WOOSTER, Ohio — There was a time when American chestnuts and elms were dominant tree species in North American forests and urban landscapes.
But then, two fungal diseases, both originally from Asia, were accidentally introduced into the United States: chestnut blight, in 1904, and Dutch elm disease, in 1928. The invasive organisms quickly spread and caused massive die-offs of both chestnuts and elms, forever altering the composition of U.S. forests the streets of towns nationwide.
Decades later, in 2002, another Asian import — this time an insect — was discovered in Detroit, feeding on and killing ash trees.
Six years later, emerald ash borer has turned from an emerging pest into one of the most significant environmental threats facing North America.
From its initial epicenter in greater Detroit and western Ontario, Canada, emerald ash borer has so far infested a total of seven Midwestern and Eastern states from Illinois to Maryland, killed millions of trees, overwhelmed municipal budgets, and practically halted the region’s once profitable production of ash trees for landscaping (in Ohio this segment of the nursery industry was worth over $20 million when emerald ash borer was found in the Toledo area in 2003).
Just like chestnut blight and Dutch elm disease before it, emerald ash borer is capable of wiping out an entire species of trees from North America, said Dan Herms, an entomologist with the Ohio Agricultural Research and Development Center (OARDC) and Ohio State University Extension.
That’s why Herms — Ohio’s representative on a U.S.-Canadian science advisory panel that makes recommendations on emerald ash borer — began looking at possible ways to preserve ash as a component of North American forests from the onset of the infestation, just in case strategies to stop the spread of emerald ash borer failed.
Survival of the fittest
One course of action, Herms said, is to develop a resistant ash tree that can ward off attacks by the voracious beetle — whose larvae feed on the water- and nutrient-carrying tissue just under the bark, starving the tree to death in two to five years.
This strategy has been employed by researchers in the cases of chestnut and elm, producing hybrids that are disease-resistant and still carry characteristics of the native trees.
Herms also conducts research on insecticide treatments to protect trees from emerald ash borer and other management tools to deal with the pest in the present.
In the search for an emerald ash borer-resistant ash tree, Herms has teamed up with Enrico Bonello, an OARDC and OSU Extension plant pathologist; David Smitley, a Michigan State University entomologist; Donald Cipollini, a biologist at Wright State University; and Jennifer Koch, a biologist with the U.S. Forest Service’s laboratory in Delaware, Ohio.
This team first established an experimental planting in 2003 in Novi, Mich., which was inside the core of the emerald ash borer infestation at the time.
They wanted to expose different species of native and Asian ashes to the insect to evaluate their resistance to emerald ash borer, identify possible mechanisms of resistance, and determine the effects of drought and other stressors on susceptibility to the invasive pest.
The planting included natives such as white ash and green ash; Manchurian ash (with which emerald ash borer shares an evolutionary history in Asia); and Northern Treasure ash — a hybrid between native black ash and Manchurian ash.
“Our working hypothesis was that the Asian ash would be the most resistant because of natural defenses resulting from co-evolution with the insect over a long period of time, considering that in Asia emerald ash borer does not devastate its native hosts,” Herms explained.
The native-Asian hybrid was included because it could provide insight into patterns of inheritance of resistance genes and facilitate their identification.
The hypothesis proved right. While native ashes and even the hybrid succumbed to emerald ash borer, most Manchurian ash was much more able to withstand the attack.
Researchers have begun to hybridize Asian and North
American ashes — the first step
in developing a tree resistant to the emerald ash borer.
But the team’s work was far from over. As the metallic-green ash borer took over northwest Ohio, Herms, Bonello and collaborators set up additional research plantings in the Toledo area to refine their studies.
The latest experimental plot was established last fall in Bowling Green, a city that in 2005 adopted a proactive management plan to deal with the arrival of emerald ash borer and has been a key partner of the Ohio emerald ash borer task force in monitoring and outreach efforts.
Now, researchers will wait for emerald ash borer adults to emerge in the spring, mate, lay eggs and infest the newly planted trees with their hungry larvae. The experiment will go on for three years.
“We hope to be able to expose these trees to natural attack by emerald ash borer in the zone of infestation,” Bonello said. “We need this so we can characterize host responses to the pest.
“We expect that the response of species that are known to be more resistant, such as Manchurian ash, will be different from the responses of susceptible native ash species, and that we’ll be able to differentiate between the two.
“Then we’ll concentrate on what’s different and see if we can use those characters as markers of resistance.”
Markers of resistance
Looking for markers of resistance, and eventually the genes that are associated with them, researchers can use them in screening programs that do not involve challenging all plants with the insect.
Once the genes are found, they can be used in targeted breeding programs that may involve hybridization of native ash with Asian ash, similarly to what has been done in the case of the American chestnut.
Although the project is ongoing, Herms said the team has already made some important discoveries related to the biochemical basis of emerald ash borer resistance in Manchurian ash, as natural defense compounds have been isolated and identified.
Based on preliminary results, Koch and colleagues at the U.S. Forest Service have begun projects to hybridize Asian and North American ashes — the first step in developing a resistant tree.