Intense battle to slow a tree killer

More Eden Prairie property owners are turning to multi-year insecticide treatments to save their ash trees, as a deadly bug tears through Minnesota.

Last year, more than 2,200 ash trees on private property in Eden Prairie were treated in a program provided by Rainbow Treecare, at a discounted price through a contract with the City of Eden Prairie. That’s a 53% jump since 2019.

Rainbow Treecare has contracts with 37 cities. Among them, Eden Prairie ranked first in overall participation in 2022 with the highest number of private-property trees treated. In per-capita comparisons, which measure the number of treated trees per 1,000 residents, Eden Prairie ranked second, with Wayzata taking the top spot.

Why it matters: The emerald ash borer (EAB) is killing ash trees in at least 39 Minnesota counties, tunneling under the bark and feeding on the part of the tree that moves water and sugars up and down the trunk. Doing nothing means all infested trees die quickly and must be removed, straining the budgets of homeowners and local governments. If still healthy, ash trees can be treated over multiple years with insecticide. This keeps trees alive longer, spreads the costly removal of infected trees over more years, and allows time for science – including experiments with biological control – to catch up.

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The City of Eden Prairie has had a detailed Emerald Ash Borer Management Plan since 2018. In it, the city estimated that, of the roughly 358,000 total trees in maintained areas in Eden Prairie, more than 50,000 are ash trees.

They are succumbing quickly to EAB, and it’s getting worse.

“The entire city is considered infested,” said Matt Bourne, the city’s parks and natural resources manager. “It’s safe to say EAB is everywhere.”

Some facts about the ash borer problem:

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• This winter, 758 ash trees have been removed on city property. Over half of these had some signs of EAB infestation.
• Also this season, 194 heavily infested ash trees have been marked for removal on private property.
• Signs of initial EAB infestation have been found on 125 private properties. Letters were sent to homeowners notifying them of survey findings, recommending they contact a certified arborist to determine a plan for their ash trees.
• The cost of treating an ash tree on private property is based on the size of the tree, according to Rainbow Treecare, but typically ranges from $70 to $200 per tree each year, and treatment is recommended every two years.

The solution: There is no silver bullet, but the Minnesota Department of Agriculture says detection is improving and more is being learned over time. Experiments in forest-type settings include the use of parasitoid wasp species that either attack the larval stage of EAB under the ash bark or kill EAB eggs that are in bark crevices. Ultimately, other tree varieties need to replace ash trees. That practice is underway in Eden Prairie parks and on other city properties, where each removed tree is being replaced with, on average, 1.5 trees of other species, Bourne said.

Eden Prairie Local News reached out to residents via the Nextdoor social network to find out why those treating ash trees decided the expense was worth it. Here are a few responses:

• “Almost any tree is worth saving,” said Joan Palmquist, a Cove Pointe Road resident who has treated a pair of front-yard ash trees for about four years. Unfortunately, she added, no treatments were provided to a larger number of trees on her homeowner association’s common property, and those trees were removed, leaving ugly stumps behind. “It’s really quite a sad sight,” she added.
• Tim Mullee, who lives in the neighborhood behind Jerry’s Foods in southeastern Eden Prairie, was just notified by the city that he should consider treatment if he wishes to keep the stately, 50-year-old, 60-foot-high, front-yard ash tree that helps shade his house. He plans to do so. “I certainly can’t buy a tree like this,” he wrote. “Our neighborhood has many large trees of various varieties and I love the sheer presence they bring. Additionally, being able to sit under the tree, watching my young kids play outside, are some of the fondest memories I have.”
• “We consider our trees an investment,” said Julie Price, who has been paying Rainbow Treecare to treat a front-yard ash tree that shades her house from the morning sun.
• Michael Anderson is treating four ash trees on his property near Round Lake. “The largest was given to me by my dad shortly after he and I had finished building our house 40 years ago,” he wrote, “so there is some sentimental reason for treating that one.”
• Finally, Tony Morimoto sent a photo of his neighborhood (below), with snow-kissed trees lining the street, to help show why trees in general help make a neighborhood great, and why people are willing to pay for treatments to keep trees around. “If the trees disappeared, so would our neighborhood,” he wrote.

Go deeper: You can find additional information about EAB on the city’s website or on the Minnesota Department of Agriculture website.