If an oak croaks in the forest and no one hears it, is it still dead?

image of TLCB centered around Grand Rapids in 2003

If the oak is a red oak and it is within fifteen miles of Grand Rapids than the answer is likely to be yes, it is dead. For the second year in a row, many oaks were attacked and killed by two-lined chestnut borer (TLCB). In an aerial survey flown this September by DNR Resource Assessment, 12,557 acres of stands with oak mortality were detected in Itasca, Cass, northern Aitkin, northern Crow Wing and southeastern Beltrami Counties. In many stands, 80 to 90% of the oaks are now dead. Many stands thinned in the last year or two now have nearly complete mortality of the residual oaks.

TLCB is a native opportunistic insect that is able to attack and kill weakened stressed oaks of any species. The trees were attacked because they were stressed by forest tent caterpillar defoliation and drought. Where additional stresses occurred, such as, construction damage or recent thinning, there was nearly 100% mortality of oaks in the affected stands. Leaves on trees attacked by TLCB began to turn brown the first week of August. Entire trees seemed to turn brown overnight. By the time the leaves start to wilt most of the damage is done for the year. Larvae of the two- lined chestnut borer feed under the bark and girdle the tree. This cuts the water supply off to the top of the tree causing the leaves to wilt and to turn brown. The larvae overwinter in the bark and emerge from the tree in May, June and early July of next year as adult beetles. They are able to detect stressed oaks and fly long distances to find suitable egg-laying sites.

Sanitation is usually suggested as a means of reducing the risk of TLCB attack. However the real, underlying problem is that the trees are stressed. There's not much that can be done in forest stands to alleviate drought stress. So control in the forest is usually impractical. The best thing to do is probably to just avoid disturbing the stands further which would add to their stress.

Movement of recently killed or dying oaks as firewood will also move the TLCB with them. Next May, June and July, adult TLCB will emerge from trees damaged or killed this year. Home owners should avoid bringing infested wood into their yards if they still have living oaks. If infested wood is brought into a yard, it must be burned before next May or kept tightly sealed with a tarp. An alternative would be to peel the bark off the wood and burn or bury the bark, but that would be a lot of work for firewood. An alternative method to protect living oaks that was suggested by Chittenden (1909) was to spread a mixture of clay and cow manure thickly over the trunk and branches of living oaks. This was intended to deter the adult two-lined chestnut borers that emerge form the firewood pile from laying eggs on the nearby living trees. It should also keep those pesky neighbor kids from climbing in your trees.

The best thing homeowners can probably do for their trees is to keep them well watered. Root growth continues late into the fall so continue watering even after the growing season ends. A layer of mulch over the root system will also help keep the soil moist and protect the root system in case we have another winter without snow.

More information on two-lined chestnut borer and management can be found on the following web sites:

MN DNR - Backyard Tree Care

USDA Forest Service