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Extreme cold may be pest control

This winter is a real killer.

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, Minn., according to an estimate by U.S. 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 infected trees in the Twin Cities area last month were frozen stiff – a good thing for ash trees that adorn communities and provide smooth, durable wood used for flooring, bowling alleys, church pews, baseball bats and electric guitars.

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 80 species of 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 little, strawlike 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 and at that least 70 percent died in their southernmost range, Georgia.

At first blush, this appears to be great news, Tobin said. Important trees, including ash, birch and oak, and such vital crops as soybeans, corn and oranges, will probably get a break from millions of gnawing mouths.

But invasive bugs are a breed apart. Built to last, they almost never experience extinction.

Female adelgids and cottony cushion scales, for example, are asexual creatures that produce nymphs without copulation.

As for emerald ash borers, that Minnesota deep freeze affected only a limited number. Chicago also has ash borers, but temperatures there fell only to 17 degrees below zero and likely didn’t faze the insects. Minus 20 is the point at which they start to die.

The lowest observed temperature this winter in Fort Wayne was minus 16, according to the National Weather Service.

“This problem is not going away,” said Rob Venette, a Forest Service research biologist who studied the ash borer.

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.

The cold weather provided a stage for a grand experiment that will help researchers determine whether significant numbers of pests can be killed off, making the problems they create more manageable, Raupp said. A decade of study is needed for any definitive conclusions.

Raupp studies the crop-eating brown marmorated stink bug. He said he’d like to be “cautiously optimistic” that winter wiped out huge numbers of them in the mid-Atlantic states.

An experiment conducted by an entomology professor at Virginia Tech gave him hope. Stink bugs placed in foam-insulated buckets had a 95 percent death rate when temperatures hovering around zero persisted for days.

The bucket simulated overwintering hideouts in Blacksburg used by stink bugs to protect themselves from the cold.

But Raupp is more guarded. Blacksburg had a few days at 4 degrees below zero, but Maryland did not.

Agriculture Department research entomologist Tracey Leskey also has low expectations. When USDA researchers visited outdoor sites in Maryland where stink bugs spend the winter, they found the same mortality rate – about 50 percent – as in earlier winters.

“Unfortunately, they’re doing just fine,” Leskey said.

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