Wood-boring insects damage woody plants by burrowing into the plant's cambium and sapwood -- layers of tissue just under the bark that contain vascular and growth cells. When the insects cause so much damage that the plant can no longer effectively transport nutrients, or when they destroy vascular tissue around the entire circumference of a branch or trunk, the plant may begin to die above the site of the damage.
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Unfortunately, some of the signs that a tree or shrub is infested with borers are most noticeable only after the insects have done significant damage to the plants. One of these signs is foliage that loses its normal color during the growing season. Affected leaves will often turn brown, but remain on the tree without dropping.
Wilt and Dieback
Affected branches in the crown of a tree may also show signs of wilt near their ends, and they may eventually die. These branches will typically show discolored foliage during the first season they're attacked and will produce no new leaves in future seasons.
Branches that have been weakened or killed by borers may not be able to stand up to wind and weather and may break or drop from the tree. Some borer species, such as the beetle larvae known as a twig or branch pruners, may cut through branches entirely as they feed, causing the branch to drop and leaving behind a smooth-surfaced cut.
Scars and Cankers
At the locations where borers are active, the plant may attempt to heal the injury that the insects cause, resulting in visible scarring or callusing of the tree's external bark. Although the larvae and their actual activity goes unseen beneath the bark, these external reactions are a sign that the insects are at work under the surface.
Because most borers are opportunistic insects that tend to attack unhealthy plants, an effective way to prevent a borer infestation is to water and fertilize properly to minimize stress on plants and to be careful not to damage trees or shrubs with mowers or trimmers.
Frass and Sap
As the borer larvae feed, they produce sawdust like droppings called frass. Some species keep frass tightly packed into their burrows so that it's not visible from the outside, but other species expel the frass and it may be visible externally in crevices in the bark or on the ground around the tree. In some cases, the frass may mix with sap and ooze from the site of the injury.
Tunnels and Galleries
Many species of borers create meandering tunnels, called galleries, through the wood of tree trunks just under the bark. The galleries are often obviously visible when pieces of loose bark are peeled away.
After borer larvae develop into their adult forms, they exit the tree in which they've been feeding by boring outward to the surface of the bark. When they do so, they leave behind exit holes that are easily visible on the plant's trunk or branches. The holes are usually oval, circular or semi-circular in shape, and they are usually scattered randomly.