Appleby said that even though the pest has been in the news since it was first identified in ash trees near Detroit in 2002, there are still people who are unaware of the problem and inadvertently transport the pest to uninfested areas. "I talked with a homeowner in Michigan who had a number of dead ash trees on her property. She said that she had never heard of the ash borer and didn't know it was a problem." Appleby described an innocent scenario that may occur in which her friend or grandson from Illinois visits and she says, "I have plenty of firewood. Take as much as you want." The wood filled with ash borer larvae, gets thrown into the trunk of the car, destined for a new area. Adult beetles would emerge from the wood during May and June and infest a new neighborhood of unsuspecting ash trees.
The natural spread of an infestation is probably no more than a half mile per year. But over the years, it is the transportation of infested firewood into new locations that continues to bring new infestations. Today the insect is commonly found in the southern half of Michigan with scattered infestations in northern Michigan, an area in Canada just east of Detroit, northwestern and several other locations in Ohio, northeastern Indiana and just north of Indianapolis, and in 2006 Illinois infestations were found in Cook and Kane counties.
"Nurseries and businesses that sell trees are monitored and controlled," said Appleby. "But all it takes is one uninformed person to transport infested firewood to bring the pests into a new area."
The beetle may have been in the Detroit area for 10 to 12 years before it was even discovered there. Shortly afterward, federal and state agencies imposed a quarantine in the infested areas, which prohibited the movement of any ash trees, logs, and firewood out of the areas. Unfortunately, prior to the quarantine, infested firewood and some infested trees were already moved to other areas. Households having fireplaces use firewood and a common practice for campers and people who own cabins in outlying areas is to take firewood to their camps and cabins. So, unknowingly infested firewood was distributed to many areas and even to adjoining states.
Appleby says that the emerald ash borer joins a long list of exotic species that have become invasive --most have been accidentally transported. "The emerald ash borer beetle is native to China, Korea, Japan, Taiwan, and parts of Russia. We can only speculate that the beetle probably arrived in the larval stage inside wood crating material from China. Shortly after arriving, adult beetles emerged from the wood and flew to local ash trees infesting them."
Although Appleby says that woodpeckers are natural predators, they feed on larger larvae but by that time the larger larvae have already done extensive damage.
From studies in the state of Michigan it is known that the insect overwinters in the larval stage under the bark of ash trees. In April the larva changes into the inactive pupal stage and then beginning in early to mid May it changes into the adult stage. The adult beetle then chews a distinctive D-shaped exit hole, about an eighth of an inch in diameter, in the bark. The adult beetle flies to ash foliage where it feeds on the edges of the leaf. After feeding for about a week, the beetles will mate. Seven to 10 days after mating the female beetle is attracted to the upper branches of a living ash tree where she deposits eggs in bark crevices. The eggs hatch in about 10 days. The larva bores into the bark and feeds just under the bark where it makes serpentine tunnels.
It is the feeding of the larval stage that is so destructive to the tree. The feeding causes a disruption in the tree’s ability to transport nutrients and so as the number of tunnels continues to increase each year, the upper tree branches begin to die and kill the tree in three to five years.
To date, white, green, and black ash trees all appear to be susceptible to attack by the emerald ash borer. In some urban areas ash may comprise as much as 20 percent of their ornamental trees. "In some Michigan communities where ash lined the streets on both sides, now only stumps remain and the residents have no shade," said Appleby. "We should learn never to plant all of one tree species in a given area but plant a diversity of tree species."
Appleby said that some communities are no longer planting ash trees. "This might be a good course to take, particularly in northeastern Illinois, but in other areas it might be too drastic not to include the planting of at least some ash. Time will tell whether or not we will be successful in limiting the spread of this very destructive ash insect."
Debra Levey Larson | EurekAlert!
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