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In June 2001, a plant pathologist was called to look at some trees on the grounds of a condominium complex in Plymouth, Michigan, just west of Detroit. What he found were dozens of sick and dying ash trees. He thought the culprit was ash yellows and was relatively unconcerned. A month later he made a follow up visit. A tree crew was on site removing the ash trees. On one of the trees, the bark had fallen away and he saw for the first time the maze of squiggling tunnels throughout the phloem. He took some wood back to his lab. The next day dozens of bright green beetles emerged from the logs, chewed their way through the plastic bags and were flying around. By then he realized he was seeing something new. He took the beetle to Michigan State University. They didn’t know what it was. Nor did Oregan State, or UCLA. No one in the entire United States had seen the beetle. Finally, someone sent a specimen to an entomologist in Slovakia. The beetle did have a name, Agrilus planipennis, or Emerald Ash Borer as it was eventually called. It was from eastern Asia. The only information was in one small paper in an Asian forestry textbook with nothing about its biology. The emerald ash borer is not a problem in it’s native Asia. The Asian trees had defenses, honed by millions of years of shared evolutionary history. The North American trees lacked these defenses.
By 2002 the ash borer had already reached all the ash trees in the southeastern corner of Michigan. Efforts to stop the invasion were troubled. Quarantines were put in place to stop all nursery stock, timber and firewood from leaving the affected counties. Healthy ashes were cut down in a six-mile-wide swath in hopes of creating a fuel break to contain the beetles’ spread. If anything, the beetle seemed to spread more quickly. Then they tried insecticides. Those can work to save a small number of trees but not a whole forest. Later, the focus shifted to tiny wasps from Asia, which kill emerald ash borer eggs and larvae. They help but don't completely get rid of EAB. Overall, the outlook for ash trees in North America is still really dire.
Twenty-four years later emerald ash borer has spread to 37 states, and 6 Canadian provinces. It has killed hundreds of millions of ash trees and threatens to kill most of the 8.7 billion ash trees throughout North America. Minnesota is home to an estimated 1 billion ash trees. Sixty-one percent of Minnesota counties are now infested.
Scientists have noticed that some ash can resist the borers. These are the lone green and robust tree in a boneyard of dead ash. These trees not only have resistance to the emerald ash borer, they can actually kill the larvae. They call these trees “lingering ash.” Only one-half of one percent of green ash trees show any resistance to the beetle. But this may be enough. Maybe these trees can breed new, resistant stock. So begins the work of decades.
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