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Tree-killing beetles unfazed by harsh winter

An adult emerald ash borer. | File photo

An adult emerald ash borer. | File photo

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Updated: April 29, 2014 6:22AM



As cold as it got this winter in the Chicago area, it probably did little harm to a tree-killing bug that is costing some communities millions of dollars.

Some people who are on the front lines of the battle against the emerald ash borer had hoped the third-coldest winter on record might have been their ally.

In Oak Lawn, where nearly 1,500 ash trees ultimately will be removed because of the beetle infestation, ash borer larvae still were thriving despite a season that had its share of below-zero temperatures.

Matt Basile, the village’s forester, recently checked some ash trees in his community and didn’t see any evidence of the cold affecting the bugs.

“I was hoping the cold would kill some of them and maybe buy us some time,” he said Thursday. “It doesn’t seem like it made much of an impact, from what I’ve seen.”

First confirmed in Illinois more than seven years ago, the tiny green beetle has been blamed for killing ash trees throughout the Chicago area. Some Southland communities, such as Tinley Park and Orland Park, expect to spend millions of dollars in the coming years removing thousands of infested trees and replacing them.

The beetles lay their eggs in the trees, and the larvae nibble away at the tissue below the bark that carries water and nutrients throughout the trees, causing them essentially to die of thirst.

Mitch Murdock, an arborist with Site Design Group, is working with Tinley Park to coordinate its ash tree removal and replacement program. He said the beetles have spread as far north as Canada and have been detected as far south as Georgia.

“If anything, (winter cold) just kicked the population back a few ticks but certainly was not enough to wipe it out,” he said. “I have to imagine they’ve been through winters like this.”

If it had been a bit colder in the Chicago area, the subzero temperatures would have helped in the bug battle, according to Rob Venette, a research biologist with the U.S. Forest Service who has been studying how the ash borer responds to super-cold temperatures.

At minus-20 degrees, beetle larvae start dying off, and the colder it gets, bugs bite the dust in larger numbers, he said.

The lowest air temperature this winter in Chicago was minus-16 on Jan. 6, according to the National Weather Service, and that simply was too warm to help curb the ash borer population in this area, Venette said.

While combined with the wind, temperatures plunged lower, baby beetles aren’t bothered by wind chill, partly because they’re nestled under tree bark and also because they’re cold-blooded, he said.

Studies in Minnesota, where the beetle has been found in just four counties, have confirmed larvae die-offs because of the cold, Venette said. But even in areas where that did occur, it’s simply slowing, rather than stopping, the infestation process.

The ash borer “is such a good reproducer,” and it will take just a year or two for the bugs to return to full strength, Venette said.

Ash trees were favored by developers because they are fast-growing and relatively inexpensive. In some suburbs, entire subdivisions were landscaped mainly with ash trees, which now are dead or dying.

Basile said Oak Lawn has removed nearly 900 ash trees — with 619 still to go — and has replaced them with 700 new trees. In that community and others, diversity of new trees will be the key to prevent a mass die-off should another bug or disease lay siege to a particular tree species, Murdock said.

He said Tinley Park officials still are tweaking tree-replacement plans but that more than 40 tree species are being considered.

“Diversity is key to maintaining the health and longevity of the urban forest, so when the next epidemic comes along it won’t wipe out a third of your trees,” Murdock said.



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