Many insects have the potential to devastate a single specimen of tree or a whole stands of trees. However, insects that bore into a tree to feed on living wood tissue are at the top of the list of destructive pests. Almost all trees and shrubs are able to be attacked from boring insects. Healthy trees can fend off initial attacks by forming callus tissue or by creating toxic compounds that will kill young insect larvae. Trees that are under some form of stress are more attractive to attack and are less able to respond defensively.
Most damage done by borers is caused by the larvae or immature stage of the insect. Eggs are typically laid in bark crevices, branch junctions or near open wounds. When the eggs hatch the small larvae chew their way through the bark to the point where the wood and bark meet. This section of wood contains the living nutritious tissues on which most borer larvae feed. These tissues, called xylem and phloem, serve as the trees’ transport system to move water and nutrients up from the roots to the leaves, and sugars from the leaves down to the roots.
When these tissues are damaged by borers the tree loses its ability to transport nutrients and also reduces its ability to respond to the borer. It is incredibly important to keep a trees’ vigor up to reduce both its attractiveness and it’s susceptibility to borers. In many cases, borers also carry diseases, such as Dutch Elm Disease that can infect the tissues in the tree and increase the rate of decline.
Most borer infestations go unnoticed until external signs of damage begin. Damage is not usually visible until after at least one year of infestation, when exit holes of the adult are seen in bark and when dieback occurs in the tree due to the disruption of the transport system.