While the recent cold spell, polar vortex, or whatever name you prefer for the frigid temperatures across the U.S. for the last week may have set record-low temperatures, it didn’t do enough to offset the spread of invasive insects thriving in warmer winters and feasting on drought-weakened trees.
In recent years, warmer winters have been contributing to the proliferation of certain insects that are harmful to tree populations, such as pine beetles and the Emerald ash bore. These and other tree-eating insects can spread rapidly under the right conditions and are already responsible for the deaths of millions of trees. Their impact has been enhanced by the mismanagement of forests over the last century, in which trees across the West were allowed to grow in unnaturally dense stands as all forms of fire were suppressed.
Upon finding the Emerald ash borer in the greater Denver metropolitan area in November, the Colorado Department of Agriculture issued an emergency quarantine to try and protect the estimated 1.45 million ash trees.
“The Emerald ash borer is a highly destructive pest to ash trees,” CDA’s Plant Inspection Division Director, Mitch Yergert, said in a press release. “In other states, it has caused significant economic impact to property owners and the nursery and landscaping industries. The quarantine is vital to limiting further infestation.”
Cold weather can act as a natural quarantine for these unwanted pests.
“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,” Dr. Robert Venette, a research biologist with the U.S. Forest Service in St. Paul, Minnesota, recently told Milwaukee Public Radio.
According to Dr. Venette’s research, “34 percent of Emerald ash borer larvae die when the temperature hits 10 below zero, 79 percent of the larvae die at 20 below zero and 98 percent perish when the temp is 30 below zero.”
Larvae mortality is affected by variables such as how long the larvae have had to develop, how insulated they are by bark and how long they are exposed to the elements. Even though in Minnesota temperatures reached deep into the negative 20s in many places, it’s unclear how many of the Emerald ash borer perished.
“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,” Lee Frelich, director of the University of Minnesota Center for Forest Ecology, told the LaCrosse Tribune.
“The European elm bark beetle, which transmits Dutch elm disease, is likely totally wiped out by the subzero temperatures,” Frelich said. “The Asian long-horned 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. That would really be a disaster.”
In Iowa, state forestry officials said the cold spell, in which lows ranged down to about 20 below zero, would probably only kill about 10 percent of the Emerald ash borer larvae.
When it comes to pine beetles, an especially harmful pest that has decimated millions of acres of forest in North America, the cold spell probably didn’t have much of a long-term impact.
“They’ve already changed their chemical physiology and increased the percentage of glycols in their systems,” Dave Leatherman, a retired Colorado State Forest Service entomologist, told the Denver Post about pine beetles at this time of year. “It’s the main component of anti-freeze we put in our cars.”
Even if temperatures had sustained -30 levels for nearly a week, which is what is needed to decimate pine beetle populations, in Colorado, the pine beetle has already run its course in much of the state.
“Aside from temperatures, the other thing that drives beetle populations is forest stand conditions,” Joe Duda of the Colorado State Forest Service told ClimateProgress. “Whether we get cold weather or not, pine beetle populations already subsided about five years ago having run their course on lodgepole pine.”
According to Duda the lengthy period of warmer and drier years since the turn of the century has not only meant that beetle populations haven’t been set back during freezes, but also that trees have been stressed, thus reducing their natural defense system of producing sap.
While pine beetles may be on the way out, Duda is worried about a new invader taking advantage of the dense forests stands and trees weakened by heat and drought.
“For the second year in a row, the number one beetle has been the spruce beetle,” Duda said. “With warmer and drier conditions we can expect that to continue.”