Scouting for Emerald Ash Borer in Mid-Summer

If you have an ash tree on your property, mid-summer is the season to look at it closely. The emerald ash borer is one of the most destructive tree pests in North America, and by July the damage it does becomes far easier to spot than it was in the spring. The insect works quietly for a year or two before a homeowner notices anything is wrong, and by the time the symptoms are obvious, the tree is often already in serious decline. Learning the warning signs now, while the canopy is fully leafed out and the tree is under summer stress, gives you the best chance of catching a problem while there are still options.

The borer itself is a small metallic-green beetle, but you will almost never see the adult. What you see is the damage from the larvae, which tunnel just under the bark and cut off the tree's ability to move water and nutrients between the roots and the canopy. That is why the symptoms show up first in the top of the tree and work downward. The classic early sign in summer is thinning and dieback in the upper canopy, where branches that should be full and green come in sparse or fail to leaf out at all. When the top third of an ash tree looks noticeably thinner than the rest, that is worth a close look.

Below the canopy, the trunk tells the rest of the story. As the tree tries to respond, it often pushes out sprouts of new growth low on the trunk and at the base, a stress reaction called epicormic sprouting that shows the tree is struggling to survive. Look also for vertical splits in the bark, and for the distinctive D-shaped exit holes, roughly an eighth of an inch across with one flat side, where adult beetles have chewed their way out. Peel back a piece of loose bark and you may find the S-shaped, winding tunnels the larvae carve underneath. Increased woodpecker activity is another tell, because the birds strip bark to get at the larvae, leaving light-colored blonde patches on the trunk.

The reason early detection matters so much is that the window for saving an ash tree is narrow. Treatments exist that can protect a healthy ash or one in the very early stages of infestation, but they are far less effective once the canopy has already lost a large share of its leaves. A tree that is more than a third to half declined is usually past the point where treatment makes sense, and at that stage the decision shifts from saving it to removing it safely. Catching the problem while the canopy is still mostly intact is the difference between a tree you can protect and a tree you will be paying to take down.

There is also a safety dimension that makes a confirmed infestation something you cannot simply leave alone. Ash killed by borer damage becomes brittle and unpredictable much faster than most dead trees. The wood dries out and loses strength quickly, and dead ash is well known in the tree care world for dropping limbs and failing without warning, which makes it especially dangerous to work on and especially urgent to address once it is standing dead over a house, driveway, or walkway. A dead ash is not a tree to put off dealing with for another season.

If your ash is showing any of these signs, the right next step is a proper assessment rather than a guess. An arborist can confirm whether the culprit is the borer or one of several other problems that thin an ash canopy, judge how far the decline has progressed, and tell you honestly whether the tree is a candidate for treatment or has crossed into removal. Mid-summer, with the canopy fully out and the symptoms at their most visible, is exactly the time to have that look done, while you still have the full range of choices in front of you.