Lingering ash

Emerald Ash Borer resistant saplings could hold the key to future regeneration

Photo by U.S. Forest Service An ash tree with a unhealthy canopy. The U.S. Forest Service is making note of ash trees that have survived in areas of Michigan and Ohio infested by emerald ash borer.

The ash trees are going to die.

Almost all of them.


But, officials at the Allegheny National Forest and the U.S. Forest Service’s Northern Research Station have hope that the landscape will include a meaningful amount of ash again in the future.

“We anticipate that following emerald ash borer-caused ash mortality, we will end up with a very low population of ash and a lower population of EAB,” ANF Acting Ecosystem Management Officer Andrea Hille said. ”

Photo by U.S. Forest Service An ash tree with a healthy canopy. The U.S. Forest Service is making note of ash trees that have survived in areas of Michigan and Ohio infested by emerald ash borer.

It seems some trees will survive. At least through the first wave.

“There are a small number of ash — less than 1 percent of all ash — that have a slight resistance to EAB,” USFS Eastern Region Geneticist Paul Berrang said. “These trees are referred to as lingering ash.”

Northern Research Station Research Ecologist Kathleen Knight has been observing EAB in Ohio for 12 years.

EAB first showed up in Michigan in 2002 and Ohio in 2004 or 2005.

“When EAB first arrived in Ohio, we started to set up monitoring plots in 2005 spread out in forests throughout the state — different densities and species,” Knight said. “We hoped that we would discover that in some places they would kill them and in some places they wouldn’t.”

Purple EAB traps — the same as the ones set up around the ANF — captured some EAB early on.

Knight explained that the insects are attracted to purple for some reason.

Scientists learned from the trapped insects, but not enough to save the trees.

“We were trapping EAB in the purple traps,” she said. “As we watched, the large ash trees were killed off.”

The smallest trees — “less than an inch diameter” — were passed over. “It doesn’t prefer those,” Knight said.

“When all those old ash trees are dead, the population of EAB crashes,” she said.

They get hard to find, but the population is not zero.

“It’s still there,” Knight said. “They’ve never completely disappeared for multiple years. They are, unfortunately, sticking around.”

As the small trees that were left alive grow, they present food for some EAB.

The resistant trees become targets.

“Those resistant trees can still be attacked by EAB,” she said. “Sometimes, the tree kills the larva.”

Counter attack

When the population is down and possibly vulnerable, ANF officials are hopeful that the invasive pests can be ushered out.

“It is difficult to predict if these low populations will be sufficient to allow both EAB and ash to survive in the long term,” Hille said. “We can help tip the balance one way or the other by releasing predators and parasites that attack EAB and by increasing the levels of resistance in the residual ash by identifying lingering ash and breeding them to produce more resistant trees.”

“There are four different species of parasitoid wasps,” Knight said. “They have been extensively tested” to make sure the cure is not worse than the infestation.

“Those have been released,” she said.

‘Wasp’ brings a certain image to mind. That is not what is being set loose on the EAB.

“These aren’t things that people would look at and think, ‘that’s a wasp,'” Knight said. “They’re very tiny insects.”

“They don’t seem to save the large ash trees,” she said. “Over the long-term, getting those parasitoid wasps established may come into a balance with EAB populations may allow those ash saplings and seedlings to survive.”

There is one important difference between the situations in Ohio and the ANF.

“In Ohio, we have a lot of ash tree regeneration,” Knight said. “On the ANF, in the monitoring plots, I have not noticed a lot of regeneration.”

Superior trees

In years past, the Forest Service had a superior tree program. The largest, straightest examples of each species, including the ash species, were identified and the locations noted.

They are not particularly resistant to EAB.

Those fast-growing, straight trees would not be the ones to, at least initially, repopulate the forest after EAB.

“The original tree improvement program — superior tree program — was developed by the U.S. Forest Service back in the 1970s,” Berrang said. “The program’s focus was on producing seed for propagation of seedlings that would be planted on the landscape and grow especially fast and produce high-quality stems.”

“It was an expensive program to maintain and the Agency can no longer afford a traditional tree improvement program,” he said. “Around 60 white ash trees were designated to establish the foundation for a traditional tree improvement program on the ANF; however, since the ANF relies on natural seedling establishment for nearly all hardwood species, little or no ash has been planted anywhere.”

Instead of the towering “superior” ash, the lingering ash will be the progenitors of future generations.

“Following this initial intensive wave of EAB, those will be the trees that have scientific and restoration related values,” Berrang said. “They could form the basis of a program to develop EAB resistant ash. The ANF is interested in finding those resistant trees that survive, and recording their locations for future restoration purposes. These trees could be used in tree improvement programs that breed ash for resistance to EAB.”

The seeds from the superior trees are still around and could provide valuable genetic material if a resistant strain is developed.

“Having seeds that have been banked gives us a lot of possibilities,” Knight said. Including someday breeding “them with the resistant trees.”

Other species

EAB does not feed on other common species on the ANF — oak, hickory, cherry, beech, birch, and maple, but it is not completely limited to ash.

“They have now started to eat white fringe tree,” Knight said. “It’s not an ash tree. It’s in the same family as ash.”

“It can also feed on the commercial olive tree,” she said.

That’s not a problem around here, but, “if it got to areas of the country where there are olive trees, it could feed on those,” she said.

Moving EAB

Having the resistance is important, even if EAB disappears from a forest.

“EAB seems to be great at hitching a ride with humans and getting around,” Knight said. “They can ride in firewood or cut logs, but they can also hitch a ride on vehicles.”

She said the research is showing a close and “interesting correlation between where roads are and where EAB is.”

The future

Will there ever again be towering ash trees on the ANF?

“We don’t know,” Knight said. “Our hope is that we would have large ash trees in our forests again.”