You are more likely to see the damage that TCD causes than the insect that transmits it.
The walnut twig beetle is only 1/16 inch long, and it makes pin-sized holes in the bark as it enters and exits.
The first noticeable symptom is usually a thinning canopy and yellowing foliage on a branch, which rapidly progresses to brown wilted foliage. The branch dies with the wilted leaves still attached.
The cankers form where the insect enters and exits the bark. They are circular to oblong, but at first are visible only when you peel the bark back. They appear as small patches on branches at first, but as the tree becomes more infested, the cankers beging to merge into larger cankers that may be visible on the trunk.
On logs, you would see cankers only if the bark has been removed. If the bark remains, you might see circular holes no larger than 1/16 inch in the bark. If you peel the bark back, you might find walnut twig beetle adults or larvae beneath it.
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