Three Rivers Parks prepare for onslaught of emerald ash borers

  • Article by: TOM MEERSMAN , Star Tribune
  • Updated: March 12, 2013 - 4:07 PM

As bugs close in, Three Rivers Park District will remove and replace about 2,000 trees on an aggressive schedule.


Emerald ash borers burrow under bark and make S-shaped tunnels as they feed on water and nutrients that the trees need to survive.

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The emerald ash borer is spreading faster than expected in the Twin Cities, and forestry workers at Three Rivers Park District are ramping up efforts to cut down vulnerable trees and replace them with different species.

The Park District’s action plan — presented to commissioners last week — also includes treating select ash trees with insecticides to prevent them from being infested by the tiny green beetle.

“The emerald ash borer is moving about as fast as anyone had predicted it could move,” said John Barten, natural resources director for Three Rivers. “We’re almost looking at a worst-case scenario.”

The iridiscent green borer kills trees by burrowing under their bark and eating away at vessels that carry water and nutrients, effectively starving the trees in four or five years.

Its first appearance in the metro area was in a St. Paul neighborhood in 2009. Although it has since been discovered in more than half a dozen other nearby areas, the pest seemed to be spreading slowly.

But that may be changing. New infestations were confirmed in late January in Lakewood Cemetery in south Minneapolis, about 7 miles from Bryant Lake Regional Park in Eden Prairie and Hyland Park Reserve in Bloomington.

That’s uncomfortably close for Three Rivers, which runs those parks and 21 other parks and park reserves on about 27,000 acres, mostly in suburban Hennepin County.

No borers have been discovered yet in any Three Rivers parks, said Paul Kortebein, the district’s senior manager for forestry.

The park system has about 250,000 ash trees in its forests, he said, including 2,000 in “active areas” such as picnic spots, campgrounds and trail corridors.

Those trees in popular areas need to be removed over the next several years, Kortebein said, because once they become infested and die, they will be hazardous.

In other states where the borer has decimated ash populations, he said, the wood becomes brittle and can lead to “mass failures of trees,” where large limbs and even whole trees collapse unexpectedly, even in good weather.

Quick spread

The small green beetles came from China, probably in crates or pallets, and were first found in Detroit-area trees in 2002. They have since spread to at least 14 states and two Canadian provinces, killing an estimated 50 to 100 million ash trees in their path.

Mark Abrahamson, entomologist and director of the emerald ash borer program for the Minnesota Department of Agriculture, said it’s only a matter of time before the borer spreads throughout much of the state, and the goal is simply to cut down trees that have been infested as soon as they’re discovered.

“We’ve done a good job in staying on top of this, but we also knew this was a losing battle from the start,” Abrahamson said. “The goal here is to slow things down.”

Because the bugs often spread when people move infested wood, regulators have imposed quarantines to restrict transfer of the wood out of the four Minnesota counties where borers have been confirmed: Hennepin and Ramsey in the metro, and Houston and Winona in southeastern Minnesota.

Other states have seen the borer show up and spread slowly at first, Abrahamson said, but usually there’s an explosion at some point.

“Every year I’m kind of holding my breath hoping this isn’t the year where it gets found in 20 new places,” he said. “That’s always kind of lurking out there.”

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