Taking a stand for trees: rebuffing the emerald ash borer

In Robert Blanchette’s lab, treatments to kill the emerald ash borer are in development.

Ash tree with part of bark removed

“There are at least a billion ash trees in Minnesota. We have to use everything we can to stop this insect.”

head shot of Robert Blanchette
Robert Blanchette, Professor, Plant Pathology

With those words, Robert Blanchette lays bare the magnitude of the task before our state if we are to control the emerald ash borer.

A professor in the Department of Plant Pathology and researcher in the Minnesota Invasive Terrestrial Plants and Pests Center, Blanchette, along with his research team, is perfecting biological means to defeat this half-inch-long pest.

Life cycle of the emerald ash borer

Egg, larva, pupa, adult emerald ash borer
  • Female ash borers lay 40+ eggs in the bark of an ash tree.
  • Eggs hatch in two weeks. Larvae emerge, then feed below the bark for 1–2 years, creating S-shaped "galleries”—this is the most destructive stage.
  • Larvae then pupate, after which they turn into adults.
  • Adults emerge through D-shaped holes in the bark.
  • The adults mate, find another tree, and start the process over.
  • The host tree will likely be dead in three to five years.
Main stem of ash tree showing woodpecker damage
 The main stem of green ash showing woodpecker damage from feeding on larvae under the bark.
Bark removed on ash tree with woodpecker damage.
Bark removed around woodpecker damage showing EAB larval galleries.
Larvae feeding on phloem/outer sapwood interface in a serpentine pattern and has not yet caused phloem necrosis.
Emerald Ash Borer larvae feeding on phloem/outer sapwood interface in a serpentine pattern. Phloem tissue has not yet begun dying (necrosis).
Bark removed on ash tree showing serpentine galleries.
An older EAB gallery shows killed sapwood tissue, which has led to a disruption of vascular function, discoloration and necrosis.

Marshaling microbes

The emerald ash borer, or EAB, can be stopped by removing ash trees from a vicinity in advance and planting other species. This method is costly in time, money and, often, emotions. Enlisting armies of biological agents to attack the insect will avoid these drawbacks.

But even if infestation and tree loss or removal looks inevitable, “We want to develop another tool for land managers to slow EAB down so they can have a smoother transition,” Blanchette says.

In Blanchette’s lab, graduate student Colin Peters researches fungi that infect insects. His work is geared to protecting black ash, a tree that forms an immense forest in northern Minnesota. The black ash is central to several Indigenous cultures, to whom it is important for making baskets or as a spiritual resource and source of medicine.

Close-up of emerald ash borer covered in fungus.

“I’m taking fungi that infect insects and testing field application methods,” says Peters. “We use a modified Lindgren trap, which is a green, funnel-shaped trap. We bait it with a pheromone that attracts EAB."

As adult ash borers navigate through the trap, they get coated with fungi. The goal is for the heavily coated ash borers to carry the fungus to trees, where other ash borers will pick it up and become infected.

“We’ve done preliminary work with one fungal species,” says Peters. “We tested it in summer 2023 in Minnetonka and saw preliminary promise with infection. Starting in summer 2024, we’ll set up [test] plots along advancing fronts of EAB and deploy auto-dissemination devices [to spread the fungus]. 

“In several years we’ll be able to measure the effect on EAB population growth and tree survival all over the state.” 

Pop Quiz

Emerald Ash Borer sitting on a green leaf with slightly open wings

Adult emerald ash borers emerge from trees through holes shaped like what letter of the alphabet?

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Emerald Ash Borer sitting in D shaped hole in the tree


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No, we wouldn't be calling attention to common old round holes.

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No, that wouldn't leave them much room to wiggle out.

Friendly fungi

Peters is also experimenting with injecting the fungus into ash trees, where it can live without killing the trees.

“The fungus has genes similar to genes that help bacteria infect guts of insects,” he explains. “I inject roots, the fungus moves up into the tree, then EAB larvae feed on the cambium and phloem and ingest the fungus.”

drawing of three ash trees

Good neighbors make good defenses

A study of urban ash trees found that treatment of a subset of trees could preserve the canopies of all of the trees in the vicinity—a strategy known as "associational protection." The study, published in 2023, was led by doctoral student Dorah Mwangola and her adviser, Brian Aukema, a professor in the Department of Entomology.

An adult emerald ash borer beetle is on an ash tree leaf.
An adult emerald ash borer beetle on a leaf.
Two grad students hang a green funnel-shaped emerald ash borer trap.
Nick Rajtar and Colin Peters hang an emerald ash borer trap.

Stop signs for invasive fungi

Three men working together outside on spore sampling equipment.
Eric Otto (DNR) and Blanchette lab members Benjamin Held and Nick Rajtar change out spore sampling equipment.

In other work, Blanchette and graduate student Nick Rajtar are collaborating with the state Department of Agriculture and 40 businesses—nurseries and Christmas tree plantations—to detect and prevent the spread of invasive waterborne molds.

“Minnesota ships nursery material in from other states, so we’re surveying state nurseries to identify introduced pathogens as early as possible,” Rajtar says. “We know there are 14 new-to-the-state species. Some are very pathogenic against rhododendrons, oaks, alders, and woody domestics.”

Blanchette’s group is also part of a project to thwart the spread of a root fungus deadly to red and white pines, as well as other conifers. Dozens of spore traps have been set up along the Minnesota-Wisconsin border to detect any of the fungus coming in from the east.

“It was found near Winona in 2014, but so far, the state Department of Natural Resources (DNR) has been able to eradicate it,” Blanchette says.

Secret sauce of success

"The Minnesota Terrestrial Plants and Pests Center (MITPPC) and Minnesota Aquatic Invasive Species Center (MAISRC) are unique—we focus on invaders that are the greatest threats to the state," says Robert Venette, director of the MITPPC and adjunct associate professor in the Department of Entomology. "Prioritization is the ‘secret sauce’ that makes both centers work. We concentrate resources where they’ll make the biggest impact."

That's one reason “the University of Minnesota is a national leader for research on invasive species,” he says.

Minnesota is meeting the challenges of EAB and other plant pathogens because it has invested in the places best suited to the task. As Blanchette puts it:

“It’s taken the University’s expertise to put together projects so we can control invasive species before they cause enormous damage to our natural resources.”

Supportive partners

The Minnesota Invasive Terrestrial Plants and Pests Center is funded largely by the Environment & Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources.

Give to the Minnesota Invasive Terrestrial Plants and Pests Center