A new University of Minnesota study has found that natural fungi may stop emerald ash borers from advancing, a key to protecting millions of ash trees in Minnesota cities and forests.

The study by scientists at the university’s Invasive Terrestrial Plants and Pests Center, published in the journal Fungal Biology, identified various fungi living in ash-borer-infested trees.

The finding is considered an important first step in finding specific fungi that may be harnessed to control the spread of the emerald ash borer and eventually saving ash trees.

“We discovered that several different species of fungi attack EAB and other insects, and they can now be further tested for their potential for biocontrol,” said Robert Blanchette, the study’s project leader and professor in the university’s College of Food, Agricultural and Natural Resource Sciences.

An adult stage emerald ash borer. A new University of Minnesota study finds that natural fungi may help stop the insect from killing ash trees. (Photo courtesy of emeraldashborer.info)
An adult stage emerald ash borer. A new University of Minnesota study finds that natural fungi may help stop the insect from killing ash trees. (Photo courtesy of emeraldashborer.info)

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PREVIOUSLY: Scientists and foresters worry about the spread of emerald ash borer from Duluth into northern ash forests.

Since first found in the U.S. 19 years ago, the emerald ash borer has become the single most devastating invasive forest insect in the nation, killing hundreds of millions of ash trees at a cost of hundreds of millions of dollars.

Ash borer larvae feed just beneath the bark, leaving behind tunnels that can stretch up to 20 inches long. Beneath the surface, fungi — some of which may be capable of parasitizing ash borers — may be carried by the larvae as they develop, or may enter the tree through the tunnel galleries.

From Rochester to Duluth, researchers gathered samples where ash trees have been infested by the insect. Scientists used DNA sequencing to identify several types of fungi, including some that attack insects and others that cause wood decay. It's hoped the fungi that kill emerald ash borer could someday be used as a sort of natural insecticide, Blanchette said.

“Before now, we simply haven’t been sure what fungi are associated with EAB infestations in Minnesota. This project identified those species and, in doing so, opened up new possibilities for managing one of our state’s most devastating tree pests,” said Ben Held, the study’s lead author and researcher in the College of Food, Agricultural and Natural Resource Sciences.

The research was funded through the Minnesota Environment and Natural Resources Trust Fund.

The unintentional Asian import likely hitchhiked from China in packing crates. It was first discovered in the Detroit area in 2002 and has expanded east, then north and now west to 32 states and two Canadian provinces and has now killed tens of millions of ash trees of all varieties. Now, with ash borers expanding their range in Duluth, the critters are on the doorstep of nearly 1 billion black ash across northern Minnesota's forests.

Emerald ash borer have been killing trees in Duluth and Superior for several years now, incldung this tree in West Duluth. A new study found natural fungi may help stop the insect. (Bob King / 2018 File / News Tribune)
Emerald ash borer have been killing trees in Duluth and Superior for several years now, incldung this tree in West Duluth. A new study found natural fungi may help stop the insect. (Bob King / 2018 File / News Tribune)

Emerald ash borers have been killing trees in Superior since about 2013. They were found on Park Point in Duluth in 2015 and spread fast, infesting the Hartley Park area and then spreading to other neighborhoods in recent years. Duluth had about 4,500 green ash just on boulevards alone — nearly 1 in 5 of the trees along city streets — and thousands more ash trees in urban forests across the city. Many have already succumbed to the insect or been cut down in a preemptive effort to slow the spread.

Because the insect expands its range at a slow rate, only a few miles annually, the rapid spread across the continent is attributed to humans moving infested ash firewood or infested nursery ash trees.

After feeding on the tree as larvae, the bug eventually hatches as a bright emerald green flying bug that can move relatively short distances before laying eggs that burrow into a new host tree. So far they have killed only ash, but that includes every North American ash species they have encountered at mortality rates approaching 99% in some areas.

There is some good news that the toughest Minnesota winters also will slow the ash borers from spreading as fast. The Minnesota Department of Agriculture found that 98% of ash borer larvae died at 30 below zero and about 34% died at 10 below zero.

Minnesota law prohibits moving infested ash as firewood or moving any firewood out of quarantine areas like Duluth. State officials are asking people to be on the lookout for ash borer expansion and to watch their ash trees for infestation.

For more information on emerald ash borers in Minnesota, go to emeraldashborer.info/state/minnesota.php, or in Wisconsin go to emeraldashborer.info/state/wisconsin.php.