In 2001, ash trees began dying in Detroit, and no one could say why. Then glittering green beetles were discovered crawling out of an ash log.
American scientists had never seen the beetles, and they reached out to experts around the world for help. A Slovakian entomologist namedEduard Jendek solved the mystery: Detroit’s ash trees were being killed by Agrilus planipennis, the emerald ash borer, an obscure species native to East Asia.
In its home forests, the emerald ash borer causes little trouble. Sadly, that was not the case in North America. The emerald ash borer has spread north through Canada, south to the Gulf of Mexico, east to the Atlantic and as far west as Colorado. In a review published last year, scientists called it “the most destructive and economically costly forest insect to ever invade North America.”
Insects are dread enemies of any tree species. They may strip away the leaves, so that tree can no longer harvest sunlight. They may destroy its trunk or lay waste to its roots, so it can no longer take in water.
To defend themselves, trees depend mostly on chemistry, manufacturing compounds that make their tissues tough to digest or their wood and leaves poisonous. Trees produce some defensive chemicals, called constitutive compounds, pretty much all the time. But trees also create a fresh supply of other insect-killers, called inducible compounds, when they sense an attack.
In East Asia, ash trees have evolved defenses against their local insect species like the emerald ash borer. To study their adaptations, scientists have grown East Asian species, such as the Manchurian ash, in greenhouses and experimental fields. They allow emerald ash borers to attack the trees, and then observe how the trees fight back.
When an emerald ash borer searches for a tree in which to lay her eggs, she avoids healthy Manchurian ash trees, much preferring a North American ash or a Manchurian ash weakened by drought. As it turns out, a healthy Manchurian ash is a hostile place for beetle progeny. When emerald ash borer’s eggs hatch in the trees, the larvae struggle to survive.
This may be why the beetle never caused much alarm: In East Asia, it left healthy ashes alone. “It’s going to kill already-dying trees,” said Caterina Villari, a postdoctoral researcher at Ohio State and an author of the new study.
Dr. Villari and her colleagues don’t know precisely how drought makes Manchurian ash vulnerable to the insects. Drought-stricken trees appear to produce constitutive compounds in the same amounts as healthy trees, but not some of the inducible compounds.
By contrast, even a healthy North American ash tree can fall prey to the emerald ash borer. North American ashes can fend against their local enemies, but the emerald ash borer presents a new challenge, one they haven’t evolved to meet.
To understand that failure, Dr. Villari and her colleagues have been comparing the defenses of Manchurian ashes with those of North American species like white ash and blue ash. The researchers have identified an assortment of chemicals that Manchurian ashes make in higher amounts.
“The fact that Manchurian ash had a higher concentration of one particular compound doesn’t mean that compound is what makes it resistant,” Dr. Villari said. “It’s a first step.”
North American ashes may not be dying because they lack a magic antidote against emerald ash borers. Instead, the trees may not be producing defensive chemicals in the right balance — or perhaps they just aren’t making them fast enough.
In one particularly intriguing experiment, Justin G.A. Whitehill of the University of British Columbia and his colleagues were able to get North American ashes to fight off the beetles. To do so, the researchers sprayed the trees with a plant hormone called methyl jasmonate.
That hormone is released by plant cells that are damaged by chewing insects. It acts as an alarm, prompting the entire plant to make inducible compounds.
Dr. Whitehill and his colleagues reported last year that when they primed North American ash trees with the hormone, the plants killed off many emerald ash borers. The effect was about as strong as a lethal dose of insecticide.
Now researchers are hoping that one day, they may be able to identify ash trees making high levels of key chemicals and to develop them into resistant stocks. Scientists may be able to pick out trees that respond with unusual speed to an emerald ash borer attack.
In the real world, North American ash trees fail to deliver this powerful punch. It’s possible that they don’t recognize emerald ash borers as a special threat, and so they fail to mount a swift response.
New studies are underway to make sense of these tantalizing results. Unfortunately, running experiments on trees is not for the impatient. The science is moving ahead steadily and slowly, but the emerald ash borer is racing for new forests