Scientists have been studying emerald ash borers since the Chinese insects started killing ash trees near Detroit 15 years ago.
They've been following the imported insects' march east, then north and now west and watching the bugs kill nearly every ash tree in their path.
But those were mostly green ash, with some white and blue ash, too - the kind of trees that once lined urban avenues, wooded parks and farm woodlots across much of the country.
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, and no one knows what's going to happen.
"We have some ideas, but nobody really knows. You really are going to be the laboratory for how EAB impacts black ash ecosystems," said Dan Kashian, a scientist at Wayne State University in Michigan.
That's why Kashian and more than 170 other scientists and forestry experts will converge on Duluth this week to share their knowledge of the emerald ash borer and the future of ash trees in North America. It's the biggest emerald ash borer symposium ever, an effort to get as much information out as possible on what researchers know so far about the pest and what questions remain unanswered that need to be tackled.
The group is especially interested in early results from experiments in northern Minnesota and Michigan's Upper Peninsula where scientists are replicating expected ash borer damage to black ash forests and seeing what might grow in their place.
Because black ash are such swampy ground trees, there are concerns that no other trees will be able to grow there, and vast swathes of northern forest will become scruffy, wet brushlands filled with tag alder and invasive buckthorn. If no big trees grow, scientists also are worried what will happen with the water table in those lowlands.
"We really wanted to focus on what options are available to manage these (black ash) ecosystems given the inevitable invasion by EAB," said Randall Kolka, a scientist with the U.S. Forest Service's Northern Research Station, who helped organize the Duluth event. "What happens if you have these straws sucking up water and then take the straws away."
The goal was to get the latest results into the hands of local, state and federal foresters and land managers as well as timber industry officials, tribal resource agencies and private landowners "struggling with what to do with their ash forests," Kolka said.
Promoters of this week's event got far more response than expected.
"We thought at first we would be hosting maybe 100 (registered for the conference) but we are approaching about 175 now," Kolka noted.
Foresters in Maine, the U.P. and northern Wisconsin are watching closely, too, with vast black ash forests in those areas as well.
"We're really the wild west for emerald ash borer research right now," said Robert Venette, a U.S. Forest Service research biologist and dean of a new invasive species research center at the University of Minnesota.
Venette said reaction to the emerald ash borer seems to fall into two camps: Fear or apathy.
"And neither of those are a constructive way to advance," he said. "We don't want to overreact out of fear and start cutting down every ash tree and replacing them with something when we don't know the impacts. But we also don't want people to ignore it and say it's a lost cause and just move on."
Black ash is the dominant player in a unique wetland ecosystem and it's not at all clear how the other players will react if the trees are killed. Experiments are finding some substitute species that might work if black ash are killed off from the lowlands of the north woods landscape.
One experiment in northern Minnesota's Chippewa National Forest has tried cutting gaps of black ash and planting other species to see what might take their place. So far, swamp white oak and some elms have thrived in wet black ash habitat, as does imported Manchurian ash.
"The hardwoods seem to be doing better than we thought in black ash habitat," Kolka said. "Some other species we thought would do well in wet areas, like tamarac, actually have done pretty poorly."
But swamp oak will have to be imported from southern state climates (they likely will survive here now with warmer temperatures) and Manchurian ash, which are resistant to emerald ash borers because they evolved together, are another non-native species.
"Do we really want to engineer our own invasion? We don't know yet how other (animal and insect) species might react to them," Venette said.
Venette has experimented on the impact of cold temperatures on the bug, and at first blush it looks like Minnesota's winters may be more than the ash borer can stand.
"We start to see mortality at 20 below zero. That won't stop it, but we think it will slow it down," he said.
At 30 below, most of the bugs die. "And we haven't seen one yet that can take 40 below," Venette said. "Unfortunately, with warmer winters, we don't see many 40 below temperatures anymore, even in northern Minnesota. So we're not counting on that to stop emerald ash borer."
The bug also gets some insulation by being burrowed under the bark where temperatures are a little warmer than outside the tree.
The Northland's winters may be enough to slow the bug down, however, so the insect and tree develop some sort of natural balance, Venette said.
"But there's just so much we don't know," he said.
Some trees survive
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, now infesting the Hartley Park area on the outskirts of the city. Duluth has 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. Experts expect most of them to perish within a few years.
A 2014 study by Daniel Herms of Ohio State University and Deborah McCullough of Michigan State University found that in forested areas near the southeast Michigan epicenter of the emerald ash borer infestation "more than 99 percent of the ash trees" bigger than saplings had been killed.
There are some ways to slow the impact. Milwaukee, for example, is inoculating big, stately trees with insecticide every other year. It's expensive, but not much more than the cost of cutting down and removing a big, dead tree. Homeowners and city officials in the Northland will have to decide which trees, if any, are worth that effort. Superior chose to simply cut down all its ash and start over with other species.
Efforts are underway to control the pests on a landscape level with parasitic, non-stinging wasps imported from Asia that have worked in some areas. But it remains unclear if the wasp can move fast enough to prevent ash borer outbreaks. The wasps generally don't thrive until there are many ash borers for them to prey on, and by then the tree damage is already done.
Contrary to early predictions, however, emerald ash borers have not killed 100 percent of the ash trees in their path. A few survive unscathed. Kashian has researched areas of southern Michigan that have seen the insect for 15 years and still finds a few living ash.
"Will it ever be the same as it was before? Absolutely not. There aren't going to be ash along city streets or as a major part of mixed forests," Kashian said. "But there will be some ash. As a species, I don't think it's going away entirely."
He likened the impact of the emerald ash borer to that of Dutch elm disease a half century ago. Most elms perished; a few survived.
Kashian said ash seems to be hit hardest where it is part of a mixed hardwood forest. In areas that were nearly all ash trees, however, the bugs killed only about 60 percent of the trees. That could mean enough black ash will survive in northern Minnesota because black ash stands tend to be entirely ash, with few other trees.
"But I was looking at green ash. We don't know yet if that's the case with black ash," he said.
Kashian said southern Michigan now has a relatively low population of emerald ash borers - they literally ate themselves out of house and home - which may allow some ash to resprout. But he figures the insect will then ramp up its population to kill new ash that do return.
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.
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.
Emerald ash borer 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. 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.
State law prevents 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. If you think your ash tree is infested, go to www.mda.state.mn.us/~/media/Files/plants/eab/eab-treeshaveit.pdf to use the "Does My Tree Have Emerald Ash Borer?" guide.