Current Issue

Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Good bye ash trees of the Potomac: Emerald ash borer (EAB), Agrilus planipennis


Emerald Ash Borer adults and larvae easily fit in the palm of your hand but when thousands attack a single tree, its fate is sealed.


Along the C & O canal, woodpeckers remove the outer bark of ash trees as they search for larvae of Emerald Ash Bores on which to feast. This results in telltale “blonding” of the bark, a key sign of EAB infestation. 

One of the jewels of the greater Washington region is the historic C&O canal. Each spring forays along the canal and its towpath produce marvelous encounters with wildflowers and six-legged wildlife. Unfortunately, but not unexpectedly, visits to the trail this spring revealed the demise of hundreds of magnificent ash trees along the mighty Potomac and Shenandoah rivers. Two years ago we recounted events leading up to the arrival of Emerald Ash Borer in Maryland.  This nefarious killer of ash trees has now been found in almost every county in Maryland including those east of the Chesapeake Bay. Nationwide it has killed more than 100 million ash trees since it was first detected near Detroit, Michigan, in 2002. It now occupies parts of Alabama, Arkansas, Colorado, Connecticut, Delaware, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Massachusetts, Maryland, Minnesota, Missouri, New Hampshire, New Jersey, New York, North Carolina, Ohio, Oklahoma, Pennsylvania, Tennessee, Texas, Virginia, West Virginia, Wisconsin and two provinces in Canada.  Since its arrival in the US, Emerald Ash Borer has killed hundreds of millions of ash trees. Estimated management costs, lost property values and lost revenues from timber exceed 1.6 billion dollars annually.

D-shaped exit holes are a diagnostic clue left behind on the bark of ash trees when adults emerge from beneath the bark in spring. 

EAB likely arrived in this country in packing material from Asia. In 2003 the first EAB tremor was felt in our region when the borer was detected in a nursery in Prince George’s County. This detection confirmed the suspicions and fears of many, namely, that the EAB could be transported and relocated with infested nursery stock. Ash trees infested with EAB were illegally shipped from an out-of-state quarantine zone to a Maryland nursery just south of Washington, DC in April, 2003. During the spring and summer beetles moved from infested Michigan-grown trees to ones grown in the nursery. Some of these trees were shipped and installed in several locations in the greater Washington, DC area.  Despite attempts to eradicate the beetle, it escaped quarantine zones in Prince George’s County and by 2015 it had arrived in Baltimore. According to the Maryland Department of Natural Resources, ash trees are the most common tree in Baltimore where the population stands at something north of 290,000 trees. Perhaps more disturbing is the fact that when the entire metropolitan area surrounding Baltimore is considered, the total population of ash exceeds 6 million trees. According to estimates by USDA the loss of ash trees in Baltimore could exceed $200 million dollars. 

Why are ash trees so valuable, especially in urban forests? Most people don’t appreciate the many benefits that accrue from trees in our cities and suburbs:

•By virtue of their shade and evapotranspiration, trees cool cities and the loss of this ecosystem service drives up summer cooling costs dramatically.

•Trees intercept rainfall, slowing its movement and allowing water to infiltrate the soil, thereby mitigating surface water runoff and protecting the Chesapeake Bay.

•Photosynthesis uses carbon dioxide as a raw material. Trees are vital for fixing carbon and removing it from the atmosphere, thereby helping to mitigate climate change.

•Trees in cities also remove pollutants from the air and help beautify our urban areas, adding significantly to property values.

•Recent studies demonstrate a link between the presence of urban trees and improved human health, including faster recovery times of patients in hospitals and reductions in rates of heart disease.

•Ashes are also a common native tree in watersheds in Maryland’s piedmont, where they provide shade and nutrients to our riparian and woodland ecosystems. Ashes provide food and shelter to more than 20 species of indigenous native creatures. Collectively, these organisms are valuable members of several ecological communities. They help the natural world go around. 

Emerald Ash Borer larvae create serpentine galleries beneath the bark. By consuming cambium, phloem, and sap wood, the borer effectively girdles the tree condemning it to death.

As far as we know, EAB attacks all native species of ash and also non-natives used for ornamental purposes in landscapes. Black ash and green ash appear to be the most susceptible to this pest and mortality of these species is a near certainty unless they are protected by insecticides. Other species of ash, such as blue and white ash, are more resistant, while the Asian host of EAB, Manchurian ash, exhibits high levels of resistance. In North America it is estimated that some 8 billion ash trees populate our lands. We once believed that only members of the ash clan were on the menu for this beast. Unfortunately, we recently learned that our beautiful native fringe tree can also serve as a host for this killer. 

Neighborhoods throughout Maryland are feeling the wrath of Emerald Ash Borer. In 2015 EAB came to college at the University of Maryland.

Since its detection in the Midwest in the early 2000’s, much has been learned about managing this pest in cities and suburbs. EAB tends to roll over the urban forest like a great tsunami, doubling the number of ash trees killed in a city each year once it becomes established. In a period of eight to ten years, tree mortality rises from 1 or 2 percent to 100 percent if intervention does not take place. Early infestations of EAB often went undetected and cities were forced to remove dead and dying trees at enormous expense to municipal budgets. While some municipalities are opting to remove all ash trees when EAB is detected, some experts are recommending a hybrid approach of removing smaller or less healthy ash trees that provide fewer ecosystem services and are less valuable, while protecting larger and high value trees with insecticides. Clever analysis by Cliff Sadof of Purdue University and Rich Hauer of the University of Wisconsin have demonstrated that this hybrid approach of removing some trees and protecting others minimizes the cost of management while preserving the benefits of urban forest ash trees.  An effort to manage EAB by releasing tiny parasitic wasps that attack and kill larvae and eggs of EAB is also underway in Maryland and other states. To learn more about this approach, please visit the May 31, 2010 Bug of the Week episode entitled “Parasitoids to the Rescue.”


After doing battle with a giant finger, an Emerald Ash Borer takes flight to find a less public place to consume ash leaves.

f you suspect your trees are infested with EAB , you can confirm or infirm your suspicions by visiting the following websites that will help you identify EAB and trees that it attacks.  These websites also provide valuable guidance for managing EAB in residential landscapes, town, and cities.

Excellent webinars on many aspects of EAB biology and management can be found at this website:

As a parting thought, if you are fond of the outdoors you might want to visit the beautiful hardwood forests along the hills and dales of our region to pay respects to our rapidly disappearing ash trees. 


The references “Emerald Ash Borer Invasion of North America: History, Biology, Ecology, Impacts, and Management Approaches” by Dan Herms and Deb McCullough; “Predicting the Movement and Potential Economic and Ecological Impacts of the Emerald Ash Borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), in Maryland Municipalities and a Discussion of Possible Management Options” by C. Sargent, H. M. Martinson, R.A. Bean, S. Grimard, B. Raupp, S. C. Bass, E. J. Bergmann, D.J. Nowak, and M. J. Raupp; and “Direct and indirect effects of alien insect herbivores on ecological processes and interactions in forests of eastern North America” by K. J. K. Gandhi and D.A. Herms, were used in preparation of this episode. Excellent webinars by Deb McCullough, Cliff Sadof, and Rich Hauer at the website above were also used.