ST. PAUL — Minnesota agricultural officials confirmed last week the discovery of emerald ash borer, an invasive tree-killing species native to Asia, in Carver and Sibley counties.

That makes them the 24th and 25th counties in the state where the insect is known for certain to be since 2009. The Minnesota Department of Agriculture announced the discovery six months after the pest was found to have spread to Mower County, the 23rd.

As in the past, officials are unsure of how the insect made its way to a new area.

"There’s not going to be a smoking gun kind of thing," Minnesota Department of Natural Resources forest health program consultant Val Cervenka said.

It might be that it did so naturally. Or it could have hitched a ride on firewood, Cervenka said, one of the most common ways that it likes to travel across the state.

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In any case, both Carver and Sibley counties have been put under an emergency quarantine order. A more formal quarantine may be established under a proposal submitted by the Minnesota Department of Agriculture. Public comments on the proposal can be submitted from now until Nov. 20, 2020.

If established, the quarantine would prohibit manufacturers from using ash wood, on whose insides of emerald ash borer larvae feed, to make pallets or crates without first treating it. It would also make removing ash wood from a those counties punishable by a fine.

The quarantine measures are among several methods that state officials deploy when attempting to combat the beetle's spread. Another prominent one is timber management.

By cutting down ash trees and replacing them with other kinds of trees, officials can cut off the food source for the insect while maintaining an area forested. Because the emerald ash borer seem especially partial to the black ash trees that dot Minnesota's swamps, Cervenka said, keeping it out of the them is imperative given the difficulty that working in wetlands poses.

Colder weather appears to have kept the emerald ash borer from moving farther north in Minnesota as well, said University of Minnesota adjunct associate professor Rob Venette.

Venette, who heads up a collaboration between the university and the MDA, also works with the U.S. Forest Service and is helping to develop a "bio-control" method of stemming the ash borer's spread. That involves the use of wasps that kill the insect in its larval stage, which otherwise has few other natural predators.

He said the state is fortunate to have strong partnerships that make its fight against the ash borer a relative success, but cautioned that "the key is that we have to keep it up."

Its unlikely in the long term that the ash borer will be eradicated, said MDA entomologist Angie Ambourn. The effect that it will have on the ash trees it feasts on will reveal itself only in time.

"Are we going to get rid of emerald ash borer? No, that's not realistic," Ambourn said.