Emerald Ash Borer was discovered in Michigan in 2002, having probably established itself years before. It is believed to have arrived in wood packing materials from overseas. Since it’s unintended introduction, EAB is now infesting 36 states, including Vermont in 2018. A trap caught the beetle on the Town Green in Middlebury in 2021, and as of January of 2024 has not been located on campus.

There is an overload of information on EAB on the web, here are some good sites. The short form is that the insect bores into the bark of an Ash, lays eggs, which later hatch under the bark. These larvae then feed on the cambium under the bark, eating the important cells that move water and nutrients throughout the tree. With enough cambium eaten, the tree is effectively strangled, dying with 3-5 years.

These galleries of eaten cambium make for the best diagnosis of EAB presence, but peeling the bark of an uninfected tree will kill it as well. External symptoms of EAB look similar to many other pests that can affect Ash, but it’s the combination of several of them together that is the primary indicator. Pictures below show the best symptoms to look for-these include sprouting of suckers (epicormics) from the lower trunk, flaking of bark from woodpeckers trying to eat the beetle, and dieback of upper branches in the crown.

Imported pests into Northern temperate forests have a long history, are devastating for multiple reasons. One is the very nature of temperate forests itself, and another is the concept of co-evolution. (https://en.wikipedia.org/wiki/Coevolution)

EAB is a great example of an obligate parasite-an organism that needs it’s host to survive. It cannot complete it’s life cycle without an Ash tree involved. This means that the parasite and the host co-evolve, coming to a bit of an understanding ecologically, adapting to each other. The easiest explanation is one of dependence-consuming and completely eliminating your host species will mean the end of you as well. This is the wonderfully named Red Queen hypothesis . These natural checks and balances keep our ecosystems and food webs intact.

But what if a pest does not evolve with the host, and is instead introduced? Havoc, pure and simple. The natural defenses the hosts had evolved over millennia are not present in other species, and the host runs rampant through the population. EAB co-evolved with Asian Ash trees, such as the Manchurian and Manna Ash, and they are not killed. Our native ash? Defenseless.

These problems are further compounded by our tree species evolving in a temperate climate. Colin Tudge in The Tree writes about ‘’the sheer violence of the physical conditions”. Our trees have evolved to face physical threats well, cold that would lay low tropical species, snow for months on end. Temperate species of trees are fewer, and the genetic adaption for survival is numbers. If some of your species die, there are still plenty waiting in the wings to reproduce and carry on the line.

Tropical trees, on the other hand, are much more diverse, so biological pests have a harder time even finding suitable hosts. Most parasites require large populations of hosts to survive, so diversity brings resilience.

But what happens when a biological threat is introduced to temperate species it’s never co-evolved with? Let’s ask American Elm after meeting Dutch Elm Disease, or Chestnut after the introduction of Chestnut Blight, or Butternut canker, or Asian Longhorn Beetle, or Beech bark disease. Should we go on?

In our communities Ash are particularly susceptible to EAB. Adult Emerald Ash Borers prefer to lay their eggs in solitary trees, such as street trees, and thrive well in monocultures of Ash, found in many typical street tree plantings. Like many pests, EAB also prefers stressed trees. This can include many conditions found in our landscapes, such as dry soils, compacted soils, excessive heat from hardscaping, road salt, and other pollutants. Ash are also already subjected to other pests as well, like many native borers, Ash Yellows, and Ash Decline, just to name a few. These stressors make for fertile breeding grounds for EAB to explode in our maintained landscapes.