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.
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.