Emerald Ash Borer adult on leaf. (Photo by David Cappaert, Michigan State University)

by Ken LeRoy and Bill Hengst

The massive white ash tree at the parking lot behind the Baptist Church at the top of the Hill has been growing there since before the Civil War. The diameter of the trunk at 4.5 feet above ground measures 66 inches, the circumference is 17.25 feet. The bark is so thick and the fissures so deep.

This ash is one of the largest of it's kind and one of the oldest trees in Chestnut Hill.

This incredible tree has weathered many a winter, has persisted through many hot summers, providing cooling shade for generations of people passing under its canopy. Today providing a shady spot to park a car, yesteryear providing a shady spot for horses pulling carts. How many lightning storms has this tree survived? How many construction events? And now? This grand old tree will likely succumb to the emerald ash borer (or EAB).

EAB was first detected in the United States in 2002, near Detroit, Mich. It since has spread rapidly throughout the forests, cities and towns in the Midwest, East Coast, and the South. As of this spring it had been confirmed in 30 states and two Canadian provinces. So far, it has killed more than 100 million ash trees. Because the borer insect is not native to North America, there are no natural predators to keep it in check. Individual ash trees can be protected, however, by treating them with an insecticide.

The ash borer is a green iridescent insect that could fit on the head of a penny. It starts feeding on the leaves of ash trees in spring and summer. The first signs of an EAB infestation is a general decline in the crown of the tree, with dead and dying branches and suckers sprouting below the dead limbs and along the trunk; then flecking off of the bark ridges as woodpeckers try to get at the larvae.

But the real damage occurs after the female borer lays her eggs under the bark of the tree. Soon the eggs hatch into larvae that burrow inside the trunk into the cambium layer, where they feed on the water and nutrients in the circulatory system, thereby starving the tree and accelerating its decline.

The larvae overwinter inside the trunk and in spring morph into new borers which bore their way out of the tree, leaving tell-tale exit holes along the limbs and trunk. And the cycle begins all over again. Each year the insect reproduces exponentially so that once EAB arrives in an area most all ash trees become infested and soon die.

For generations, ash trees have been an important part of the landscape on private properties in Chestnut Hill, and also in community settings such as the Wissahickon Park. For several years Philadelphia’s Parks and Recreation Department (PPR) has been proactive, implementing a strategy to prevent the future loss of all ash trees in the Wissahickon and in the other large Philadelphia parks.

In 2015, PPR treated 175 of the “best” ash trees in the Wissahickon by injecting an insecticide directly into each tree (“best” meaning still healthy with no major structural flaws or damage). This method of application was chosen because it causes little or no environmental exposure. The agency plans to retreat these same trees next summer.

In addition to this, PPR removed approximately 200 trees in the Wissahickon in 2015 and 2016 that were considered a public safety hazard due to their health or location. The removal included a significant number of ash trees.

Take a drive up 309 north, and then follow 202 north into New Hope and New Jersey, if you want to see for yourself dead and dying ash trees. Their leafless silhouettes standing stark among the other trees – an ecological disaster. Pennsylvania, once Penn’s Woods, has lifted all quarantines for transporting ash firewood. There are no plans for remediation. Some municipalities, like Philadelphia, are treating their ash trees. Not so much to conserve them, but to delay the removals they will be faced with later. Dead ash dry out quickly, become brittle, and impose a hazard.

Property owners can protect their ash trees from the borer by treating them with systemic insecticides. The most common insecticides are: Imidacloprid as a soil drench for annual control, and Emamectin benzoate as a trunk injection for two-year control.

Consult your local arborist for details.

The EAB tsunami has crashed on our shores. What started in Horsham has spread through Ambler into Fort Washington, and now into Chestnut Hill, on West Roumfort Road in Mt. Airy, and Rittenhouse Town in Germantown.

Green and white ash trees, the most common in our area, are equally affected – both male and female trees.

We live in a globalized world today, where species of plants, insects, diseases, and humans invade other continents and oceans. After the tidal wave has subsided, what will be left? Will the borer decline over time? Will research programs develop ash hybrids resistant to the borer?

Living in a post-ash world, we recommend planting a diverse selection of canopy trees. Diversity is the key to a healthy, vibrant tree community.

Ken LeRoy is a certified arborist/horticulturist who lives in Mt. Airy. Bill Hengst is a garden writer who also lives in Mt. Airy.