A bug is winning the battle over ash trees

The ash trees we once thought would be the best replacement for diseased elms are headed for near-extinction, forcing the City of Eden Prairie to change its strategy.

Once focused on controlling the spread of the invasive, half-inch-long emerald ash borer (EAB), the city has, since November, turned its attention to public safety and the removal of diseased ash trees that might fall on streets, trails, and other infrastructure. Controlling the spread is no longer an option.

The fast-moving beetle arrived in Michigan from Asia in the late 1990s via infested shipping pallets. First discovered in Minnesota in 2009, the insect larvae kill ash trees by tunneling under the bark and feeding on the part of the tree that moves nutrients up and down the trunk.

Prior to the bug’s arrival in Eden Prairie in about 2017, the Minnesota Department of Natural Resources (DNR) estimated this community had 50,000 ash trees on developed and maintained land alone. Thousands more stood in the city’s natural areas. 

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But the insect has treated Eden Prairie like a fast-food drive-through. What started as 23 EAB-infested trees marked for removal in 2017-18 grew to more than 1,000 in 2020-21, according to city staff, and 1,300-plus in 2021-22. Today, Eden Prairie is considered to be “generally infested,” as is most of the southeast quadrant of Minnesota. City officials say the infestation is at a peak, and untreated trees will likely be dying or dead within three years.

In fact, if you have an ash tree in your yard that is not already being treated for EAB, there’s a strong likelihood that it’s already infested, says Jonathan Osthus, the EAB technical assistance coordinator at the Minnesota Department of Agriculture (MDA) and a 2004 graduate of Eden Prairie High School. You might not even know it, as the ash borer’s activity happens beneath the bark.

The scope of the issue is huge. The U.S. Department of Agriculture (USDA) says the emerald ash borer is responsible for the death and decline of tens of millions of North American ash trees, which originated between 34 and 56 million years ago. EAB has been detected in 36 states and the District of Columbia.

The Journal of Integrated Pest Management in 2017 called EAB the most destructive non-native forest pest to ever invade the United States economically due to widespread tree mortality, associated cost of quarantine regulations, and impact on related industries, homeowners, and communities. According to the publication, it’s considered worse than chestnut blight and Dutch elm disease, adding that mortality in heavily infested areas could reach 99%.

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Three types of ash trees are native to Minnesota. The green ash is the most common in this area, but Minnesota also has black ash and white ash. All are threatened by EAB, which has already been found in 53 of Minnesota’s 87 counties – including nine counties this year alone. “It’s going to continue to spread, and it will eventually be found throughout the entire state,” said Osthus. 

Treatment and an eye toward public safety

When its strategy focused on controlling the spread of EAB, the city conducted an annual tree survey, marking and condemning infested trees and requiring their removal during EAB’s dormant season to prevent further spread.

The new strategy focuses on condemning only severely infested trees that pose a risk to the public or infrastructure. Winter surveys will target areas of high infestation and risk. Residents will be allowed to remove trees at any time of year and will be given more time to do so.

Diseased trees considered a hazard to the public or infrastructure are generally those in the front yard, nearest the street, according to Erika Commers, the city’s forestry specialist. 

“If we mark an ash tree in your front yard and there is another one or two in your backyard, we recommend getting those trees removed as well,” she said, “because they will still pose a risk to the property and occupiers when branches are dead. Removing a live tree tends to cost less than removing a dead-standing tree. “

With the help of grant funding, the city is still treating about 1,200 ash trees on public land, on a three-year treatment cycle in hopes of prolonging the trees’ lives. It has also contracted with Rainbow Treecare to provide discounted treatment for residents’ ash trees through 2026. More than 2,000 ash trees on private property are being treated, according to the city, but the window is closing for residents who are still considering the treatment option.

“We continue to encourage folks to consider next steps for their ash trees now, as time is running out for any options other than mortality,” Commers stated.

“Treatment has shown to be highly effective against EAB, even if neighboring trees show severe signs of infestation,” she said. “To qualify for treatment, a tree needs to be healthy with around 75% live canopy remaining. If you recently bought a property, check for a small metal tag that will indicate the tree was treated, and often provides the year. 

“Label recommendations are to treat on a two-year cycle, but the city has been treating it on a three-year cycle to stretch funds and provide treatment to more trees,” added Commers. “This has been effective for seven years now with very minimal to no EAB evidence on those trees. It is certainly not a lost cause to try to retreat a tree if it has been a few years since the last treatment, as long as the tree is healthy.”

Replanting urged, but with diversity

Looking ahead, climate volatility and global trade make it likely that other species of American trees will eventually be threatened by disease or other invasive pests, according to Osthus. Replanting of lost trees is critical, especially with tree diversity in mind, he added, urging homeowners to think beyond the currently popular maple trees. 

“Diversity is key to how we go about things,” Osthus said.

Commers added that Eden Prairie will likely see a small decrease in its overall tree canopy cover, since ash trees were a staple for developers and homeowners during Eden Prairie’s development boom of the 1980s and ’90s. Ash trees make up about 15% of the tree canopy in Hennepin County, but it’s not known what percentage is specific to Eden Prairie. However, she hopes the canopy shrinkage is temporary as newly planted and established trees begin to grow. Within its managed areas, the city has been replanting an average of 1.5 new trees for every diseased tree removed.

The city also runs a tree sale in coordination with Tree Trust for Arbor Day, with trees available to residents at a discount. The sale is online and opens in March; watch the city website for more information. 

Ash trees have supplied America with hockey sticks, baseball bats, tool handles, furniture, and more. But what Eden Prairie residents may miss most when ash trees disappear is simply their place in nature, which drew many city dwellers to the suburbs in the first place.

So, it may be worth a paragraph to wax nostalgic about the disappearing ash tree. Nature writer Donald Culross Peattie perhaps did it best in the book “Trees of Eastern and Central North America” when he described the ash as “Nature’s last word”:

“How many thousand-thousand of untold White Ash trees are the respected companions of our doorways, kindliest tree in the clearing beyond the cabin? No one can say,” he wrote. “But this is a tree whose grave and lofty character makes it a lifelong friend. White Ash has no easy, pretty charms like Dogwood and Redbud; it makes no over-dramatic gestures like Weeping Willow and Lombardy Poplar. It has never been seen through sentimental eyes, like the Elm and the White Birch. Strong, tall, cleanly, benignant, the Ash tree with self-respecting surety waits, until you have sufficiently admired all the more obvious beauties of the forest, for you to discover at last its unadorned greatness.”