WASHINGTON — This winter has been a real killer — for bugs.
The deep freeze, with arctic blasts from the polar vortex, has put invasive insects on ice in dozens of states. That includes the emerald ash borer, a pretty bug that does ugly things to ecosystems it invades.
Up to 80 percent of the ash borers died when January temperatures dipped below minus 20 degrees in St. Paul, according to an estimate by US Forest Service biologists, who have been conducting studies on the impact of cold weather on the bugs for the past three years.
Their estimates were affirmed when state researchers found that nearly 70 percent of ash borers collected from trees in the Twin Cities area last month were frozen stiff .
Across the country, other destructive pests are dropping dead, including the hemlock woolly adelgid, which preys on Christmas trees in the Appalachian Mountains; the kernel-munching corn earworm, found in nearly every state; the citrus-destroying cottony cushion scale that migrated to Maryland from Florida; and the gypsy moth, which chomps on trees and is spreading from the Northeast to the Midwest.
The bugs found their way to the United States from all over the world and thrived in the relatively warm winters of recent years. At least two of the pests mounted great migrations from the Deep South to Virginia and Maryland.
For now, at least, the freeze has stopped them in their tracks. Researchers in the Appalachians of West Virginia and Maryland found hemlock adelgids whose straw-like mouths were stuck to the pine needles from which they suck nectar.
Based on surveillance, researchers believe more than 95 percent of hemlock adelgids were killed in the Northern Appalachians.
But invasive bugs are a breed apart. Built to last, they almost never experience extinction.
Winter’s blow to the pests is more like a reprieve, said Mike Raupp, a professor of entomology at the University of Maryland, ‘‘a little correction’’ that thinned their ranks and probably will slow them down when warm weather returns.