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Spring Pest Watch
Article #170, May 2011
By Bill Cook

     Spring time is when dormant ecosystems return to full vigor.  This includes fungi, bacteria, and other pathogens.  Soon enough, the bugs will be here, too.  Most pathogens and insects are a natural and desirable part of a forest, even though some of us are not fond of them.  And then, there are a few non-native pests that deserve special attention.

     For the following reasons, and for more reasons to come, don’t move firewood.  The good old days are now gone when moving firewood was a harmless activity.  This may be hard to believe, but it’s been true over and over again. 

     Now is the time to avoid pruning and damaging oak trees.  Every wound is a potential entry for oak wilt.  This is a disease that is most definitely not wanted.  It’s expensive to eradicate and the visual impact is significant.  Treatment in residential areas is particularly difficult.  The highest risk season runs through July but infection is possible throughout the growing season. 

     An oak wound is one of the very few situations where a tree wound should be painted, and as soon as possible after the wound.  While the wound is moist, the oak tree is vulnerable.  If you have oaks along rights-of-way and roadsides, beware of maintenance crews.  Some crews are notorious for spreading oak wilt.  Others are quite careful.  The same is true for tree care contractors. 

     A tree infected with oak wilt will lose all the leaves within a few weeks, sometimes more quickly.  Leaf loss will begin at the top of the tree.  However, oak wilt can be confirmed only through a laboratory test or presence of fruiting bodies under the bark.  A trained eye and knowledgeable assessment may sometimes be adequate.  Oak trees can succumb to a number of pests, some of which can appear like oak wilt.

     Emerald ash borer (EAB) is common throughout most of the central states and has now been found in several locations in the Upper Peninsula and Wisconsin.  Again, firewood is one of the most common ways this beetle has been transported. 

     EAB begins its work at the top of an ash tree and works its way down.  The light green larvae feed on the live tissues under the bark.  Fluid movement is interrupted and the upper leaves wilt and die.  The process may take several years and looks similar to other ash maladies.  Near the end, the tree will produce a bunch of shoots around the base of the tree trunk.  Woodpeckers may frequent the trees looking for EAB larvae to eat. 

     Each spring, hundreds of purple trap boxes are hung in ash trees as part of a survey.  If enough EAB adults are in the area, some may find their way to the trap boxes.  In the fall, an empty trap is good sign but not a sure indicator of EAB absence.  If you think you have seen EAB, contact a forester or the Michigan Department of Agriculture. 

     Gypsy moth populations continue to fluctuate with spring weather conditions.  In regions where gypsy moth has been around for several years, the populations become less explosive as native predators and parasites begin to “naturalize” the species.  The larvae populations reach their peak in both numbers and individual size around the Memorial Day weekend, which can be inconvenient for backyard grillers and picnickers. 

     Gypsy moths prefer to eat the leaves of oaks, aspens, and apples.  However, most other species will be attacked if larval populations grow large.  In the northern regions, gypsy moths do not usually kill trees, unless the trees are already stressed from drought, old age, poor soils, or other predisposing condition. 

     Beech bark disease will soon have killed most of the beech in Michigan, the last large North American stronghold for the tree species.  Fortunately, a small percentage of beech are resistant and seeds have been collected.  Hopefully, the genetically resistant species can be introduced back into the forest. 

     Historically, our northern forests have already seen widespread impact from exotic pests such as Dutch elm disease, white pine blister rust, and the larch casebearer.  Elsewhere, North American forest ecology has been altered by diseases such as chestnut blight and sudden oak death. 

     In the future, we’ll almost certainly see additional forest-damaging species such as the Asian long-horned beetle, hemlock woolly adelgid, and other insects and diseases.  The number of severe forest threats is increasing.  One of the best defenses is proactively managing the forest for maximum vigor and tree diversity.  The forest is more complex than what you might think.  Consult a professional forester. 

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Bill Cook is an MSU Extension forester providing educational programming for the Upper Peninsula. His office is located at the MSU Forest Biomass Innovation Center near Escanaba. The Center is the headquarters for three MSU Forestry properties in the U.P., with a combined area of about 8,000 acres. He can be reached at cookwi@msu.edu or 906-786-1575.

Prepared by Bill Cook, Forester/Biologist, Michigan State University Extension, 6005 J Road, Escanaba, MI  49829
906-786-1575 (voice),  906-786-9370 (fax),  e-mail:  cookwi@msu.edu

Use / reprinting of these articles is encouraged. Please notify Bill Cook.
By-line should read "Bill Cook, MSU Extension" Please use the article trailer whenever possible.

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