Polar Blast May Help At Least Slow Spread Of Emerald Ash Borer, Other Invasives

It’s been so cold in the upper Midwest this week that even a few minutes outside invites frostbite. But there is least one upside to the polar vortex deep freeze: The extreme cold is likely wiping out millions of invasive tree-killing insects across a broad swath of territory from South Dakota to Minnesota, offering hope for at least a temporary reprieve from their incessant spread into a steadily warming north.

A swelling army of invasive insects—including the gypsy moth, introduced to the U.S. from Asia in the late 1800s, and the emerald ash borer, which arrived in Michigan in 2009 (most likely on shipping palettes from China)—have killed millions of trees across the northern states. They continue to colonize new territory, aided by warming trends that are rendering more areas hospitable to them. The consequences for forest ecosystems and cities alike have been swift and severe. The emerald ash borer has already wiped out 100 percent of ash trees in parts of Ohio and Indiana, says Robert Venette, a research biologist with the U.S. Forest Service’s (USFS) Northern Research Station. As infestations spread northward, the one-of-a-kind black ash swamps of northern Minnesota and Wisconsin could become treeless marshlands—leaving nearby areas more prone to flooding. Gypsy moths have defoliated millions of oak, birch, cottonwood and other tree species, causing millions of dollars’ worth of damage. The hemlock woolly adelgid, a sap-sucking native of eastern Asia that has decimated hemlock trees throughout the U.S. Northeast, is now encroaching into Midwest forests. “It’s a pretty big impact,” says Lee Frelich, director of the Center for Forest Ecology at the University of Minnesota, referring to all the region’s invading insects. “Hundreds of millions of trees have been killed.”

Although these insects are hardy enough to survive an involuntary trip across the world and thrive in unfamiliar territory, extreme cold is often their Achilles’ heel. The hemlock woolly adelgid cannot withstand temperatures below –4 degrees Fahrenheit, a 2008 USFS study found. Other invasive insects evolved in cold environments in their home countries and are equipped with impressive wintertime defenses—40 percent of the emerald ash borer’s body comprises an antifreeze-like substance, for example. And its larvae can tuck themselves under bark, where it can be as much as 5 degrees F warmer than the air. But even these cold-weather superstars are no match for the kind of temperatures now seizing much of the northern U.S.

Researchers will not know exactly how much damage this cold snap has inflicted on invasive insect populations until they get out and examine them in the spring. But a survey conducted after the 2014 polar vortex hints at what they might uncover. That winter temperatures dropped to –23 degrees F in Minneapolis and Saint Paul. When Venette peeked below the bark of ash trees in the Twin Cities, in most places he found 60 to 70 percent of emerald ash borer larvae had perished. Because this year’s polar vortex has brought even colder temperatures, the die-off is likely to be considerably larger: up to 80 percent could be killed in some areas, say experts including Andrea Diss-Torrance, the invasive forest pests coordinator at the Wisconsin Department of Natural Resources. Venette has subjected ash borers to various temperatures in the lab and found the colder it gets, the bigger the die-off. Only 34 percent of ash borers succumbed at –10 degrees F—but at –30, the figure soared to 98 percent. That is above the low much of Minnesota hit Tuesday night (in some areas it was even colder), and parts of Wisconsin endured on Wednesday; meteorologists reported it was the coldest the region had been in at least 20 years.

EDIT

https://www.scientificamerican.com/article/polar-vortex-could-knock-back-invasive-tree-killers-mdash-for-a-while/