There are two types of things to look for when assessing an ash tree for emerald ash borer (EAB) infestation - tree symptoms and insect signs. Tree symptoms are the tree's response to a stressor (e.g. insects, diseases, drought, or lack of nutrients). Insect signs are the physical evidence of insect activity on the tree. These signs and symptoms are not usually seen at early stages of infestation, but become visible as the infestation progresses. EAB usually attacks the upper part of the tree first, so it can be difficult to see insects signs in the first year or two of infestation. In later stages, or in smaller trees, these signs are found lower on the tree trunk.

Crown dieback:
The tree crown may start to thin the year following the first EAB attack; thinning starts at the top of the tree and becomes progressively worse over time. Stand back from the tree and look for loss of leaves, yellowed leaves or dead branches at the top of the crown.

Ash canopy dieback from emerald ash borer. Epicormic shoots are seen at the base of the trunk. Image: Leah Bauer, USDA Forest Service Northern Research Station, Bugwood.org

Epicormic shoots (water sprouts, suckers):
Epicormic shoots grow from the base of the tree, or sometimes along the trunk, when trees are stressed, dying or already dead. Different types of stresses to the tree can cause both epicormic shoots and crown dieback to occur, but if these symptoms are seen along with any of the following signs or symptoms, and you are near to an area with EAB, suspect the tree is infested.

Bark splitting:
Vertically-oriented splits in the bark (5-10 cm long) occur as a result of callous tissue that forms around EAB feeding galleries. Freezing can cause similar symptoms; however, when the splitting is because of EAB, the insect's feeding gallery is often visible beneath the split. At early stages of infestation splitting is more likely to be found in the top part of the tree.

Splitting in ash tree bark with emerald ash borer feeding gallery underneath. Image: Kathleen Ryan, Silv-Econ Ltd.

Woodpecker activity:
Woodpeckers peel layers of bark off of the tree when foraging for EAB larvae (called flecking); the trunk appears light coloured where the bark is peeled off and holes are evident where the woodpecker has extracted insect larvae or pupae. Where woodpeckers are present, this an especially useful indicator of EAB-infested trees.

Flecking on ash bark by woodpeckers foraging for insects. Small holes visible where insects were removed from the tree. Image: Chris Gynan, Silv-Econ Ltd.

Emergence holes:
EAB emergence holes are found a year (or more) after the tree is first attacked by EAB. They occur where adult beetles emerge from the tree by boring through the bark. The emergence holes are an obvious D-shape and 3.5-4 mm wide. At early stages of infestation these exit holes are in the upper part the tree and can be difficult to see. There are other insects that attack ash trees and make exit holes, though most leave a round or oval exit hole.

D-shaped emerald ash borer exit hole in ash tree bark. Image: Kathleen Ryan, Silv-Econ Ltd.

Larval galleries:
Feeding galleries may be visible under splitting bark, or when the tree's bark is peeled away. They are typically serpentine-shaped and when there are many EAB in the tree the trunk may be covered with galleries. Like exit holes, at early stages of infestation most of the galleries are in the branches of the tree.

Multiple serpentine emerald ash borer galleries on ash tree. Image: Kathleen Ryan, Silv-Econ Ltd.