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Will emerald ash borer decimate the Upper Midwest's ash trees?

"Growing Together" columnist Don Kinzler says our awareness of the invasive insect can help stop the spread.

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Emerald ash borer adult beetles are metallic green and about one-half inch in size.
David Samson / The Forum
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FARGO — Picture a city in which three-fourths of its trees are standing dead. This bleak vision can become a reality if emerald ash borer continues marching across the Upper Midwest.

Abbreviated EAB, the invasive insect was first detected in the United States in 2002, and has since spread to 35 states, where it has killed hundreds of millions of ash trees. According to the University of Illinois, the insect kills 99% of the trees it invades.

Although it only attacks ash trees, nearly 80% of the trees in some North Dakota cities are ash, and Minnesota isn’t far behind, with ash comprising 60% of many cities’ tree types. If EAB continues marching onward, dead trees will be everywhere.

That doesn’t need to happen, because our awareness of the insect can help stop the spread. May 24-30 is Emerald Ash Borer Awareness Week in North Dakota, and the state Agriculture Department and Forest Service and North Dakota State University Extension are working together to increase knowledge about the threat this pest poses to the region. Today’s column is based on information provided by these groups as part of the awareness week.

How close is EAB? It’s already present in Minnesota, South Dakota and Manitoba, and is within 70 miles of North Dakota’s borders. While EAB spread appears to be progressing more slowly in recent years, all it would take is an infested bundle of firewood to quickly bring EAB within North Dakota.

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Humans bear the greatest responsibility for the quick spread of the insect through the affected states. Transporting firewood that is harboring this bark-boring insect drastically accelerates the spread of EAB. Firewood should be burnt where it’s purchased, or buy firewood that’s been certified for sale.
The adult EAB beetle is metallic green with coppery reflections and is one-half inch in size. Eggs laid in bark crevices hatch into larvae that tunnel beneath the bark, creating winding, S-shaped galleries.

The larvae eventually emerge from the tree, creating D-shaped exit holes from trunks or branches.

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A small D-shaped hole indicates the presence of the emerald ash borer.
David Samson / The Forum
Click here to see other recent "Growing Together" and "Fielding Questions" columns from Don Kinzler.

As the larvae bore below the bark, the tree’s water and nutrient flow are interrupted, causing branch dieback and eventual death during a two- to five-year period. EAB attacks young and old ash alike.

EAB isn’t the only cause for ash decline, and so far all the North Dakota trees suspected of having the insect were damaged by other pests or diseases. An overall state of decline is a symptom of EAB, and investigators specifically look for D-shaped exit holes on the bark of the smaller branches and stems, and for serpentine feeding galleries underneath the bark.

Woodpecker behavior can also help detect EAB. It has been observed that in EAB-infested forests, bark-foraging birds like woodpeckers will forage more heavily on ash trees than other species. This is the principle that allows managers to use the signs of bark-foraging birds on ash trees to identify locations that may contain EAB.

It’s long been wondered whether EAB could survive North Dakota’s cold winters. New information shows that there is enough variation in the insects to allow a portion of the beetles to survive extreme cold conditions, as they have in Winnipeg. Prior to this recent finding, it was believed that temperatures reaching minus 30 degrees Fahrenheit would kill 98% of the EAB. So, whether ND’s cold climate will spare the state is unknown.

Because ash is under such threat from EAB, very few ash trees are sold or planted anymore, with the goal of diversifying tree types planted in shelterbelts, communities and yards. Many cities are proactively removing and replacing unhealthy or declining ash trees.

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Insecticides aren’t currently recommended unless an EAB infestation has been verified within 15 miles, or the ash trees are in an EAB-infested area. Applying insecticides when no beetles are in the area is futile, as the insecticides have a limited protective life.

For more information about EAB and its control, visit https://www.ag.ndsu.edu/publications/lawns-gardens-trees/emerald-ash-borer-biology-and-integrated-pest-management-in-north-dakota.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.

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