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Dying Oak Trees

 Steven Katovich, USDA Forest Service,  Dieback of terminal branches is a common symptom of damage caused by Agrilus borers. This picture is of a white oak with dieback from twolined chestnut borer attack.

University of Minnesota Extension, Stearns County News
August 31, 2016           

Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns, Benton & Morrison Counties


Dying Oak Trees
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (08/31/16) — Central Minnesota oak trees have been facing many different challenges the last few years, some even causing death.  Disease and insects are both causing the decline of many of our century year old oaks. One pest that has seen a spike in population and is causing some significant damage in Central Minnesota, specifically the Morrison and Crow Wing County area, is the two-lined chestnut borer.

The two-lined chestnut borer, Agrilus bilineatus, is a native insect that attacks stressed or weakened oak trees. The borer seems to prefer red oaks, which can cause confusion because red oaks are the species most susceptible to oak wilt as well.  According to Andy McGuire, DNR Forester based out of the Little Falls Area Office, “Environmental factors are the biggest issue but it’s hard to find just one reason why it is so prevalent this year. It could still stem from the years of drought we had three to four years ago.” 

Typically the first visible signs occur in mid-July, with sparse, small, or discolored leaves.  This is followed by dieback, usually from the top of the tree down. There is a saying “dead, red, and green”, indicating the pattern of loss in the tree from the top down. The leaves on infected branches will uniformly turn red-brown but remain on the tree for several weeks.  Leaves on non-infected branches will remain perfectly green. This is key, as trees infected with oak wilt will first show the bronze, red-brown discoloration from the tip or margin of the leaf, while the rest of the leaf still remains green for a little while. Also, leaves on trees infected with oak wilt will begin to drop, even when some are still partially green. 

Unfortunately management is difficult, especially in woodlots.  Best practices are to minimize any stress your oak trees may be under. Andy McGuire, DNR Forester, recommends waiting a year or two to remove the trees if planning on using it for firewood, because it would require several years of continuous infestation to completely kill the tree. If a landowner decides to remove the trees, it should not be done April through July; rather, wait until winter to minimize compaction and also disease. It is also important to confirm the damage is caused by the two-lined chestnut borer versus oak wilt. Management is different. 

High value trees within someone’s yard can be treated with a systemic insecticide; however first efforts should be cultural such as watering in drought, reducing compaction, mulching, and proper pruning. Treatment using a systemic insecticide is only recommended if the tree is in the initial stages of decline with less than 40% canopy loss.  Systemic products containing imidacloprid or dinotefuron can be used, however a certified arborist should be consulted as application method and timing is critical. Always read, follow, and apply any insecticide according to the label. Remember, maintaining good plant health is your best management plan against two-lined chestnut borer.

As always, it is important to not transport firewood throughout the state and across state lines. Although it may seem harmless, insects and disease will be hitch-hikers and humans end up causing the spread of such unwanted pests or disease into other parts of the state.

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PHOTO: Steven Katovich, USDA Forest Service,

Dieback of terminal branches is a common symptom of damage caused by Agrilus borers. This picture is of a white oak with dieback from twolined chestnut borer attack.