Two years into Minnesota's emerald ash borer invasion, St. Paul has decided that not all ash trees are equal.

The city probably will inoculate ash trees to protect them from the borer around Midway Stadium, in key downtown locations such as Mears Park and along the edge of Phalen Golf Course, where leafy branches block errant golf balls from dinging cars on Hwy. 61.

But near the bull's-eye of the borer's foothold in the city, sacrificial trees have been girdled to attract the insect. This winter, they will be cut down and burned to destroy any larvae.

St. Paul's nuanced approach to a pest that is expected to eventually kill most of Minnesota's ash trees signals how cities have refined their response since the insect was discovered in the capital city in 2009. Once, talk centered on tree removal, but the high cost of removal has cash-strapped cities considering more subtle approaches.

Since 2009, the borer has spread in the metro area to Minneapolis, Falcon Heights and Shoreview. Outstate, it's been found in La Crescent, in the southeast corner, in two locations near Winona and in rural Houston County.

Inoculate, chop or wait?

Some cities, like Richfield, have chosen to inoculate trees in an attempt to postpone removal. Others are replacing them now. Some, including Shoreview, are simply waiting to see what happens.

Shoreview had a confirmed borer case this summer, to officials' surprise. "We thought it would come through Roseville before it came to Shoreview," said public works director Mark Maloney. The city will remove infected trees.

In Bloomington, ash trees have been added to the city's list of forbidden nuisance species. The city has rated the 8,200 ash trees on city-owned land as excellent, fair or poor. Those rated poor, as well as healthy trees in construction zones, are being removed in the winter, when the adult insects are inactive.

"We can't stop it. We hope to control it," said city forester Paul Edwardson.

In contrast, budget-conscious Richfield officials opted to spend $50,000 annually for three years to inoculate the best of its 3,200 boulevard ash trees. Chemical treatment should last for three years before trees need to be inoculated again, postponing the need to spend as much as $5 million over five years to remove dying trees. "Residents of Richfield love their trees, but for us, it's mostly financial," said Chris Link, operations manager for the city's street and forestry division. "We're eight to 10 miles away from the [borer infection] in Minneapolis, and we decided to be proactive and start on the east side of the city and try to get ahead of it."

Attracting the bug on purpose

St. Paul and Minneapolis both girdle test trees near borer infestations in hopes they will keep the insects in the area. While the borers attack healthy and stressed trees, a girdled tree acts as a magnet. Damaged trees are cut down and destroyed in the winter.

St. Paul is removing and replacing about 1,000 of its roughly 30,000 boulevard ash trees each year, sometimes taking out all trees on a block. Cy Kosel, forestry supervisor for St. Paul's park and recreation department, said the city will let homeowners willing to pay for inoculations keep a tree if they treat it on schedule.

St. Paul has decided that wholesale treatment of trees is not economical and perhaps not environmentally wise. But this summer, using a state grant, it inoculated about 300 trees in borer-infested areas if the trees were 10 to 20 inches in diameter and in good shape.

Minneapolis is removing weak ash trees and replacing boulevard ashes for homeowners who request it, but is holding off on chemical treatment.

The city just finished an inventory of ash trees on its golf courses and will talk with managers about those worth spending money on to treat, said Ralph Sievert, director of forestry for the park and recreation board. Trees can be successfully inoculated even after they have the borer, as long as they aren't too damaged.

Mary Jane Smetanka • 612-673-7380