What are wood borers and how do they damage trees?

Wood-boring insects are insects that bore into the sapwood and sometimes the heartwood of trees and shrubs. Typically, wood-boring insects, referred to from here on as wood borers, infest weakened, dead, or dying trees and play an important ecological role in the decomposition process in forests. However, some wood borers can be damaging pests, such as those that infest otherwise healthy living trees or those that infest recently harvested logs. Wood borers are pests in harvested logs when they bore into the heartwood, damaging wood for use as lumber or wood products.

Damage from wood borers is primarily done by the larvae (the immature stage of the insect). Wood borers tunnel and feed under the bark in the living wood of the tree, called the cambium. A diagram of the components of a tree can be found here. When wood borers infest living trees, this destroys the water and sap conducting tissues and results in girdling. Other issues related to wood borer damage can include structural weakness, branch dieback, general decline, disease transmission, and death. When wood borers tunnel into the heartwood it can cause degradation of wood or wood products and can impact the timber value.

: Galleries beneath the bark of an ash tree caused by the emerald ash borer. Photo by Eric R. Day, Virginia Polytechnic Institute and State University, Bugwood.org

: Galleries within the heartwood of a tree bored by Asian longhorned beetle. Photo by Kenneth R. Law, USDA APHIS PPQ, Bugwood.org

Several different types of insects are categorized as wood borers, the most common being beetles, moths, and wood wasps. In the beetle order (Coleoptera), the families Cerambycidae and Buprestidae are the primary wood borers. Bark beetles (discussed in Module 3) are generally not considered wood borers because while they feed under the bark, they do not bore into the sapwood or heartwood of the trees. For the moth and wasp orders, it is the families Sesiidae and Siricidae, respectively, that are most commonly wood borers. The scope of this module is specifically focused on potential invasive wood borers in the beetle family Cerambycidae.