Borers can refer to a number of insects with a larva that feeds inside the bark of our trees and large shrubs. Borers are most often beetles but can also be clear-winged moths as in the case of the lilac-ash borer. There are many extremely damaging varieties of borers, including the Emerald Ash Borer, which is currently devastating our native ash tree population, and the Zimmerman pine moth, which is devastating our native pine forests. Borers of all species feed inside the bark on the cambium or living tissue of the tree. Since tree cambiums are primarily how trees move food and water around their systems, this can easily stunt or kill the trees. Borers leave holes in the bark when they emerge as adults. This is the best way to discover their presence. Although the insect that made the hole is long gone, often another generation will attack the same tree.

Most borers will only attack stressed trees and shrubs, so keeping our landscapes properly watered and fed is the first way to control this destructive pest. However, some borers attack perfectly healthy trees, and since there are many borers on our woody plant material, knowing the host tree or shrub is very helpful. Borers that are beetles can almost all be controlled with Fertilome’s “Tree and Shrub Systemic Insect Drench,” but this product isn’t as effective at killing moths. Use this product once a year at the base of the tree or shrub. To control moth borers, a product containing permethrin, such as Hi Yield’s “38,” must be sprayed on the trunk every few weeks when the adults are emerging or laying their eggs. This is April and May for the lilac/ash borer, but it will differ for different borers.