Last week's subzero temperatures were bad for the emerald ash borer.

The invasive pest is a major threat to ash trees, a common tree species in parks and neighborhoods throughout the Twin Cities metro area. And while people across the Midwest muddled through a polar vortex and the Twin Cities reached 28 degrees below zero, those in charge of battling the ash borer welcomed the possibility of relief.

A study from the Minnesota and U.S. departments of agriculture found that temperatures of 20-degrees below zero have the potential to kill nearly 80 percent of ash borer larvae. The mortality rate reaches 98 percent at 30-degrees below zero.

"The longer the better and the colder the better is what they say," said Brian Rumpca, assistant public works supervisor in St. Paul Park.

But will the short cold snap have long-term effects on the pests?

"It really just slows down (the spreading) process, it doesn't eradicate it," said Mike Adams, assistant parks and recreation director in Woodbury.

The most recent period of extreme cold just wasn't long enough to do lasting harm to local ash borer populations, he said.

"It does set back the spread of EAB, but that's only a temporary delay of that pest unfortunately," Adams continued. "We would need weeks of those kinds of kinds of temperatures to make a dramatic impact on those populations."

Cottage Grove city forester Quinn Palar agreed that the well-below-zero temperatures may have killed off some ash borer larvae, but what they really did was temporarily take some of the pressure off.

"It's not something like, ope, they all died," Palar said. "I think it probably had some effect on the EAB population and it's a positive one for those that have to manage it, but I think it'd be hard to measure the extent and basically it would buy us some more time."

One stretch of extreme cold hasn't changed the cities' outlooks when it comes to battling the borer. They continue with the eradication process, which includes tree removal and replacement or, in some cases, treatment.

"It's not going to get any better," Rumpca said. "It's just going to get worse as time goes on."

Palar echoed Rumpca's statement.

"It give us just a little more time before those population get so large that they're really affecting the trees," Palar said.