Emerald Ash Borer
Emerald Ash Borer (EAB) is an invasive pest that kills ash trees. It was first discovered in the United States in Michigan in 2002. It was found in Naperville in 2008.
The larval stage of EAB feeds on the inner bark of ash trees, disrupting the tree's water and nutrient circulation system, ultimately killing the tree. The only way to save an ash tree from EAB is to provide treatment. Learn more about the insect at the EAB Information Network.
An adult EAB on an ash leaf.
Damage from feeding larvae.
Ash trees are native to Illinois and commonplace in Naperville. Before EAB was discovered in Naperville, there were over 16,000 ash trees on the parkways. At the time, even more ash trees existed on private property. It is estimated that over 50,000 ash trees once lived on private property prior to the arrival of EAB.
Do you own an ash tree? Use this guide to identify ash trees in your area.
In 2009, the City enacted a containment strategy designed to slow or stop the spread of EAB. The strategy involved treating all ash trees in a half mile radius around known infestations. Despite the effort, the infestation continued to spread throughout the City (containment of EAB has proved nearly impossible nationwide). In 2012, facing a City-wide infestation, City officials announced a comprehensive EAB management program. The comprehensive EAB management program consists of four major components:
- The annual inspection of ash trees to assess their health
- The treatment of healthy ash trees
- The removal of poor ash trees that no longer benefit from treatment
- The replacement of removed ash trees with a diversity of species
How are we doing?
The comprehensive management program has proven to be an efficient, cost-effect method to manage EAB.
Ash Tree Survival
Since treatment began in 2012, 66% of the trees have survived. This equates to losing just over 4% of the trees each year. By contrast, communities that do not treat have been forced to remove as many as 50% of their ash trees in a single year.
Each summer, the surviving trees are assessed for EAB damage. The vast majority of the survivors are in "good" or "fair" condition and exhibit only minor damage. Trees in "marginal" condition show significant damage while "poor" condition trees are damaged enough that continued treatment is not viable. Poor trees are scheduled to be removed and replaced.
A Tale of Two Treatments
Two treatment products are used to fight EAB. Larger trees receive a trunk injection of Tree-Age every 2-3 years. Smaller trees receive an annual soil injection of imidacloprid. Both treatments are administered in the spring and early summer. About half of the trees receive Tree-Age.
Both treatments are effective, but Tree-Age has consistently performed better than imidacloprid. Trees treated with Tree-Age had a greater chance of survival over the past 9 years and also appear healthier and more vigorous during the annual condition inspection.
Removing and Replacing Ash Trees
The continuing removal and replacement of "poor" condition ash trees from the parkways is an important part of the EAB management plan. Treatments are keeping the vast majority of ash trees alive, which is enabling the gradual removal and replacement of ash trees.
Had the City never treated against EAB, annual removals were anticipated to reach into the thousands and strain the City's budget and operational capacity. Instead, treatments have kept the tree removal work load at a manageable level. This has enabled our forestry crew to keep up with other work including tree trimming and the removal of other dying trees.
Annual removals peaked in 2013, one year after City-wide treatment began. Removals have since reduced to a level that can be can be managed by the public works department.
Choosing the right replacement tree is very important. Replacement trees are selected from a diverse group of species, many of them native. Increasing species diversity will make the urban forest less susceptible to any single pest in the future.
Gradually transitioning away from ash trees will allow residents to enjoy the benefits of mature ash trees today while simultaneously seeding a diverse and strong tree canopy for the future.
The Cost Benefit
Treating against EAB helps preserve the urban tree canopy and reduces strain on the City's crews, but it also makes financial sense. The cost to keep the ash trees alive is significantly less than the economic and environmental benefits provided by the trees.
Naperville's parkway ash trees are estimated to provide $2,700,000 in environmental and economic impact. The benefits significantly outweigh the $899,000 annual cost of treatments, removals and replacements.
How was the annual benefit of $2,700,000 determined? The City uses the I-Tree assessment tool, a nationally-recognized method for calculating the value of urban trees. The model considers a tree's effect on property values, energy consumption, air quality, carbon sequestration, and storm water interception. You can calculate the benefit of your own trees using the tree benefits calculator.
For an individual tree, treatment is significantly cheaper than removal. On average, it costs about $30 a year to treat a tree, while the average cost to remove and replace an ash tree is nearly 20 times higher at $600 a tree.
The goal of the EAB program continues to be cost-effective preservation of the urban canopy for today and tomorrow. This includes continued treatments, removals, and replacements.
Research indicates that the EAB population crests like a wave. The initial population starts low but builds exponentially as EAB feeds on untreated ash trees in an area. Untreated ash trees die off rapidly over several years until they are wiped out. Having decimated its own food source, the EAB population then begins to crash.
Aggressive treatment, like the City has been engaged in for the last 9 years, is crucial for ash tree survival during the explosion of the EAB population. Once the population begins to decline, a reduction in treatment dosages is possible. Determining a decline in EAB is difficult, but it is likely already occurring in Naperville. For this reason, in 2018 the City began treating some trees on a 3-year Tree-Age cycle instead of a 2-year cycle.
Reducing Program Costs
Starting in 2018, a select group of trees was shifted to a 3-year Tree-Age treatment cycle. The performance of this group has been tracked as compared to the trees on a 2-year cycle. As positive results have been seen with the 3-year cycle, the City will continue to transition more trees to the longer treatment interval.
Strategic adjustments to treatment levels will bring cost reductions over time. Gradually transitioning Tree-Age treatments from a 2-year cycle to a 3-year cycle will reduce costs by about 30%. As the EAB population continues to decline, the City will continue to find ways to reduce cost by further reducing treatments.
The only alternative to treatment is the complete removal and replacement of all ash trees. Stopping treatment now would cause all the ash trees to die within 5 years or less. A five-year removal plan would cost more than twice as much as treatment.
Use the tool below to compare the cost of treatment (Transition to 3 Year Cycle) to the cost of removal and replacement of all ash trees.