Our Trees Under Attack

Bob Vaughn, UM Extension Master Gardener in Anoka County, April 10, 2015

Adult Emerald Ash Borer

Adult Emerald Ash Borer

Emerald Ash Borer, Oak Wilt and Dutch Elm disease are three serious threats to the landscape trees and forests of Minnesota.

An infestation of Emerald Ash Borer (EAB) has been confirmed in Anoka County. Ramsey, Hennepin, Houston and Winona counties also have this pest. EAB is not a disease but is a serious pest invader for ash trees. The EAB is a non-native metallic wood boring beetle. It is small, about 3/8th inch long, and is a bright metallic green color.  The larvae of these beetles feed in the cambium layer just under the bark.  This layer moves water up to the leaves and sugars down to the roots.  Once this layer is damaged all the way around the trunk (girdled), the tree will die.

Some of the symptoms include woodpecker damage, die back of branches in the crown, epicormic sprouts (suckers) on trunk, small 1/8th inch D shaped exit holes. More information on EAB can be found at www.emeraldashborer.info. This site can help you identify ash trees and EAB, there is also information on treatments, selecting an arborist and a guide to help you determine what you should do if you get this pest. If you suspect EAB in your trees contact Arrest the Pest hotline (651)201-6684 or 1-888-545-6684 or www.mda.state.mn.us/invasives . Moving firewood from an infected area to another area is the main cause of this pest spreading quickly.  Left to its own sources, beetle infestation spreads very slowly.

Oak Wilt exists now in Anoka County.  Oak Wilt is a fungal disease that affects oak trees.  Oaks are the most plentiful and valuable shade trees in our state.  Northern Red Oak and Northern Pin Oak are most susceptible.  Burr Oak and White Oak are less so.  Oak Wilt is much more controllable than Dutch Elm Disease. This is because 90% of Oak Wilt is spread underground by root grafts.  Trees up to 50 feet apart commonly have grafted roots.  If we follow the “no prune” model in April, May, and June we can prevent new outbreaks of Oak Wilt.

The other 10% of Oak Wilt is spread over land by the picnic beetle.  The beetle carries fungal spores and is attracted to injured trees. These beetles are not borers so they need an injury on tree to spread disease.   Transporting firewood can spread this problem as fugal mats can be under bark of logs. The beetles rarely travel more than 1 mile per year. Once an infection in an oak tree is identified, root graft barriers should be put in place.  These barriers consist of a trench or vibratory plowing 5 feet deep around infected trees.  A secondary trench should be plowed around the nearest trees without symptoms.

Dutch Elm Disease (DED) is also a fungal disease.  It is spread by two bark beetles: the native elm bark beetle and the smaller European Elm Bark beetle. DED disease took a heavy toll on the urban forest in the 1970’s.  Minneapolis lost thousands of stately elms.  This disease is harder to control because 90% is spread over land by beetles, 10% by root grafts.  Moving firewood is also a factor with this disease because fungus can grow in dead logs and beetles are attracted to dead or distressed trees.

So the moral of this story is to buy your firewood where you use it.  Don’t spread these pests at 60 MPH. The insects rarely travel more than 1 mile per year, without our help.