Behind the Dogwood’s Bark

What is a dogwood borer? A good answer might be “a borer that attacks our flowering dogwoods (Cornus florida).” The dogwood borer (Synanthedon scitula) is a native insect found in the Midwestern and Eastern United States, from Canada to Florida. As an adult, it is a small clearwing moth resembling a wasp, with a blackish blue and yellow banded body about 3/4 inch long. It is the borer’s larva stage that causes damage to our native trees.

This pest, named after the first plant it was found on, doesn’t limit itself to dogwoods. It will attack oak, mountain ash, birch, chestnut, willow and pecan trees as well. In the 1980s it was discovered infesting apple trees, too. Studies have indicated that on apples, the dogwood borer prefers the burr knot tissue where grafts try to grow new roots, especially on certain apple rootstocks. From there, it will find its way to other healthy tissue.

Usually the damage from the borer is not severe enough to kill the tree, but successive years of heavy damage could do just that. This is especially true if the tree is stressed by other factors, such as drought, physical damage or being in full sun. Dogwoods are an understory tree, meaning their native habitat is at the end of a forest under other trees. The University of Tennessee reports that dogwoods growing in full sun exhibit more dogwood borer infestations than those in their proper habitat.

The moth lays its eggs singly on the bark, with peak adult activity occurring around mid-May. Activity occurs throughout the summer, with a second peak time of early August. The cream-colored larvae hatch and look for a way to enter the tree through wounds, cuts, cankers or other areas that are broken and unprotected. Young borers are 1/16 inch long, while the mature larvae are 3/5 inch long. The borers tunnel their way through the cambium layer of the tree’s trunk or branches and the bark. During the winter the borer larva hibernates in its tunnel. It comes out in the spring, pupates and emerges as an adult clearwing moth.

Damage to the tree is visible as pieces of bark peeling way from the trunk, especially the lower trunk. The tunneling under the bark will in itself not usually kill a tree, unless it is a young tree with a small-diameter trunk. The damage can, however, let in fungus and bacterial infections. Heavy infestation and repeated infestation over a few years will take its toll, especially if the tree is stressed in other ways.

Cultural control can have some success. Do not plant dogwoods in full sun — young trees are especially vulnerable. Prevent physical damage to the bark from lawnmowers, string trimmers and pruning cuts where borers can enter the tree or females can lay eggs. Borers are attracted by the resinous smell released from the wounds. Water regularly, treat all wounds or injuries and prune in winter only.

In addition, insecticides with a residue such as permethrin applied to the trunk and damaged areas beginning in mid-May provide effective control. Apply to the lower limb down to the soil line, and always apply the pesticide according to the label directions.

References
Dogwood Borer, Eric Day, VCE Publication 2808-1010
Dogwood Borer, Jim Walgenbach, NC State Extension
Dogwood Borer Infestation, Frank A. Hale, University of Tennessee Extension
Insect Borers of Fruit Trees, Bruce A. Barrett, University of Missouri Extension