The emerald ash borer has been discovered in Stearns County, meaning it continues to spread northward across Minnesota despite a cold winter that many hoped would slow its march, state officials said Tuesday.

The discovery places the tree-killing beetle some 50 miles northwest of what was thought to be the edge of the infestation, according to the Minnesota Department of Agriculture.

The department placed Stearns County under quarantine Tuesday, prohibiting transport of firewood or other ash wood out of the county. The state has now placed 18 counties under quarantine, mostly around the Twin Cities and Duluth.

The beetle, which was first found in Minnesota in 2009, is a certain death sentence for ash trees. Its larvae burrow inside a tree's bark until they eventually cut off its circulation like a blood clot.

The beetle's larvae can't survive long when temperatures drop to 30 degrees below zero, as they did this winter, which gave researchers hope that a generation of ash borers would freeze to death and delay the insect's spread to other parts of the state. But given how far the beetle was found from a known infected area, it was almost certainly brought into Stearns County by someone who carried in firewood or other ash products, said Mark Abrahamson, the department's plant protection division director.

"This highlights the importance of quarantines and the need to limit the movement of firewood," Abrahamson said.

Larvae of emerald ash borers are a death sentence to ash trees.

Most states in the Midwest, including Wisconsin, Michigan, Iowa and Missouri, enacted statewide bans on moving firewood years ago, unlike Minnesota, which has implemented bans only county-by-county as the borer is found. Illinois threw in the towel in 2015 and stopped enforcing its ban, saying it was a lost cause. Michigan removed its ban last year after recognizing that the pest had already reached every part of the state.

The ash borer has spread to 35 states, killing hundreds of millions of trees. With an estimated 1 billion ash trees in Minnesota, a large share of the state's canopy is at risk.

Greg Stanley • 612-673-4882