Emerald Ash Borer is a shiny green beetle first found in southeast Michigan in 2002. Since then, the invasive species has killed hundreds of millions of ash trees across North America.
Toby Petrice is a research entomologist with the USDA Forest Service Northern Research Station based in Lansing, Michigan.
He’s worked on a lot of invasive species over his 30-year career with the Forest Service, but much of his time right now is devoted to Emerald Ash Borer [EAB]. Specifically, he’s working on using classical biological control to try and slow its spread.
“We go to the native range of that pest species, and we find natural enemies there that attack it. Then we import those species into the new area that is colonized,” said Petrice.
Classical biological control
Petrice says they’ve identified four species that could be solutions to the getting the EAB population in the U.S. under control.
Right now, Petrice and a team of researchers are looking at the effect parasitoid wasps has on the EAB larva.
“These species actually sting their prey and when they do that, they either inject eggs into the prey, or they lay eggs on the body of the prey, and then their progeny consumes the prey,” explained Petrice.
It’s a pretty gruesome, but effective tactic.
Petrice says three species they’ve brought in, two of which attack the EAB larva and one that attacks the eggs, have been establishing really well.
“Actually, here in Michigan where I do a lot of my work, these are some of the first release sites for those parasitoids. They're doing really well. They are reducing emerald ash borer numbers. They do especially well when the trees are smallish, so less than four to five inches in diameter. That's when they really do the best,” he said.
Now, your first reaction to bringing in one non-native species to control the populations of another non-native species may not be a good one.
There is a history of predators brought in to control an invasive species only for it to have a negative impact on native ones.
“In the past, there weren't really any regulations. People thought the only good insect was a dead insect, so they thought bringing in any natural enemy was fine,” said Petrice.
Petrice stresses standards for doing that have changed.
Any non-native species researchers are looking to purposely introduced must go through several years of laboratory testing to ensure that they would only target the invasive species and not any native ones.
“You go through several layers of regulation. You have to go through the North American Plant Protection Organization. You go through APHIS PPQ [Plant Protection Quarantine], you get Fish and Wildlife Services approval, you also get public comments, you get input from Tribal nations as well. It’s pretty rigorous getting permission to release these,” said Petrice.
As Petrice sees success with getting Emerald Ash Borer species under control, that’s only part of the equation.
At this point, EAB is here to stay.
More resilient ash trees
Petrice has also been working with some scientists that specialize in tree breeding to produce ash trees that are more resilient to EAB.
When EAB hits a forest, there are typically a few ash trees that survive the first invasion of the beetles.
Petrice says it isn’t clear why some ash trees do better than others. Some of them are referred to as lingering ash because they’re not 100% resistance to EAB, they’re just not immediately killed.
“We're taking cuttings from those trees. The group in Delaware is then grafting them and producing new trees from those cuttings,” said Petrice. “Then from there, they test them for resistance to emerald ash borer, and they also breed them to produce progeny that are resistant to emerald ash borer.”
Petrice says it’s still a work in progress, but there have been promising results.
“It looks like it's working. They're actually planting some of these trees in different field trials,” he said.
While that work is ongoing, other researchers are trying to figure out how restore ecosystems that have lost ash trees to emerald ash borer.
Preserving the ecosystem
Dr. Dustin Bronson is a research plant physiologist with the USDA Forest Service Northern Research Station in Rhinelander.
EAB has been making its way north in Wisconsin over the last two decades. Bronson says it’s a matter of time before ash trees start dying.
“Usually, it's between about six to eight years between when the EAB arrives and the stand is completely dead,” said Bronson. “It's a very short timeline, when you think about all the needed management activities that it takes to get some new cohorts of trees established.”
In much of the region, ash trees tend to be the only tree species due to historical logging of other species and Dutch Elm Disease wiping out elm trees.
When EAB destroys those the remaining ash trees, those stands will likely lose most, if not all, of the canopy.
Bronson says without remediation, these sites will transition to wet meadows. This would result in habitat loss for many wildlife species.
“When we think about the needed cool waters that trout need, the canopies of ash trees are providing that cool water. When those canopies aren't there, because the ash have died off, those trout streams are going to heat up,” said Bronson. “Whether it's birds that require nesting cavities in these trees or mammals that use these forests as kind of wildlife corridors, these are all really important functions that the forest provides. And again, when the trees aren't there and the forest isn't there, the wildlife are going to be missing out.”
Bronson has been working with a team over the last couple of years to research which other trees species could be planted to diversify the forests.
His goal isn’t to prevent ash trees from dying off, but protect the function of the ash trees in the forests.
There’s a couple of different challenges they need to work around.
The trees need to be hardy enough to withstand our freezing winters now, but adaptable enough to thrive 100 years from now when the Northwoods' climate is expecting to more closely resemble the climate now in southern Wisconsin.
Bronson and the team have been collecting seeds from trees mostly to the south. He says they’ve got a potential list of species that might fit the bill, including some surprises like Bur Oak, Swamp White Oak, Silver Maple, and River Birch.
“A lot of these species, like let's say the Silver Maple or the River Birch are species that we don't traditionally find here in northern Wisconsin. They might be south of the tension line, or they might be along, let's say, the Mississippi corridor, but through a lot of our testing, we find that they've done quite well,” said Bronson. “So this is, again, where global warming and things like that in some ways can help us in terms of having species just to the south be able to survive up here in the north.”
Another challenge is ensuring these trees can grow in these sites, something that’s made more challenging by another invasive species like reed canary grass.
Establishing new growth
“Foresters that aren't able to get out before EAB arrives to get these plants established, to get these trees tall enough, they may happen to be dealing with a lot of vegetation competition, a lot of invasive plant growth,” said Bronson.
If EAB wipes out ash and removes the canopy, the understory will have a lot more sunlight to help it grow and spread.
Part of Bronson’s research is using herbicide in areas where reed canary grass is established. It’s an invasive grass that can grow to about six feet tall.
Bronson isn’t trying to wipe out the invasive. The plan is to use enough herbicide to fight the grass back long enough to get tree seedlings established and tall enough that they aren’t competing for sunlight.
“No one likes the use of herbicides, but I think, oftentimes, we're looking at, do we want to keep these systems as forested or not? If it's the competing vegetation that's preventing that from happening, what are the tools that we have available to deal with that,” said Bronson, “If we can use an aquatic based herbicide once and get these trees established, that might be a tradeoff that a lot of management groups are willing to take.”
That’s the main goal of all this work, to give land managers as many tools as possible to protect forests from invasive species.
“I think people need to know how hard that foresters work to provide functions of the forest. I mean, field foresters, are up against a myriad of challenges,” said Bronson. “Usually, it all comes back to what the function of we want that forest to do, what the ecological purpose that we want to get from that stand. That can be really challenging. I think our goal as researchers is just trying to provide them as many tools and information so that they can make the right calls.”
Emerald Ash Borer was first found in Wisconsin in 2008. It has slowly been making its way north. It’s been found in all 69 of the state’s 72 counties.
