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Troubling times ahead for Nebraska's ash trees
EDITOR'S NOTE -- This is the first installment of three detailing the destruction and death caused by the emerald ash borer to billions of ash trees across North America. In McCook in early September, Dr. Mark Harrell of the Nebraska Forest Service visited with arborist Bruce Hoffman.
McCOOK, Neb. -- Ash trees are in serious trouble.
The loss of most of the country's ash trees is inevitable.
The damage and death caused by a shiny jewel-green beetle called "emerald ash borer" can be slowed but not stopped, said Dr. Mark Harrell of Lincoln, forest health program leader for the Nebraska Forest Service, during a stop at Common Scents in McCook recently. He said, "We can slow it some, with education."
Since the first discovery of the emerald ash borer (EAB) in the United States, in southeastern Michigan during the summer of 2002, it has killed millions of ash trees in North America. It threatens to kill most of the 8.7 billion ash trees throughout North America
EAB is not native to the United States; in its natural range in eastern Asia, it's found in low densities and is a tolerable nuisance. Outside eastern Russia, northern China, Japan and Korea, it's an invasive species with few, if any, natural predators, and billions of green ash, black ash, white ash and blue ash trees to feast on -- and kill.
The adult EAB beetle feeds on ash foliage, causing little damage. It's the larvae (the beetle's immature stage) that feeds on the inner bark of ash trees, disrupting the tree's ability to transport water and nutrients, and ultimately killing the tree.
In the spring, female EABs deposit their eggs in bark crevices or cracks. About two weeks later, the eggs hatch into larvae that chew through the bark to the cambium where they feed and develop. In the fall, most mature larvae excavate chambers to the outer bark where they develop into pupae. (Some larvae may overwinter in their tunnels, feed through another summer and emerge as adults the following spring.)
In the spring, the pupae develop into adult beetles that chew through the bark and exit through holes they've shaped into distinctive capital D shapes. Males and females ... eggs and larvae ... pupae and adults ... the insidious cycle continues. "The female lives only four to six weeks, long enough to lay eggs," Dr. Harrell said.
It's the long, white, wormy larvae that sound the tree's death knell, scribbling the death notice for its host ash tree within its twisted, serpentine tunnels.
After discovering EABs in an Omaha city park on June 6, 2016, Nebraska became the 27th state to fight an active EAB infestation. Which shouldn't surprise anyone because neighboring states Iowa, Missouri, Kansas and Colorado are all waging the same battle.
Tree experts believe that most of the long-distance spread of EAB is from people transporting ash firewood. Dr. Harrell says, "An ash tree dies and sometimes the owner doesn't know why. It makes good firewood," Dr. Harrell said. "We can transport EAB hundreds of miles that way."
EAB is known to cover short distances by hitching a ride on or in vehicles, or even on tree workers and their equipment. An emerald ash borer may fly a mile or so if it needs to, Dr. Harrell said, but it's not moving itself over great distances.
Studies in Michigan have shown that an EAB infestation can spread about 20 miles a year.
Dr. Harrell said that typically EAB damage goes unnoticed for three to four years. "It's not easy to see," he said.
A few trees will die each year in the next three to five years, while the infestation spreads.
By year eight, the mortality rate of the ash trees within an infestation area takes off. Within the next four years, "the bottom falls out," Dr. Harrell says, and 70 percent of ash trees will die.
Within 15 years, the only ash trees still standing will be those that received treatment. "Treatment is generally effective," Dr. Harrell said. "It can keep trees alive for quite a few years, but ... they will eventually die, of the beetle or damage from the treatments."