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Duluth braces for emerald ash borer infestation

Emerald ash borer beetle.

It has been nearly four months since emerald ash borers were discovered on Park Point, and residents gathered Tuesday evening at Duluth City Hall to discuss the city's emerging plan for how to combat the destructive beetle and save at least some local ash trees from near-certain death.

The arrival of the invasive insect in the Twin Ports was first documented in 2013, when emerald ash borers turned up in Superior. Duluth has been on high alert ever since, watching for the seemingly inevitable spread of the beetle, and in October four trees on Park Point were found to be infested.

An emergency quarantine was declared, and all transport of ash branches or wood from the Point to other parts of the city was forbidden.

In a pre-emptive move, the city already has cut down 34 ash trees that could be harboring ash borer eggs or larvae due to their proximity to where the invasive insects had been found on Park Point, said Dale Sellner, buildings and grounds supervisor for the city of Duluth.

But while the city of Superior's solution to dealing with the pest primarily has been to cut down its ash trees, Duluth is looking to develop a more nuanced long-term strategy, and members of the Urban Forest Commission met to share the details of a draft management plan Tuesday.

Left to their own defenses, ash trees throughout the city are likely to succumb to the non-native insect. The adult beetle feasts on the leaves of ash trees, but the insect wreaks its greatest damage in larval form. Larvae burrow under an ash tree's bark and feed on its phloem, effectively girdling the tree and depriving it of essential nutrients.

In Michigan, where the beetle made its first U.S. appearance in 2002 — presumably arriving in shipping pallets from its native China — more than 99 percent of ash trees with trunks larger than 2.5 centimeters in diameter have been killed as the infestation has spread.

The beetle lacks any natural predators stateside, but successful insecticide treatments have been developed. These involve periodically injecting trees with emamectin benzoate to protect them from infestation.

A little more than 20 percent of trees planted along Duluth's boulevards are ash — 2,404 trees in all.

A draft management plan proposes to remove most of those trees. But mature ash trees with trunks that measure at least 12 inches in diameter at a point 4½ feet above ground-level would be candidates for protection with insecticide, assuming they are in otherwise sound shape. The study identifies 911 ash trees as candidates for protective treatment. If the approach proves successful, it could save about 38 percent of the ash trees now growing along Duluth's boulevards.

Louise Levy, an arborist and owner of Levy Tree Care, praised the city's proposal to protect mature ash trees but said a hard-and-fast approach to determining which to cut and which to save should involve more than measuring trunk diameters.

"What I would like to be assured of is that the distribution of those ash trees that will be protected is throughout the city of Duluth," she said, suggesting that smaller trees might warrant treatment if they are in areas with relatively few mature trees.

Sellner said the most recent audit of boulevard trees the city conducted was flawed and would need to be redone before a more detailed plan can be developed. He said the audit was conducted largely by temporary workers and volunteers. When cross-checked by city staff, 20 to 30 percent of the data was found to be incorrect.

Sellner called the situation "unfortunate" and said: "I could dance around the subject. I'm not going to. It just needs to be redone."

Prior to any removal of boulevard trees, they would be marked, and neighboring residents would receive at least 30 days' notice of the city's plans. Individuals would have the option to save trees slated for removal by paying to have them treated with insecticide instead.

New trees of different species would be planted to replace ash trees the city removes in its efforts to control borers.

The current draft management plan deals solely with boulevard trees, not those growing in city parks or on other city property.

The plan indicates that the city has no intention of requiring the removal of ash trees from private property unless damaged trees are deemed a threat to public safety.

Rick Hanson, owner of Rick's Tree Service, recommended Duluth exercise caution as it shapes its response to the emerald ash borer.

"I like to be real about things," he said advising the city to consider the long-term cost of treating trees with insecticide.

"This injection isn't just a one-time fix. It's every other year for the rest of its life," he said.

In the end, Hanson suggested Duluth could be fighting a losing battle, as with Dutch elm disease.

"Would I rather see an ash tree live? Yes. Do I think this is a terrible thing? Yes. Is the writing on the wall? Yes. What can we save? How much money will we spend? Could that be put toward other trees, new trees? I think so," he said.

Levy noted that much has been learned about how to cope with emerald ash borers in recent years and suggested more advances could be on the horizon.

Sellner said he takes encouragement from the fact that Duluth's detection of an emerald ash borer infestation is at the earliest stage of discovery ever documented in the nation. He said most communities don't realize the pest is present until it has been active for three to eight years.

Dealing with the invasive beetle will be expensive. Sellner noted that even if the city chose to cut down all ash trees on its boulevards, at $500 per tree, it would cost upwards of $1.2 million.

Sellner acknowledged that continued treatment to protect trees from the pest will likely cost more per ash over the long haul but said: "There's value in a mature healthy tree. You have to ask yourself what's the true cost of losing that tree."

The Urban Forest Commission will consider comments it has received on the draft emerald ash borer management plan before forwarding it. The document will then go to the Duluth Council probably in March or April for possible further modifications and/or final adoption.

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