The tree-canopy-killing emerald ash borer continues to spread north and west across Minnesota.

The invasive and relentless beetle, which has destroyed millions of ash trees throughout the country, was found this week in Carver and Sibley counties. The pest has now established a foothold in nearly a third of the state, spreading from hot spots that formed years ago in the Twin Cities and near the Iowa and Wisconsin borders.

There's no way to stop an ash borer infestation, said Jeffrey Hahn, University of Minnesota entomologist. The best hope is to treat infected trees and slow down the spread long enough for a solution to be discovered that can save at least some of Minnesota's 1 billion ash trees.

"We are trying to buy time," Hahn said.

The ash borer has been one of the most devastating tree pests to hit North America since it was first found in Michigan in the early 2000s. The beetle kills ash trees like a blood clot, burrowing under the bark to feast and lay eggs. Eventually, enough larvae build up in the inner tree to block the tree's flow of nutrients, starving it.

Minnesota is particularly vulnerable, with the most ash trees of any U.S. state. The Minnesota Department of Natural Resources (DNR) estimates that one in five trees in many cities are ash, and they make up as much as half of the tree canopy in parts of western and central Minnesota.

It's possible the borer could wipe out every ash tree in the U.S. and even North America. But there is still hope, Hahn said.

Studies out of Michigan are finding that a particular type of ash tree — blue ash — isn't quite as susceptible to the beetle and may be able to survive in small numbers, he said. In parts of Asia, ash trees have been living with the borer for millennia. It may be possible to someday breed ash trees that are resistant to the borer and can survive Minnesota winters, he said.

Bio-control agents, such as parasitic wasps that attack the beetle, are also starting to show some signs of promise.

"These wasps are never going to eradicate them, but they might be able to get them down to numbers where the ash will survive," Hahn said.

Insecticide treatments can also help keep ash trees alive in yards and along city roadways for years after the borer is present, as long as they are reapplied regularly. When the treatment stops, the borers typically march right back in and kill the tree.

The quickest way the beetles infect new areas is by hitchhiking inside firewood. The Minnesota Department of Agriculture has placed Carver and Sibley counties under quarantine, making it illegal to transport firewood or ash products out. It is now illegal to carry firewood out of 25 counties in Minnesota.

Several states have banned the transportation of firewood to try to slow the invasion of the beetle. Some, such as Illinois and Michigan, have since given up on those bans, repealing them after the borer still made it to every corner of those states.

Along with insecticide treatments, many cities in Minnesota have tried cutting down ash trees before the borer can infect them. Cutting the trees down does little to stop the spread, but it can make it more manageable on city budgets to remove and replace the trees over several years rather than all at once when the beetle arrives.

Cities and trees are going to look a lot different in the next three to five years as the ash borer takes hold, said John Knisley, city planner in New Ulm, where more than 20% of the trees are ash.

The city will remove about 335 ash trees this year at a cost of up to $500 a tree, Knisley said. After those trees are removed, there will still be roughly 2,200 ash trees left on city property. And it will take 10 to 20 years for replacement trees to mature to what the ash are today, Knisley said.

"That is a long time to wait," he said.