Swirling within the polar vortex is a glimmer of good news: The cold snap might have been enough to kill emerald ash borer larvae and delay the invasive insect’s destructive spread through the Midwest.
Despite protection from the naturally occurring antifreeze in their bodies, emerald ash borers can die when exposed to extreme cold, according to a 2010 U.S. Department of Agriculture study conducted on the University of Minnesota’s St. Paul campus.
When larvae reach zero degrees Fahrenheit, five percent die, according to the study. Mortality climbs as temperatures drop, reaching a 34 percent death rate when larvae reach minus-10 degrees and 79 percent at minus-20. At 30 below zero, 98 percent perish.
This week’s historic cold snap brought much of the Midwest down into the fatal temperature range, but it wasn’t quite cold enough to eradicate the pests, said Mark Abrahamson, a research scientist with the Minnesota Department of Agriculture who co-authored the study.
“We got pretty close,” he said. “We were just on the edge.”
Fatality is affected by a number of variables. Cold tolerance depends on how much time larvae have in the fall to develop, which can make the insects hardier.
Larvae burrowed into older ash trees with thicker bark are better protected. Bark offers between 3 and 7 degrees of insulation, said Lee Frelich, director of the University of Minnesota Center for Forest Ecology.
In La Crosse, the lowest temperature recorded during the three-day cold snap was minus-25; the mercury dipped to minus-26 in Viroqua and in St. Charles, Minn., according to the National Weather Service.
“I think it was cold enough that there will be some reduction in the populations of the insects but not cold enough to wipe them out,” Frelich said.
Species native to the Midwest evolved to survive the frigid winters, but invasive species are more sensitive to the cold, Frelich said.
The European elm bark beetle, which transmits Dutch elm disease, is likely “totally wiped out,” by the subzero temperatures, as is the hemlock woolly adeglid. The Asian longhorned beetle, which was detected and eradicated from Chicago, would not have survived the polar vortex, either.
“But with a warmer climate in the future, all of these things could spread,” he said. “That would really be a disaster.”
In La Crosse, where the emerald ash borer was found in 2012, city workers have already begun tagging and removing ash trees. Crews started on the far North Side, and their work has taken them south past Jackson Street. The city has about 4,500 ash trees on its boulevards, and all will eventually have to be removed. Property owners may choose to treat their ash trees with borer-resistant insecticide, but the treatments are an annual commitment at least.
Most of the ash trees in the city were planted decades ago as replacements for elms lost to Dutch elm disease, making them large, mature trees with thick bark capable of insulation, said Cinthia Johnson, forestry coordinator for the city Parks and Recreation Department.
“It will be interesting to see how the weather affects them, but we’re really not going to know until the spring,” Johnson said.
But even as residents prepare to kiss their ashes goodbye, science may again provide an alternative. Environmental geneticists have successfully developed a hybrid by combining native ash trees with Chinese varieties, which are resistant to the emerald ash borer, Frelich said. But don’t go looking for the tree in local nurseries just yet.
“It takes 30 years of research and marketing to bring a tree like that to the public,” he said.