By Bronislaus B. Kush
Worcester, MA (August 31, 2011)- Trees infested with the Asian longhorned beetle have shown remarkable resilience in physically coping with the insect’s devastating, decade-long onslaught, a recently released study by researchers from Harvard University and the U.S. Forest Service shows.
Surveys conducted in two areas around Ararat Street on Indian Hill in December of 2008 and in September of 2009 revealed that the 200 or so inspected trees, though heavily infested, had continued to grow. The researchers also observed that – though the invasive insects were in the region for at least 10 years – the ALB did not “aggressively” kill trees.
In fact, the study’s authors said they could find no signs that trees in the sample areas were “killed outright” by the beetle. “There were no ‘kill trees.’ That’s glaring,” said researcher David A. Orwig, a forest ecologist at Harvard Forest, Harvard’s 3,500-acre center for forestry and ecological studies in Petersham.
The study by Mr. Orwig and Kevin J. Dodds, an entomologist with the federal Forest Service, was published Monday in the Canadian Journal of Forest Research. Despite the ability of the trees to weather the numerous borings inflicted by the bug’s larvae, the researchers noted in their report that the arbors would eventually succumb to the damage.
The study backs the current strategy of federal, state, and local authorities to weed out infested trees out of fear that the insect might spread beyond the current 98-square-mile containment zone in Central Massachusetts and ravage woodlands throughout the Northeast. “This outbreak (in Central Massachusetts) seems to be under control,” said Mr. Orwig, who has been studying trees for 16 years. “But it’s pretty tricky to keep it that way.”
The study, in many ways, reflects the findings and thinking of officials at the U.S. Department of Agriculture, the lead agency in the battle to contain the beetle both locally and nationally. But it breaks new ground in attempting to identify tree species particularly vulnerable to the insect. According to the report, the ALB invaded many different types of hardwood trees that were located on city streets. But in the more heavily-forested areas in the northern part of the quarantine zone, the beetle disproportionately attacked large maple trees.
The authors estimated that nearly two-thirds of infested woodland trees in one of the areas studied were maples. In the second, about one third were. Those observations strengthen fears by USDA officials that the ALB, if unchecked, could cause substantive damage to New England’s tourism industry, which attracts at least a million visitors annually and generates about $1 billion in revenue. Red maples are widespread in the region and are a central component of the landscape that tourists like to visit in the fall.
Like USDA officials, the authors also fear that the beetle may decimate the population of sugar maples, which drive northern New England’s maple syrup business. Mr. Orwig said he is unsure why the beetles are attracted to the larger maples. But he added that the identification of that species as the number one choice of the insect could help those involved in the eradication efforts to map the travel patterns of the ALB.
Mr. Orwig said he was also surprised to find that the impacted trees weren’t as heavily infested as he was led to believe. He explained that most of the trees were drilled with dozens of larvae borings, not the hundreds that he had expected. Mr. Orwig said it’s not impossible to rule out that the insect instinctually knows that there’s a limit to the number of larvae that a tree can sustain. “We just don’t know,” he said.
The larvae, scientists said, live from one to two years within a tree. Once they become foliage-eating adults, the insects live for only about a month. “It is clear that it’s the larvae that causes the damage,” Mr. Owig said. However, in many cases, the researchers noted that the ALB’s impact on the growth of trees seemed “negligible.” Some trees with many visible exit holes, for example, exhibited reduced radial growth but nonetheless had live crowns.
The live crown is the top part of the tree that bears green leaves. The ratio of a live crown to its height is used by arborists as a way of gauging a tree’s health. The study also documents, for the first time, that the insect can thrive in deeper woodlands. In its native China and Korea, the insect usually lives on the fringes of forests, and, when it invaded the United States, the ALB inhabited urban settings.
The Worcester infestation is the only one in which the beetles migrated from street arbors to closed canopied forests, the study revealed. “From our work, it became apparent that ALB was readily moving through forests and attacking trees, making it a threat to forests of the region,” Mr. Dodds said.
Mr. Orwig said he’s hopeful that the beetle can be contained. He noted that all the outbreaks across the United States seem to be rooted in the 1990s when regulation of shipments of wood-packed materials from overseas was not as tight as it is today. The Worcester outbreak is the largest to date in North America. More than 29,000 trees have been removed in an attempt to stem the insect’s proliferation.
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