Two-lined Chestnut Borer (TLCB) is an opportunistic insect pest that attacks a variety of species. It was given the name “chestnut” borer in recognition of its status as a primary pest of the American chestnut tree.

The chestnut has been almost completely wiped out by a fungal disease, yet the insect has retained its name to this day even though there are few chestnut trees for it to attack. A pair of faint white to golden colored lines on the main body and wings of adults are useful identifiers.

In mid-July, the first visible symptoms of TLCB infestation occur. Infested oaks may be recognized by the sparse, small and discolored foliage, which is followed by the dieback of branches. Leaves of infested branches turn uniformly red-brown. The leaves on non-infested 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.

Susceptible Hosts

The TLCB is primarily a pest of Quercus and Castanea species. It is considered one of the most serious insect pests on oaks, and the species most frequently attacked are: chestnut oak (Q. prinus), white (Q. alba), black (Q. velutina), red (Q. rubra), scarlet (Q. coccinea), and burr (Q. macrocarpa)

Insect Pest

Agrilus bilineatus

Symptoms

• Larvae create meandering galleries on the surface of the wood, which are visible if patches of bark are cut off infested branches or stems.

• Impaired water movement within infested trees causes leaf wilting and dieback from the top of the tree downward. There is a “dead, red, green” appearance from the top down (i.e. dead leaves, wilting leaves, and green leaves)

• The dead, brown leaves usually remain attached to the tree, even after normal leaf drop in the fall.

Signs

Insect Larvae

• The larvae are white with an enlarged head, slender, noticeably segmented, flattened, and about 2.5 cm. long when full grown.

• Larvae are about 1 1/4 inches long when fully grown

• Larvae have two spines at the tip of their abdomens.

Adults

• Beetles are 6-10 mm in length and approximately 2 mm in width

• Adults are bluish black in color

• This pest gets its name from the two pale lines which run the length of the wing covers

• Adults emerge through D-shaped exit holes, and generally begin to appear in early June

Distinguish from Look-alikes

• Oak anthracnose symptoms may occur on an annual basis. Leaf symptoms generally progress from the bottom of the canopy upward, and while leaves may fall, branches are not generally killed and the leaves do not turn uniformly brown.

• Oak wilt attacks various oak species, and may be confused with TLCB. In the red oak group, oak wilt generally kills trees rapidly (4-6 weeks) and leaf symptoms include defoliation whereas with TLCB the leaves persist on the tree. In the white oak group oak wilt causes more random leaf symptoms within the canopy, although defoliation is still an indicator. Look for individual branch dieback and beetle galleries in the white oak group to help differentiate these disorders.

The best protection from the TLCB is prevention. Because this pest attacks weakened and stressed trees, care and attention should be given to maintaining tree health by watering during drought, mulching, and minimizing damage to the root zone that could cause root injury and stress.

When symptoms of TLCB are visible it is nearly impossible to save the infected limbs, so consider whether the remaining portions of the canopy are worth trying to preserve. As a rule of thumb, if more than a third of the tree is infested the chances of saving it are significantly reduced.

Infested branches and deadwood should be removed, so give attention to what the tree will look like before deciding on subsequent treatments. High value trees at risk for infestation by the TLCB may be treated preventively with Xytect insecticide.

A soil drench in the fall or early spring will provide season long protection. Additional protection can be obtained through cover sprays of insecticides to the trunk and main limbs. Trees infested with TLCB may be similarly treated, although trees with serious decline are unlikely to be saved.

University of Wisconsin Extension

Minnesota Department of Natural Resources

USDA Forest Service