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Thanks, Polar Vortex, for Exterminating Some Tree-Killing Bugs in Wisconsin

R Venette

Arctic weather conditions have been hard on us humans in recent days, but there might be positive impacts when it comes to curbing unwelcome bugs.

Credit R Venette
USDA Forest Service forester Paul Castillo cuts down an ash.

Dr. Rob Venette studies the cold-weather tolerance of invasive critter, specializing in the Emerald ash borer (EAB), at the University of Minnesota. The beetle has been taking a toll on ash trees across the region. Venette says bugs generally succumb at higher rates in Minnesota, but Wisconsin’s temperature plunge could take a toll on the EAB.

“When Emerald ash borer first showed up in Minnesota in 2009, we immediately started to ask these questions about what winter temperatures might do to this insect, and we’ve been following it ever since then,” Venette says.

Part of his research involves storing EAB-infested logs in multiple outdoor locations in different parts of the state state over the winter.

“They have different exposures to the cold. And then we’ll bring them back in the spring, peel them and look at the larvae. It’s pretty clear that they were damaged during the winter,” Venette says.

Credit Susan Bence
Emerald ash borer larva

Scientists have learned that when the Emerald ash borer, in its larval stage, is subjected to extreme cold, starting at 10 degrees, larvae are affected.

  • 34% of EAB larvae die when the temperature hits 10 below zero.
  • 79% of the larvae die at 20 below zero.
  • 98% perish when the temp is 30 below zero.

So, in Milwaukee, where temperatures over the last few days reached as low as 14 degrees below zero, which would kill about a third of EAB larvae.
In Waukesha County, temperatures dipped to 17 degrees below zero - almost enough to kill off three-quarters of larvae.

“What our research is showing so far is that it just has to get to that low temperature for a few seconds and that should be enough to kill Emerald ash borer,” Venette.

The entomologist has also documented below zero temperature impacts on other pesky bugs.

“These are pest insects that are also a problem in Wisconsin; so when we talk about things like codling moths that can periodically be pest on apples, those only need to reach minus 10; for Gypsy moths which are an issue in the state, those begin to die as temperatures get around minus seventeen. So there are other insects that are being set back when we have these really cold temperatures,” Venette says.

He and his students came up with a Tough Buggers fact sheet of pest insects affected by cold weather.

Susan is WUWM's environmental reporter.
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