ST. PAUL — The state’s population of ash trees should have been ruined by now.

Instead, the invasion of a tree-killing beetle has dramatically slowed, leaving millions of ash trees still standing.

“We got kind of lucky,” said Jeff Hahn, an entomologist with the University of Minnesota Extension Service.

Hahn is not declaring victory over the emerald ash borer, which is expected to eventually destroy most of the state’s 1 billion ash trees. But he is noting that on the 10th anniversary of the bug’s arrival, its advance has been slower than was originally predicted.

In Michigan, in only 10 years, the beetle wiped out 30 million ash trees, and spread to almost all of its 83 counties. But after a decade in Minnesota, the destruction has been limited to 17 counties, mostly in the metro area and southeast Minnesota.

“It’s remarkable that we still have ash trees in the metro area,” said Rob Venette, director of the U’s Minnesota Invasive Terrestrial Plants and Pests Center.

The beetle was first spotted in St. Paul in 2009. Its arrival led to fears every ash tree in the metro area would soon be killed.

Experts believe that Minnesota’s cold snaps to minus-30 degrees help kill the beetle. Michigan’s winters are more mild — compare St. Paul’s record cold of minus 41 degrees to Detroit’s minus 13.

Hahn said Minnesota also had the advantage of planning for the invasion.

When the beetle arrived in Michigan, it was the first infestation in the U.S. No one knew what it was or its potential damage.

“They saw all these ash trees dying and said, ‘Oh, that is weird,’ ” Hahn said.

In Minnesota, the warnings spread through a network of tree-service workers, master gardeners and city administrators.

They began to inform homeowners about saving their trees through the use of insecticide that can protect trees for two years. Once an ash tree is infected, it can take four to eight years to die.

Ash trees are common — especially in urban areas. The Extension Service says ash trees make up between 10 and 40 percent of urban trees and in some communities account for 60 percent of trees.

Dozens of cities have spent millions to thin out healthy ash trees, replacing them gradually with other species.

The culling of municipal ash trees won’t drastically slow the spread, said Hahn, because of all the ash trees remaining in urban areas.

But he said cities are saving money by spreading the replacement costs over many years rather than all at once.

Minnesota also was able to spread the single most important message: Do not move firewood.

Left on their own, the bugs can only fly about a half-mile. But when they ride along in firewood logs, they can travel across the country.

“Buy wood locally,” said Hahn, “and burn it locally.”

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