Ash borer: Best defense is a strong offense

Three Rivers Park District is not waiting for its 27,000 acres of prime parkland to see the green of the emerald ash borer's wings.

Believing the insect attack is inevitable, the west suburban park district plans to cut down 200 ash trees a year -- even healthy ones -- for the next 10 years. For each of the 2,000 trees removed, a new tree will be planted to replace it.

Invoking a lesson learned from the 1970s Dutch elm infestation, the district will leave heavily wooded natural areas alone, even though ash trees make up a third of the park forest, and focus on removing trees in heavily used areas -- beaches, picnic areas, campgrounds and playgrounds -- before they get the bug.

A few cities have embarked on strategies to cut and replace healthy trees, but none approaches the size of Three Rivers, which has launched its largest-ever preemptive cutting effort.

On the 40th anniversary of Earth Day, Minnesota is poised for an expansion of an emerald ash borer infestation that so far has touched parts of St. Paul, Minneapolis and Falcon Heights, according to the Department of Agriculture. Adult beetles typically emerge in mid- to late May, and the pest's spread may be revealed then, said department spokesman Michael Schommer.

Getting ahead of the destructive metallic green insect by methodically cutting down trees every year will even out park district labor costs and avoid pulling staff away from normal duties to deal with emergencies, said Paul Kortebein, manager of forestry and horticulture for the park district. But no one is predicting that the district's efforts will spare it from an infestation.

"When it gets here we won't have to deal with so many trees that are dead or dying at the same time. It makes the operation of the parks a lot smoother," Kortebein said.

Replacement trees -- oaks, maples and basswoods grown at the district's nursery -- are 6 to 7 inches in diameter and up to 15 feet tall. Planting new trees makes it easier to take down healthy ash trees, he said. "Right now we have been targeting some of the trees that maybe do not appear as healthy but, yes, we will be taking down some healthy trees."

Slowing an inevitable pest

The half-inch beetles kill trees by eating away the trunk under the bark, girdling the tree and stopping the flow of water and nutrients to its branches and leaves. New shoots sprouting at the base of the tree -- below the belt of chewing bugs -- is a sign of an infestation. Woodpecker drilling is another clue that the insects are beneath the bark.

The insect is native to China and Korea. Exactly when it made its way to North America is unknown. In the last several years it killed millions of ash trees in 10 eastern states and reached Michigan in 2002.

An outbreak in western Wisconsin and the Twin Cities was discovered last year.

The state strategy is to slow the movement of the bug, Schommer said. "It's proven futile to eradicate it."

The Department of Agriculture has placed a quarantine on Hennepin and Ramsey Counties, prohibiting the movement of ash limbs or logs to other counties.

Using lessons from the '70s

Three Rivers has targeted ash trees in actively used parts of the parks so people can safely picnic, camp and play on playgrounds, Kortebein said. If 200 trees are cut every year, it may mean that individual parks will lose 10 to 20 trees a year.

If an infestation hits, 200 trees could be lost in a single park, Kortebein said.

If ash trees in natural areas are infested, they will stand until they fall on their own, Kortebein said. That approach contrasts with the 1970s, when the district eradicated all trees with Dutch elm disease. Heavy logging equipment that disturbed soil and new trees "did as least as much damage as good," he said.

Since large groves of green and black ash grow in some natural areas, their loss will noticeably detract from the parks' beauty, atmosphere and animal habitat, Kortebein said.

"The question is what is going to replace the ash, and my fear is we are going to end up with a lowest common denominator forest," with non-native, invasive species taking over, he said. Green and black ash trees, which are native to Minnesota, will be planted again as part of the district's forestry program because they are fast-growing.

Some big ash trees on golf courses in the district could get chemical treatment to prolong their lives, Kortebein said. "If we lose one or two on a fairway it would change the character of the golf course pretty drastically."

Laurie Blake • 612-673-1711