In the early 1900s, North America lost nearly every American Chestnut to the chestnut blight. My grandparents have likely never seen a mature one, though they are estimated to have numbered 3 billion. Most people of my generation have rarely if ever seen an American Elm, once an extremely widely-planted shade tree which was almost killed off by Dutch Elm Disease from the 1920s to 1970s (and beyond). Now it seems that my grandchildren may be lucky to see an ash tree on this continent, as the Emerald Ash Borer (EAB) threatens to wipe out the entire genus, Fraxinus.
The Emerald Ash Borer (Agrilus planipennis) was first discovered in North America in 2002, and since then has caused the death of around 40 million ash trees. While it seems to slightly prefer some ashes to others, it will attack any member of the genus. The beetle causes destruction in its larval phase, when it lives just under the outer bark and chews winding trails or “galleries” through the layer of tissue called phloem, which moves sugars from the leaves to the roots. Since the beetle lives under the bark, infestations can go unnoticed until the tree is visibly distressed.
Sprouts from the base are a common symptom
I saw this recently on the museum grounds. In an ash tree, sprouts from the base are a common symptom of EAB infestation. The phloem is so damaged the roots have all but stopped getting nutrition from the leaves, and the tree sends new shoots from below the damaged area. Clearly, these few sprouts won’t suffice, and by the time such sprouts appear it is usually too late to save the tree.
D-shaped hole of emerging beetle
A second sign presented itself with a closer look: the characteristic D-shaped hole where the adult beetle emerged. I saw around ten such holes on this tree. By prying some loose bark back with my knife I was able to catch a glimpse of the galleries left by the larvae.
Gallery or trails of larvae
The adults left to lay eggs on other nearby ash trees, of which there are plenty. 19% of the City of Chicago's trees are ashes, and there are an estimated half-million privately owned in the city.
So far, the only effective treatment has been systemic insecticides. They must be applied before an infestation occurs, must be re-applied every few years, save only the treated tree, and kill all the other insects which feed on ash trees. Because of the expense and complications involved, only certain "high-value" trees are being treated, and most agencies prepare for the EAB’s arrival by replacing ashes with other trees. The EAB has a limited range and moves slowly, so it may be possible to impede its spread by treating and/or removing trees in areas not yet affected, in a strategy similar to a fire-break.
Even if you don’t own or manage ash trees, you can still help. Always use locally-sourced firewood, so if any EAB larvae or adults are in the wood they stay in an already-infested area instead of being driven somewhere that had yet to be affected. It is likely that the original U.S. infestation was a small number of individual insects that arrived in wooden packing crates from Asia, where the insect is a minor pest. The cost of removing or replacing or treating trees could well run into the billions of dollars-- largely taxpayer dollars as governments manage large ash populations and dead trees cause hazards in populated areas-- so it is best for everyone if we leave wood where we find it and do what we can to limit the spread of this invasive species.
Trust me, I wish I could end on a hopeful note, or even just give a spoonful of sugar with this bitter pill, but the destruction caused by invasive species far exceeds the limited resources of time, money, and personnel available to combat them. With luck, the EAB won’t kill all 2 billion ash trees and we’ll still have some for our grandchildren to appreciate, but now would probably be a good time for you to get to know the ashes in your area, whether the EAB is already present, and who to call if you spot damage in unaffected areas.