Two-lined chestnut borer

photo: Two-lined Chestnut borerThe two-lined chestnut borer, Agrilus bilineatus, is a native beetle that attacks weakened oak trees. It attacks all oak species found in Minnesota, with red oak its preferred host. When trees and stands are healthy, TLCB confines its attack to low-vigor trees or broken branches. When drought stress, construction, and/or defoliation have reduced tree vigor, oaks are predisposed to TLCB attack. Under severe stress conditions, widespread outbreaks of TLCB can occur.

The best management against TLCB is prevention; keeping trees healthy and vigorous will allow them to fight off invading borers on their own. Do this through proper watering if possible. Also avoid compacting the soil, changing the soil grade or water drainage pattern, damaging the bark, allowing significant amounts of defoliation by insects, or anything else that may stress the tree. Anything that weakens tree health encourages borers.

Adult beetles seek out and lay eggs on weakened oaks in late May and June. From June to August, larvae feed on the inner bark of live branches and stems. This destroys nutrient- and water-conducting tissues, causing the foliage to turn brown and hang on the branches. Larvae create meandering galleries on the surface of the wood that are visible if patches of bark are cut off infested branches or stems. Larvae are white with an enlarged head, two spines at the tip of their abdomens, and a slender, segmented body. They are about 1¼ inches long when fully grown. Larvae pupate under the bark in the fall. Adult insects emerge through D-shaped exit holes in the bark the next May and June.

The first visible symptoms of TLCB infestation occur in mid-July. Infested oaks may be recognized by their sparse, small, and discolored foliage, which is followed by the dieback of branches. Leaves of infested branches turn uniformly red-brown. The leaves on noninfested branches remain green. Infested oaks have a distinctive pattern of dead and live leaves on them. Branches in the upper crown are dead and leafless; branches in the middle crown are dying and have red-brown wilted leaves; branches in the lower crown are alive and have green leaves. In other words, TLCB infested oaks have a "dead, red, and green" pattern from the top of the tree down its branches.

By the time branch flagging becomes fully evident in August and September, the attack is finished for the year. The dead, brown leaves usually remain attached to the tree, even after normal leaf drop in the fall. When a tree is killed, surrounding oaks are often attacked by TLCB and Armillaria root disease and killed in the following year, creating a pocket of dead trees.

Management options