Borer-killing ash trees may turn the tables on insidious bug

The view of the small wooded area on the northeast corner of our property has been dominated by a mess of trees in the process of falling since autumn. The tree that instigated the situation by breaking about 8 feet from the ground and falling into another tree that then leaned into a third tree is, unsurprisingly, an ash.

Like many homeowners across the county, in the past dozen or so years we’ve lost many dozens of ash trees since the first emerald ash borer in the state was spotted in Newburg in 2008. Forests and natural areas are nearly unrecognizable from a decade ago as the ash population has been decimated.

There has been little reason for optimism when it comes to emerald ash borer, an invasive species from Asia that likely arrived in the U.S. in shipping palettes. Perhaps the only good news has been that trees that have been treated annually with specific insecticides have shown good resistance to morbid damage from boring insects. The method is effective for selected smaller trees in yards, but not practical for a forest.

Recently, however, a glimmer of hope has emerged. The U.S. Forest Service has identified some ash trees that seem to have a natural resistance to the insects. A team led by forestry worker Kathleen Knight has identified large, thriving ash trees in Ohio and Michigan, but the story gets even more exciting. Jennifer Koch, a research biologist with the Forest Service who studied samples from the so-called lingering trees, discovered that they not only seemed to have resistance to the insects, they actually kill emerald ash borer larvae.

And further research has provided even more reason to be hopeful. Seedlings produced by breeding lingering ash trees in a lab are actually more resistant to emerald ash borer than their parents.

But the work that needs to be done to bring ash forests back is long. The lab results need to be proven in the field with a large number of resistant seedlings, and then the most resistant of those trees could become the parents of a new generation of ash trees.

That’s work that is likely to take decades, which is not long in ecological terms. If this story seems vaguely familiar, it’s because it’s very similar to the trajectory of elm trees, which were decimated starting in the 1930s from a boring insect that spread a fungus that causes Dutch elm disease. The disease is still a problem, and trees that have been treated for years to prevent it still succumb to it every year. But hybrid cultivars have been developed and some American elms have indeed been identified as being resistant to the disease.

Most of the ash trees that once graced the forests, parks and back yards of Ozaukee County are lost forever, but the search for lingering ash trees continues. Scientists urge homeowners not to remove healthy ash trees. They might just turn out to be a crucial part of bringing back an important species of trees.