Deep freeze expected to slow spread of emerald ash borer


The invasive pest that's eaten away thousands of Minnesota ash trees may have met some competition, for now.

Rob Vennette, research biologist at the U.S. Forest Service’s Northern Research Station in St. Paul, told the Star Tribune that the recent cold snap across the state could kill off 80 percent of the emerald ash borer population.

After burrowing beneath the bark and feeding on wood in the summer, ash borer larvae quietly spend the winter inside ash trees. During harsh Minnesota winters, it's a death wish.

University of Minnesota grad student Lindsey Christianson, who's conducting research with Vennette, put larvae in a freezer that goes as low as -112 degrees, the Star Tribune reports. She found larvae tend to freeze when temperatures dip between -20 and -30 degrees, but some survive beyond that.

FOX 9 reports at -20 degrees, the mortality rate is about 50 percent. At -30 degrees, the mortality rate is just 90 percent.

That gives the pest bad odds during the latest stretch of extreme cold. The Twin Cities saw a low temperature of -23 degrees Monday and some places in northern Minnesota saw readings in the -40s.

However, in the long run, the partial kill-off won't eliminate the threat to the state's 1 billion ash trees. The surviving beetles will continue to reproduce, but at a little slower pace. This gives those who are planning to remove trees a little more time.

Venette told FOX 9 that subzero temperatures could buy a delay of two to three years.

Another weapon slowing the spread of ash borers are tiny stingless wasps launched two years ago by the state agricultural department. The parasitic insects are a natural predator of ash borers and evidence shows they're attacking.


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