Emerald ash borer has devastated urban and natural forests in most states where it occurs, causing 100 percent mortality of susceptible ash species. Although emerald ash borer has been in Minnesota since 2009, little mortality has been observed and spread has been slower than in other states. Forest Service researchers evaluated the role cold temperatures might play in regulating populations of this insect. In general, the insect survives exposure to temperatures from 0 to -20°F by producing cryoprotectants, so in many places, cold temperatures may not have an impact on this insect. However, as within-tree temperatures fall below -20°F, mortality rates begin to increase. With the Polar Vortex in January 2014, air temperature in the Twin Cities fell to -23°F, a temperature that could cause up to 80 percent mortality. Samples taken from cut logs and standing trees that had been outdoors during the winter showed that 60 to 70 percent of larvae had been killed in most locations. Winter mortality of emerald ash borer is unlikely to eliminate this insect but may give natural resource managers more time to respond to this insect in areas where extremely cold winter temperatures (less than -30°F) are common.