Oak trees showing signs of drought stress, beetle infestation

Oak trees stressed by recent drought have been showing symptoms of infestation by twolined chestnut borer, according to the Minnesota Department of Natural Resources.

Twolined chestnut borer is a native wood-boring beetle. The larvae feed beneath the bark of oak trees. Trees stressed and weakened by drought are especially vulnerable, while healthy trees are usually not infested.

“Even when hot, dry weather is replaced by rainy days, it can take years for trees to recover after a drought,” said Val Cervenka, DNR forest health coordinator. “After two consecutive years of drought conditions in 2021 and 2022, we expect to see oak trees dying from twolined chestnut borer attack for the next few years.”

Symptoms of an infestation often begin in mid-July and initially include dead and dying leaves at the top of the tree. Dead leaves can stay on branches for months. During the year following the attack, the top of the oak tree will be dead and leafless; leaves in the middle section die, become orange-brown and stay on the tree; leaves at the bottom will still be green. If the tree has been infested for more than a year or two, it might be possible to find small, D-shaped exit holes in the trunk where the adult borers have come out of the tree.

Oak wilt and twolined chestnut borer symptoms can be similar: red oaks with oak wilt will rapidly drop most of their leaves within six to eight weeks, and leaves may be green or only partly brown when they fall from the tree. In contrast, dead leaves on trees attacked by twolined chestnut borer will hang on to the tree and remain brown. For more information, head to the forest health page of the DNR website (mndnr.gov/TreeCare/Forest_Health).

“Unfortunately, preventing an attack of twolined chestnut borer on stressed oaks is difficult,” Cervenka said. “However, you can take steps to reduce stress during drought, which decreases the likelihood of twolined chestnut borer attack.”

To reduce stress in yard trees:

• Mulch and water trees properly. If rainfall is inadequate, watering oaks weekly may be the best method to prevent twolined chestnut borer attack.
• Avoid adding soil over roots, do not fertilize stressed oaks, and do not allow herbicides to contact oak leaves.

To reduce stress in woodlands:

• Thin trees crowding oaks to reduce stress caused by competition for resources. However, wait until the trees have recovered from recent drought because thinning during or shortly after a stress event can increase the chance of attack from twolined chestnut borer.
• Avoid wounding oaks from April to July when oak wilt spreads easily.
• Add a diversity of tree species to make woodlands more resilient to change.

Eradication of twolined chestnut borer in woodlands is not possible. However, over time, borer populations will naturally decline. If many, high-quality oaks are affected, landowners can work with a forester to set up a salvage harvest during the winter before the trees degrade. Contact a local DNR forester (mndnr.gov/Woodlands/CFM-Map.html) for questions about managing oaks in woodlands.

For more information, head to the tree care page of the DNR website (mndnr.gov/TreeCare).