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Protecting Our Trees from Invasive Species

Dan and Cheryl Forrest, Master Gardeners

The Minnesota Department of Natural Resources defines an invasive species as “species that are not native to Minnesota and cause economic or environmental harm or harm to human health.” This article defines and identifies the invasive species that can cause harm to the economy, environment, or human health.

Protecting Our Trees from Invasive Species

The Minnesota Department of Natural Resources defines an invasive species as “species that are not native to Minnesota and cause economic or environmental harm or harm to human health.” We’re looking at a two part definition.  In the first part we see that these species are not native to Minnesota.  We often use such terms as exotic, alien, introduced, etc.  There is an implication that all invasive species come from outside the United States.  This is not always the case.  These new pests simply come from outside Minnesota.  In the second part of our definition the key word is harm.  We are concerned with those pathogens, plants, animals or insects that can cause harm to the economy, environment, or human health.

Emerald Ash Borer (EAB)

The number one new pest facing our trees in Dakota County is the emerald ash borer (EAB).  The nearest confirmed sighting is in the area of Fort Snelling in Hennepin County and Highland Park in Ramsey County.   The general rule that the insect can travel by itself somewhere between 13-15 miles in a given year implies that within a short period of time we will have confirmed sightings within  Dakota County. Do you know the symptoms to look for?  The first thing to look for is increased woodpecker activity.  These birds know a tasty treat when they find one.  This increased activity normally happens in the second year after EAB infestation and is followed by vertical splits in the bark and sometimes defoliation in the tree canopy.  A closer inspection could then reveal the large s-shaped galleries under the bark and the D-shaped exit holes.  These symptoms generally confirm that the tree has been infected.

Gypsy Moth

The second pest that we all need to be aware of is the gypsy moth.  It attacks several varieties of trees but here in Minnesota aspens and oak seem to be the favorite.  The gypsy moth caterpillars are capable of defoliating acres of trees.  It has arrived in Minnesota!  Not necessarily here in Dakota County, but it has invaded out state.  Lake and Cook counties in the Arrowhead region have reached the point where they will be experiencing the first steps in a quarantine.  How will it affect us?  Most of us will not be directly affected.  But if we have property in these counties or intend to vacation there we will feel the impact.

Thousand Canker Disease

Thousand Canker Disease is a third new pest that affects black walnuts.  It is a fungus carried by the walnut twig beetle.  When it enters the bark it leaves behind a fungus that causes a canker.  If you have any black walnut trees you need to be concerned and be able to recognize it.  For the rest of us you do need to know that it is now the law that black walnut wood of any size or shape cannot be imported into Minnesota.  A quarantine is in effect that makes this illegal.

Asian Longhorn Beetle

The fourth pest emerging in Minnesota is the Asian Longhorn Beetle.  While poplars, maples and box elders seem to be the preferred trees in Minnesota it also feeds on several other varieties which makes it especially dangerous.  Signs and symptoms include crown die-back, shallow depressions in the bark where the eggs are laid, sap seeping from these egg niches, pencil-size round exit holes and a sawdust on the top of branches or on the ground surrounding the tree.

Brown Marmorated Stink Bug

Two new and emerging pests do not actually kill the tree but both will damage the fruit the tree produces. The first is the Brown Marmorated Stink Bug.  It has been found in several Minnesota counties and is considered a pest because it feed on fruit and vegetables.  The insect is also considered a nuisance as it invades houses and other buildings in the fall seeking warmth.  When disturbed, it emits a foul odor.

Spotted Wing Drosophila

Spotted Wing Drosophila (SWD) is also a new and emerging pest and has been found in 20 counties in Minnesota.  It is similar to a fruit fly.  But unlike the typical fruit fly that feeds on damaged fruit, SWD feeds  on intact, healthy, ripening fruit, especially thin-skinned berries.  The female can pierce the soft skin and lay its eggs.  In doing so there is also a possibility of an introduction of rot and fungus. So far in Minnesota the favorite has been raspberries.  It has been known to attack apples and other tree bearing fruits.

Oriental Bittersweet

In our training one invasive plant was discussed.  Oriental Bittersweet has been found in Dakota County in Burnsville and Eagan.  It is a vine that can grow over 60 feet long and will girdle and smother trees and shrubs.  It is spread by rhizomes and seeds, mainly through birds ingesting and then eliminating the seeds.  Through this natural action, entire plant communities have been known to be overwhelmed.  iI becomes our job to eradicate it.

What Can I Do?

We still need to answer one important question.  What should be done if you suspect you have found one of these tree pests?  There is an “Arrest the Pest” hotline you can email at or by phone at 888-545-6684.  Doing so could help us protect our native trees.

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