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.
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 30% 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.
All oaks of high value that have been subjected to stress due to abiotic or biotic disorders, construction injury or other reasons should be targeted for preventive treatment with Transtect or Xytect. Xytect and Transtect are most effective when applied to healthy appearing oak trees that have not been attacked.
Transtect soil applications or systemic basal bark sprays can be applied throughout spring and into early to mid summer. Transtect has the advantage that it moves into the tree within a 2-3 weeks, this gives the practitioner an opportunity to assess the canopy after full leaf out for any signs or symptoms from borer attack. Transtect will provide one season of control.
Xytect soil applications are applied in early spring as soon as soils thaw out and no later than the first week of May. Xytect can also be applied in late summer or fall to provide protection the following season. Xytect will provide one season of control.
Mectinite or Xytect 10% as a tree injection treatment for trees that cannot be treated with foliar sprays or other systemic options.
Cambistat improves the health of trees which improves their ability to withstand stress induced insect and disease attacks. For best results, Cambistat should be applied as part of an overall management strategy.
Other Treatment Practices
- Cambistat improves the health of trees which improves their ability to withstand stress induced insect and disease attacks. For best results, Cambistat should be applied as part of an overall management strategy.
- Adequate water is a key factor in maintaining healthy trees. A slow, deep watering event once per week during dry conditions will help maintain soil moisture levels and minimize the stress that invites the two lined chestnut borer.
- Mulch is very beneficial for trees because it reduces competition with turf and moderates soil temperature and moisture levels. The addition of 2-4 inches of wood chips or shredded bark under the drip line can have a very beneficial effect.
- 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.
- Promote health and vigor with, root collar excavation and prescription based fertilization practices
- Preventive treatments are more effective. Treat trees preventively if adjacent trees are displaying symptoms from TLCB attack.
- If >30% of the tree is showing symptoms saving the tree is difficult.
- If <30% of the tree is showing symptoms treatments may work, but not always.
- For infested trees, treatments will result in an improvement in tree condition typically in the 2nd full season following treatment as the tree repairs previous damage and prevents future damage.
Signs of Damage
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.
- 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
- 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
- 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
Trees At Risk
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 oak (Q. alba)
- Black oak (Q. velutina)
- Red oak (Q. rubra)
- Scarlet oak (Q. coccinea)
- Burr oak (Q. macrocarpa)
- Overwinters as larvae and pupae under bark.
- Adults emerge through D-shaped exit holes in late spring through summer (500-1000 GDD) and lay eggs in bark crevices •Larva feed on vascular system of the tree.
- One generation per season.
University of Wisconsin Extension
Minnesota Department of Natural Resources
USDA Forest Service
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.