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.”

Adding to the unpredictability, said Abrahamson, is that it often takes a few years before the borers do enough damage to be spotted. People may notice split bark on the trees, top branches that don’t leaf out, and woodpeckers that show up to feast on the bugs.

The time lag between infestation and discovery means that many more areas could already have the bugs, said Abrahamson. “The problem with this thing everywhere is that we’re always behind it some amount of time,” he said.

The borers are also cold-hardy, he said, and temperatures need to plummet to 30 below to kill significant numbers of them.

Phased-in cutting

Barten said it may seem odd to cut down healthy ash trees since there’s no infestation yet in Three Rivers parks, but it’s better than waiting for all of the trees to die at once. The plan is to remove 10 to 15 percent of the ash trees in the active use areas each year, he said.

“It spreads out our costs, it spreads out the visual impact, and it allows us to go back in to replant some of those trees from nursery stock over a number of years,” he said.

Many cities are following similar plans, including Minneapolis and St. Paul, where several thousand ash trees have been removed from both infested and non-infested neighborhoods. Last week the Minneapolis Park and Recreation Board marked about 50 more ash trees that need to be removed from private property in the Prospect Park neighborhood near the University of Minnesota.

Kortebein said crews removed about 90 ash trees from the picnic and campground areas in Baker Park Reserve a few months ago, and will replace many of them next fall with different species. He said most of the estimated 2,000 that need to be removed are also clustered in popular activity areas of Hyland Park Reserve, French Regional Park in Plymouth and Elm Creek Park Reserve in Maple Grove.

Tens of thousands of other ash trees in the “natural areas” of the parks that receive much less public use will not be removed or treated, he said.

The district will also try to prevent about 100 “specimen” ash trees from becoming infested by treating them with chemicals, Kortebein said. These include some at Baker Golf Course, Cleary Golf Course in Scott County, Baker campground and Noerenberg Gardens on Lake Minnetonka.

The insecticide needs to be applied every other year to remain effective, he said, and plans call for the trees to be treated for the next 10 to 15 years.