater than 99% larval control, often with protection lasting two to three years from a single application. Research has also shown that moderately infested trees can recover when treated before canopy decline becomes severe. (auf.isa-arbor.com)
For high-value ash trees in residential landscapes, parks, campuses, and municipal settings, trunk injection remains the most reliable and cost-effective treatment available.
Other Systemic Insecticides
Additional insecticides such as imidacloprid, dinotefuran, and azadirachtin can provide varying levels of protection when applied properly. These products may be useful in certain situations, but they generally require more frequent applications and have shown less consistent performance than emamectin benzoate. (CSU Office of Engagement)
Early Detection and Monitoring
Ash trees treated before significant canopy loss have the highest probability of long-term survival. Once canopy decline exceeds roughly 30%, the likelihood of successful recovery decreases substantially. Monitoring for woodpecker activity, bark splitting, D-shaped exit holes, and canopy thinning allows treatment to begin before irreversible damage occurs. (CSU Office of Engagement)
Biological Control
Researchers have released several species of parasitic wasps that attack Emerald Ash Borer larvae. While these biological controls help slow population growth across larger landscapes, they are not considered a standalone solution for protecting individual landscape trees. (Wikipedia)
Treatments and Methods That Have Not Been Proven to Work
Biodynamic Preparations
Some biodynamic practitioners have suggested clay, manure, sand, or herbal preparations painted onto trunks. While these treatments may have value within biodynamic philosophy, there is currently no peer-reviewed evidence demonstrating that they prevent Emerald Ash Borer infestation or protect ash trees from larval feeding beneath the bark.
The beetle’s larvae live inside the cambium layer under the bark, making surface-applied preparations unlikely to affect them in a meaningful way.
Compost Teas and Foliar Sprays
Improving soil biology and tree vigor can benefit overall tree health, but there is no evidence that compost teas alone prevent Emerald Ash Borer attack. Healthy ash trees are still readily attacked and killed when EAB populations become established. (Wikipedia)
Fertilizer Treatments Alone
Many homeowners assume stronger growth equals better resistance. Unfortunately, Emerald Ash Borer attacks healthy trees as readily as stressed ones. Fertilization may improve vigor but does not stop larvae from tunneling beneath the bark.
Essential Oils, Garlic Sprays, and Home Remedies
Numerous internet remedies claim to repel Emerald Ash Borer. To date, none have demonstrated reliable protection under field conditions. These approaches should not be considered substitutes for proven insecticide treatments.
Wrapping Trunks or Painting Bark
The beetles can lay eggs in bark crevices throughout the tree canopy and upper trunk. Physical barriers such as trunk wraps, paints, or homemade coatings have not been shown to provide meaningful protection against infestation.
A Practical Arborist Perspective
For property owners hoping to save valuable ash trees, the evidence is remarkably clear. Trunk injections using emamectin benzoate are supported by years of university research and real-world success. While improving soil health, mulching, irrigation during drought, and general tree care can support overall vigor, none of these practices replace proven Emerald Ash Borer treatments.
In arboriculture, there is a significant difference between a treatment that “might help tree health” and one that has been scientifically demonstrated to stop Emerald Ash Borer. When the goal is preserving mature ash trees, decisions should be based on evidence rather than hope.
For many ash trees in Eugene and throughout the Pacific Northwest, proactive treatment before infestation becomes severe may be the difference between decades of continued shade and the eventual cost of removal and replacement.
An important consideration regarding biodynamic trunk coatings is that Emerald Ash Borer adults do not confine their activity to the lower trunk. Female beetles may lay eggs throughout the tree, including on major scaffold limbs and upper portions of the trunk. If the intended mechanism of a biodynamic preparation is to repel adult beetles from landing or laying eggs, it raises a practical challenge: the treatment would likely need to be applied across the entire tree surface where beetles may deposit eggs, not merely near the base of the trunk.
On mature ash trees, complete coverage of the trunk, scaffold branches, and upper canopy is generally impractical. Even if such coverage were achieved, there is currently no peer-reviewed research demonstrating that biodynamic coatings repel Emerald Ash Borer adults or prevent successful infestation. This does not necessarily mean the preparations provide no benefit to overall tree health, but it does mean their effectiveness as a direct Emerald Ash Borer control method remains unproven.
From a biological standpoint, any treatment intended to repel egg-laying adults must intercept the insect before eggs are deposited. Once larvae hatch and tunnel beneath the bark into the cambium, surface-applied treatments are unlikely to affect them. This is one reason systemic insecticides, which move through the tree’s vascular system and target larvae feeding beneath the bark, have consistently outperformed surface treatments in scientific trials.
Another consideration is the composition of many biodynamic trunk coatings, which often include clay, soil, compost, or manure-based ingredients. Agricultural and horticultural literature has long recognized that soil, clay, and manure can serve as reservoirs or vectors for a variety of microorganisms, including potential plant pathogens. Depending on their source and handling, these materials may already contain fungi, bacteria, or other organisms capable of causing disease under favorable conditions. While many microorganisms in these materials are harmless or even beneficial, applying untreated biological materials directly to tree bark introduces a complex microbial community whose effects are not always predictable. For this reason, claims that such coatings inherently protect trees from pests or disease should be evaluated carefully and supported by evidence.
From a biological standpoint, any treatment intended to repel egg-laying adults must intercept the insect before eggs are deposited. Once larvae hatch and tunnel beneath the bark into the cambium, surface-applied treatments are unlikely to affect them. This is one reason systemic insecticides, which move through the tree’s vascular system and target larvae feeding beneath the bark, have consistently outperformed surface treatments in scientific trials.
If a property owner or other responsible decision maker wishes to proceed despite this uncertainty, the decision ultimately rests with the person who has the authority to authorize such an experimental treatment and who is willing to accept whatever outcome may result.
