Trees can decline for a number of reasons: insects, disease, soil compaction, winter injury, drought stress, and many other factors. Trees declining from many of these reasons may show general symptoms of tree stress (shown below) and these do not specifically indicate EAB. However, these indicators can be early signs of although such symptoms will occur when EAB attacks ash trees.
Canopy Thinning and Crown Dieback:
As larvae feed beneath the bark they damage tissues responsible for transporting water and nutrients. Initial damage appears as thinning in the upper canopy of the tree and branches can die over time.
Other conditions (disease, soil compaction, etc.) can cause canopy die-back in ash trees, so canopy thinning alone does not definitively indicate EAB.
(See the page on diseases of ash trees for some additional information)
When trees are stressed, they may try to grow new branches and leaves wherever they still can (suckering). Stressed ash trees may have new growth at the base of the trunk or on main branches.
As with canopy thinning, other sources of stress can cause suckering around the base of ash trees, and this symptoms does not specifically indicate EAB.
In addition to the two symptoms above, trees infested with EAB are often sought out and attacked by woodpeckers. However, woodpeckers will attack just about any tree full of insect larvae. Thus, while woodpecker activity can be an indicator of EAB, it does not specifically confirm an EAB infestation. Extensive woodpecker damage is sometimes referred to as “flecking” or “bronzing”.
Specific Signs of EAB:
If you see the general symptoms mentioned above, take a closer look at your ash trees to check for the specific signs of the emerald ash borer shown below:
D-Shaped Exit Holes:
As they emerge from ash trees in June and July, adult emerald ash borers leave behind distinct D-shaped exit holes. These holes are approximately 1/8″ wide and can be oriented in any direction (i.e., the flat side may be facing upwards, downwards, etc.). These D-shaped holes are a strong indicator of EAB.
Exit holes of native borers will be round or oval and can vary in size.
S-Shaped Galleries & Splitting Bark
Trees attacked by EAB have distinct S-shaped or “serpentine” galleries (tunnels) beneath the bark. These tunnels are approximately 1/8 inch wide and are packed with frass (a fine mixture of sawdust and insect excrement).
As EAB larvae feed, trees attempt to create callus tissue around larval galleries, which can cause the bark to split vertically. The S-shaped galleries and larvae can often be seen beneath split bark.
In addition to the D-shaped exit holes and S-shaped larval galleries, finding EAB larvae, pupae, or adults will confirm an EAB infestation.
A helpful video illustrating how to identify the signs/symptoms of EAB can be found here.
A printable document describing the signs and symptoms of EAB is available from Michigan State University.