Bark Borers in Hornbeam

Two-Lined Chestnut Borer

• Although two-lined chestnut borers (Agrilus bilineatus) pose a greater threat to several other hardwood tree species across the Eastern and Central United States, the blue-black, 1/5- to 1/2-inch beetles occasionally attack hornbeams. Named for the bright-yellow lines edging their wing covers, these beetles target trees already compromised from environmental stress or weakened from disease and insect infestation. Their activity increases following extended drought or severe, tree-damaging storms.

Life Cycle

• Two-lined chestnut borers infest hornbeams between April and August, depending on their location. They feed on canopy leaves prior to mating on the branches or trunk. One or two weeks after the females deposit eggs in bark crevices, white wormlike larvae hatch and tunnel into the tree's cambial tissue to feed. After molting four times, the mature 1-inch larvae overwinter in bark chambers, pupate and emerge as winged adults from D-shaped exit holes to repeat the cycle the following year.

Damage

• As the larvae eat their way through a hornbeam's inner layers, their tunnels block the flow of food from the canopy and water and soil nutrients from the roots. The tree begins dying back just above the tunneling sites. By late summer, a borer-infested hornbeam exhibits scattered, wilting brown leaves. This dead foliage, the last the affected branches ever produce, often remains on the tree for months. Peeling back the bark on the dying branches reveals tunnel-riddled wood. A severe borer attacks may be fatal in the first year. More commonly, however, a hornbeam survives for two or three years before succumbing to the pests.

Cultural Management

• Late-summer pruning of borer-riddled hornbeam branches below the last wilted leaves helps a hornbeam weather infestation. Bury or burn the pruned branches before the adults emerge. To boost your tree's vigor and reduce future attacks, spread a 4-3-4 granular fertilizer formulated for acid-loving plants evenly over the root zone in spring, applying 1 pound for each inch of diameter on a trunk of 3 inches or less. For a larger tree, increase the amount to 3 pounds per inch. Follow up with a half-strength application in late fall. Watering whenever the top 3 or 4 inches of soil feels dry and maintaining a 2- to 2 1/2-inch layer of organic mulch around the tree guard against drought stress.

Chemical Management

• Treating a hornbeam with ready-to-use carbaryl insecticide labeled for use on bark helps rein in an existing borer infestation. Spray the trunk and branches thoroughly one or two weeks before you expect the adults to begin emerging. Spacing two additional applications two weeks apart destroys many of the surviving adults, along with eggs and larvae. Always wear protective clothing and follow the manufacturer's recommendations when working with insecticide.