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How do I know if a tree has emerald ash borer in it?

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, pathogens, drought, 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, becoming visible as the infestation progresses.

Tree symptoms
The presence of the following symptoms does not necessarily confirm that EAB is infesting the tree, it could be other stressors causing them. However, if an ash tree is in or near to an infested area and has more than one of these symptoms then EAB should be considered.

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[3]. Stand back from the tree and look for foliage loss, yellowed leaves or dead branches at the top of the crown. Crown dieback can also occur as a result of disease (e.g. ash yellows, anthracnose and verticillium wilt), poor site conditions, injury or other insects[36].

ash crown dieback from EAB 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,

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 from EAB attack[28]. A number other stressors, such as injury, poor site conditions or other pests or pathogens, can also cause epicormic shoots to grow[36].

Bark splitting:
Vertically-oriented splits in the bark (5-10 cm long) occur as a result of callous tissue from EAB feeding galleries[3]. Freezing can cause similar symptoms; however, when it's a result of EAB the 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 bark from EAB 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 EAB larvae or pupae. Woodpecker activity simply indicates insect activity within the trunk of the tree, however, other ash-boring insects usually only attack already stressed trees, so, if the tree was previously healthy EAB should be suspected. Where woodpeckers are present, this an especially useful indicator of infested trees.

woodpecker activity on ash bark 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.

Insect signs
Emerald ash borer lives most of its life as a larva feeding under the bark of the ash tree and this makes it difficult to detect in early stages of an infestation, especially in trees with thick or rough bark (i.e. older trees)[28]. Emerald ash borer usually attacks the upper trunk and lower parts of the main branches first, then in successive attacks the insect signs are seen lower in the trunk[3,22].

Emergence holes:
EAB emergence holes are found a year (or more) after initial attack when adults emerge from the tree by boring through the bark[28]. The emergence holes are an obvious D-shape and 3.5-4 mm wide[37]. At early stages of infestation, binoculars or a spotting scope may be needed to see these exit holes in the top of the tree. In later stages of attack, or in smaller trees, exit holes are seen lower on the trunk. There are other boring insects that attack ash and create exit holes, though most leave a round or oval exit hole. Another, native, Agrilus beetle attacks ash and has a D-shaped exit hole, however, its exit hole is much smaller (less than 2 mm)[38].

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

Larval galleries:
Galleries may be visible under splitting bark, or when the tree's bark is peeled away. They are typically serpentine-shaped, though sometimes in smaller branches they are straighter[28]. When EAB density is very high, the trunk may be covered with galleries. Galleries of other insect species may be found, however, they are unlikely to be serpentine in shape[36,37].

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


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