The property our house sits on is about 3 acres in area, about one-third of it wooded. Among the trees in the woods are native ashes. I became aware of them when, a decade or so ago, a storm knocked down a few trees, and the tree cutters we hired to clean up pointed them out. They are beautiful and resilient trees. Or, more accurately, they were. The ones on our property are dying, as are ashes throughout the east and midwest. This is because a pest from Asia, the emerald ash borer, was introduced to the US in about 2003, and native ashes have no resistance to it. The larvae of these insects eat the cambium beneath the bark that carry nutrients, as you can see above. This damage to nutrient transport then kills the trees. Once the borer was discovered in Michigan, there was an attempt to contain it, but it was futile. The borer is too mobile. It has spread throughout the midwest, the northeast, and through Appalachia as well. It was predicted that the borer could cause the extinction of all 16 species of native ash in North America.
When one of our ashes died last year, we called in the tree guys again to cut it down. They left the wood so that we could use it in our fireplace. We split the word last weekend, and saw the tunneling patterns typical of the emerald ash borer. We have a couple of other ashes on our property, and it is apparent that they are sickly, so they are likely infested also. This infestation is not the only incidence of species devastation of trees on this continent by foreign pests: this has also happened to native chestnut and elm over the past century.
However, there is now some hope for being able to save native ash species by a breeding program initiated by tree researcher Jennifer Koch. While there was general despair over the survival of native ash once the emerald ash borer escaped containment, Koch considered the possibility that within the diversity of ashes in wild forests, there might be genes that could confer resistance to the pest. After all, Asian ash trees had adapted to the pest, and while millions of years of evolution separates those species from American ones, some native trees might show signs of natural resistance.
Her team searched out trees throughout Ohio looking for ashes that were not dying despite infestation, which they called “lingering ash.” While such ash trees can resist the borer for a year or two, they will still eventually succumb. But different trees have different strategies for resistance. Some produce chemicals that killed the larvae. Others were less attractive to the egg-laying adults, and so on. By breeding the trees with different resistant characteristics together, Koch realized it may be possible to produce a strain that could survive this pest infestation.
The efforts have borne some fruit. The top-performing offspring of two lingering ash trees kill up to four times as many larvae as their parents, Koch and colleagues have found. “Our best tree so far was 11 out of 11” larvae killed, Koch says; the 12th egg failed to hatch. Now, she’s planning to use those top performers to breed an even more resistant generation. If those trees consistently kill 80% to 90% of larvae that attack them in field trials, Koch says they can begin restoration efforts. She hopes restoration plantings could start in about 10 years.
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Despite the progress she’s made, Koch says it sometimes “feels like the weight of the world is on us.” Hers remains the sole U.S.-based effort to revive ash trees through breeding. (Breeders in the United Kingdom are seeking genetic resistance to a fungus attacking ash trees there.) But she’s starting to recruit help by building a network of government and nonprofit organizations to grow and plant resistant ash trees. The collaboration—which includes the conservation organization American Forests; the Holden Arboretum in Kirtland, Ohio; and Fender Musical Instruments, which has long used ash wood for guitars—has plans to establish an ash tree nursery in Detroit. Some of Koch’s trees were supposed to be growing there by now, but the shipment was delayed by the pandemic and is now planned for spring.
So there is hope, but it’s a slow-motion race against the spread of the pest, and research money is drying up. On the other hand, other researchers may try Koch’s strategy to try to save other native tree species that are being devastated by foreign pest infestation, in particular American hemlock. Unfortunately, none of this will help to save the trees in our yard, but it’s hope for the future.
That’s what I’ve got for tonight. Comments are below the fold, but first, here’s a word from our sponsor:
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