Emerald Ash Borer in the United States

By: Rachel Kerr

Imagine a peaceful, sunny day in the forest. 

Birds chirping, leaves swaying, critters crawling. Maybe the forest you imagine has a stream trickling in the background. Maybe it has deer lurking around bushes, searching for food.

But imagine that the forest is slowly dying and no one can do anything to stop it. A hidden creature is lurking within the ash trees and most cannot detect it, unless they know to look for the D-shaped exit holes that the emerald ash borers leave as they emerge.

These ash borers may not have come to Oregon yet, but it only takes one infested log coming from Ohio or Michigan to eventually devastate the ash trees of Oregon. 

Ash Trees in the United States

There are roughly 8 billion ash trees in the United States, with white and green ash being the most common, and black ash being less common. 

White ash are considered one of the most valuable ash trees for their shock resistance, which is perfect for baseball bats. 

Green ash are planted as ornamental trees more often than other species of trees. 

According to the USDA Forest Service, roughly $4.2 billion in forest products are lost annually to invasive pathogens and insects. 

"Affected ash usually die within three to four years after they are infested with EAB. 

They eat in serpentine tunnel patterns inside of the tree  and emerge as adults from the tree bark in late spring."

Emerald Ash Borer Enters the Scene

The Emerald Ash Borer (EAB) or Agrilus planipennis was first discovered in the United States in 2002 in Detroit, Michigan. 

The EAB are native to Asia, found in Northern China, Japan, Korea and Eastern Russia. It is believed the EAB arrived from Asia when infected ash material was unknowingly transported to the United States.

Emerald ash borers are a metallic green color and they are roughly the size of a cooked grain of rice when they are adults. 

Female emerald ash borers lay eggs in the bark of ash and these eggs hatch roughly two weeks after they are laid. Larvae eat their way through the phloem of the tree, which is the vascular tissue that conducts sugars down from the leaves. This affects the tree's ability to get nutrients to the trunk of the tree, leading to its slow death. 

Affected ash usually die within three to four years after they are infested with EAB. They eat in serpentine tunnel patterns inside of the tree and emerge as adults from the tree bark in late spring. 

Adults leave D-shaped exit holes on the outside of the ash and they eat the leaves of the tree as they start mating and laying eggs. 

EAB Current Effects in the United States

The emerald ash borer infests all ash native to the United States. EAB has been found in roughly 35 states and is mostly concentrated in the Midwest.

According to the Arbor Day Foundation website, "The emerald ash borer has destroyed 40 million ash trees in Michigan alone, and tens of millions throughout other states and Canada." 

The National Park Service website says that there is a -76% average change in ash populations in the National Capital Region as of 2021.

The loss of each ash tree is felt in the community for the loss of their beauty. Ash trees are used all around the United States and Canada to beautify urban environments and to help reduce carbon emissions through photosynthesis.  

The loss is also felt in arthropods for the loss of a food source. In an article written by Ken Johnson on May 4th, 2021 for the University of Illinois, he said, "Nearly 300 species of arthropods (mites and insects) feed on ash trees." 

Enrico Bonello is a professor of Molecular and Chemical Ecology of Trees, in the Plant Pathology Department of The Ohio State University. 

He grew up surrounded by forests in the Alps in his native home of Italy, which is what inspired him to pursue a career in studying forests. 

Professor Bonello has a PHD from Oxford, England and a Masters Degree from the University of Padova, in Italy. 

Professor Bonello said that in Ohio, the emerald ash borer "was killing trees left and right," and that it looked like "a nuclear bomb went off" after it arrived.

Future Fixers

When asked about the state of the United State's forests, Professor Bonello said that the "emerald ash borer is well established in North America." He also said that the emerald ash borer "changes the structure and composition of particularly mixed hardwood forests in the Northeastern United States." 

Professor Bonello also said that the invasion of the EAB followed a sigmoid curve, which signifies an initial exponential increase, an approximately linear phase that contains the inflection point where the growth rate is maximal, and is followed by an asymptomatic phase where the curve plateaus. He believes that the United States is past the first exponential increase, and it is not quite to the plateau. 

The Ohio State University ran a study comparing the chemical and molecular ecology of Asian and United States ash. The Ohio State University intends to see what specific genes and forms of chemical resistance the Asian ash had to combat the EAB. 

The Ohio State University had hoped to breed United States ash with the genes they found in the resistant Asian ash through hybridization by combining single stranded DNA or RNA molecules together to form a double stranded molecule. 

The Ohio State University also wanted to see what traits that the United States ash had that the Asian ash also possessed, so that they could try and further exploit those helpful traits. This would have been a more accepted method, as it does not introduce foreign genes into the native gene pool of United States ash.

A study published by Oecologia in 2014 described the effect of methyl jasmonate (MeJA) on ash trees affected by EAB. The study found that it helped decrease the probability that an adult will emerge from the ash. The study stated, "MeJA application decreased adult emergence in susceptible ash species, comparable to levels achieved by insecticide application." 

Methyl jasmonate induces plants to activate their natural defense mechanisms, which could explain the effect that this organic compound had on the ash used in the study. 

The USDA is researching the use of stingless wasps for the biological control of emerald ash borer. These wasps attack the larvae or eggs of the EAB. The four wasps they have used so far are: spathius galinae, spathius agrili, oobius agrili and tetrastichus planipennisi. 

The USDA considers it a success so far, and plans to introduce these wasps into every EAB infected state to curb the growth. 

Professor Bonello said, "Naturally speaking, it would be very difficult for the EAB to get to the West Coast" if nature ran its course, because of the lack of ash and forests in the Great Plains and the great distance between the Pacific Northwest and the Midwest where the EAB has been found. 

Professor Bonello said that "human activities still remain a very plausible route for the insect to move around, especially in firewood." He predicts that if the emerald ash borer did make its way to the Pacific Northwest and Oregon, it would be because of human activities.

Photo By: Gustav Gullstrand