Large ash trees can be saved from emerald ash borer with timely insecticide treatment. Here, the ash tree on the left received injections of emamectin benzoate, while the tree on the right did not. (Photo originally published in Sadof et al 2023, Journal of Integrated Pest Management)

By Cliff Sadof, Ph.D.

When a mega-invasive pest like emerald ash borer (Agrilus planipennis) arrives in a new urban forest, misinformation can spread in ways that cause needless expense and a massive loss of trees. Tired of seeing this pattern recur with the spread of emerald ash borer, my colleagues Deb McCullough, Ph.D., of Michigan State University and Matt Ginzel, Ph.D., of Purdue University and I gathered common myths we’ve encountered over the last two decades working with this pest since its detection in the U.S. In an article published in August in the Journal of Integrated Pest Management, we then addressed and confronted each of these misconceptions with our own research and that conducted by colleagues, published in refereed scientific journals.

Two decades after the arrival of the emerald ash borer (Agrilus planipennis) in North America, plenty of misinformation remains about how best to manage its impact. A group of experts on this pest offer bust several myths about emerald ash borer in a new article in the Journal of Integrated Pest Management. (Photo by Steven Ausmus, USDA Agricultural Research Service)

Emerald ash borer (EAB) is the most destructive forest insect to invade North American forests. Before EAB arrived, ash trees were common in city landscapes, parks, and along roads. This is largely due to the trees’ rapid growth, few serious pests, and ability to contend with the stresses of urban environments. Unfortunately, feeding by EAB larvae destroys the inner bark of a tree that carries nutrients from leaves to roots and the sapwood that carries water from the roots to the canopy. This starves and kills ever increasing parts of the tree from the top down. Dead branches dry out, become brittle, and are hazards for nearby property and people. Recent estimates suggest that over the next three decades, the loss of over 135 million urban ash trees will saddle U.S. communities with $1.8 billion annually in additional maintenance costs. To date, at least 36 states and five Canadian provinces are dealing with EAB, including Oregon, where EAB was detected in 2022. The impacts and spread of EAB are a problem that cannot be ignored.

After two decades of research, much has been learned about this once obscure insect. Although it seems all ash species native to North America can be colonized, host preference and resistance of trees to EAB vary. For example, healthy blue ash (Fraxinus quadrangulata) trees are resistant to EAB, and white ash (F. americana) trees show intermediate resistance, while green ash (F. pennsylvanica) trees are highly preferred and readily killed. Although research is under way to identify and propagate individual ash trees that demonstrate some level of resistance to EAB, such efforts cannot replace the hundreds of millions of ash trees that have already been killed.

Chemical control of EAB remains essential for maintaining the health of existing urban forests in areas invaded by this pest. Systemic insecticides that are applied to the base of trees are transported within the xylem as water moves to branches and leaves. Adult beetles that feed on leaves of a treated tree are killed, as well as larvae feeding beneath the bark. Ideally, systemic insecticide products are applied in spring or early summer to control newly emerged adult beetles, who must feed on leaves throughout their 3–6-week life span. Controlling recently emerged beetles (within 2–3 weeks of emergence) can prevent female beetles from completing maturation and laying eggs. This can also create some level of “associative protection” when EAB females that emerge from nearby untreated trees feed on toxic leaves of treated trees.

EAB populations and their impact on ash trees fluctuate in predictable patterns that can inform long-term chemical protection strategies. For example, long term studies showed spring trunk injections of emamectin benzoate provided three years of nearly complete protection, even as EAB populations and associated damage increased exponentially and nearly all untreated trees died. An essential element of protecting trees, however, involves beginning treatment while trees remain healthy and can transport insecticide to the canopy. As untreated ash trees die, the carrying capacity for EAB drops because there is little live ash phloem for larval development. This may provide an opportunity to extend the time between insecticide treatments. Such long-term strategies will hinge on the ability to monitor trees and respond when early symptoms of EAB attack are noted.

Symptoms and signs of emerald ash borer (Agrilus planipennis) infestation include : a) late-stage larva extracted from its gallery, b) serpentine “zig-zag” galleries that etch the outer sapwood as larvae feed on inner bark (phloem); c) bark split over an old larval gallery; d) holes in the bark left by woodpeckers preying on late stage EAB larvae, e) bark scraped by woodpeckers preying on EAB larvae, f) stump sprouts and epicormic shoots on heavily infested trees. (Photo originally published in Sadof et al 2023, Journal of Integrated Pest Management)

Our analysis of the literature clearly demonstrates that it is more economically advantageous for communities to preserve ash trees using systemic insecticides compared with removal and replacement. Allowing nature to take its course is a budget-busting option that compresses costs of tree removal into 3-6 years when standing dead ash trees must be removed to protect property and people from falling limbs. This fact can be readily illustrated through using a web-based cost calculator that uses local estimates of treatment, replacement, and removal costs.

Regional cooperation on the national EAB.info website has been crucial in providing a reliable, centralized resource for extension information. Available information includes a regularly updated regional bulletin that discusses advantages and limitations of various control chemistries and application techniques.

Cliff Sadof, Ph.D., is a professor in the Department of Entomology at Purdue University. Email: csadof@purdue.edu.