In 2002, scientists from Michigan State University discovered the cause of a recent decline and death of many ash (Fraxinus) trees in Southeast Michigan. It was discovered that the dead trees where infested with a new insect which was subsequently identified as coming from China and was given the common name, Emerald ash borer or EAB. The name comes from the fact that the smallish insect is an iridescent green or emerald in color.

The theory is that the insect was introduced into the area inside wooden packing crates that came from China. These were evidently made of ash wood that had not been kiln dried or otherwise treated to kill grubs inside the boards. The crates were subsequently dumped into a landfill where the grubs matured, and flew off to infest the nearest ash trees. Nobody knows the exact date when the insects were introduced.

Unlike native borer insects which almost universally attack old, stressed trees, EAB infests young, vigorous plants. Like other exotic pests, it thrives in its adopted land because there are no natural predators or diseases that keep it under control. So, it has gone wild and, to date, has killed many millions of ash trees in the Midwest. It is still on the move and there is no current reason to believe that it will not continue to devastated ash trees throughout their natural range.

Ironically, it is estimated that the EAB would only spread a very short distance each year on its own. It is not a strong flying insect so it would move slowly from woodlot to woodlot. So why has it become such a devastating pest?

Unfortunately, although it may move only a mile or two per year on its own, EAB is capable of traveling at speeds of 70 miles per hour or the back of a pickup truck or automobile. The key way EAB is being spread is in firewood.

When dead, infested ash trees are cut down, the larvae of EAB continue to live inside. If the wood is not burned quickly, the larvae will continue to grow, turn into adults, chew their way through the bark and fly away. People taking firewood from infested areas to campgrounds, cabins or other places unwittingly spread the critter. Sadly, even though movement of firewood has been banned, a certain segment of the population still transport it anyway.

So, what can we do about EAB. Sadly, the answer is that many, many more millions of ash trees will die before a solution may be found and implemented. In the home landscape, individual ash trees that are vital to the garden may be protected by treatment with systemic insecticides. These chemicals are either drenched into the root zone of the tree or mechanically injected beneath the bark. This must be done every year...basically forever, at least as it stands now. The treatments cost money and they must be initiated BEFORE the tree starts showing signs of infestation.

Update 2013: In a radio interview, Dr. Deb McCullough of Michigan State University gave a nice update on EAB.

  • They now think that EAB was probably introduced into Michigan as long ago as the late 1980's. It takes several years for the insect to build to levels that are needed to kill an ash tree.

  • She mentioned that EAB is now found in 22 states and is spreading throughout the natural range of ash (Fraxinus) trees.

  • Federal scientists are trying to find and introduce natural enemies to the EAB. This includes species of parasitic wasps. If this is to work at slowing down the destruction, it will take many, many years for these critters to build to large enough populations.

  • In areas with well established populations of EAB, 85 to 99% of the ash trees are eventually killed.

  • Treatment of individual or small groupings of ash trees in the home landscape may be protected by systemic insecticide treatments. Unfortunately, these may be somewhat expensive and must be applied every year on trees that do not yet show any signs of damage from EAB.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


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