Oak trees stressed by recent drought have been showing symptoms of infestation by twolined chestnut borer, according to the Minnesota Department of Natural Resources (DNR). Trees stressed and weakened by drought are especially vulnerable, while healthy trees are usually not infested.
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.
The DNR has a helpful fact sheet (PDF) about twolined chestnut borer. Also view the forest health page of the DNR website.
Residents can contact the city forestry division with questions.