树，极寒天气的受益者

AUDIE CORNISH, HOST: The record-breaking temperatures of the polar vortex caused much of the U.S. to shut down earlier this week. With so many schools and offices closed and flights canceled, the hit to the economy was in the billions. But the deep freeze had at least one surprising upside, putting warm smiles on the faces of entomologists. That's because it may have been cold enough to kill some damaging species of insects, including the tree-killing emerald ash borer.

From Chicago, NPR's David Schaper reports.

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DAVID SCHAPER, BYLINE: Chopping into an ash tree with a hatchet in his frigid bare hands, Tom Tiddens(ph) peels back the bark, looking for emerald ash borer larva. Tiddens is an entomologist and supervisor of plant health care at the Chicago Botanic Garden. And in this wooded area just off a busy thoroughfare, emerald ash borers have been eating and destroying scores of native ash trees.

The wood under the bark shows the telltale signs: squiggly S-shaped trails where the larvae have been feasting, disrupting the flow of water and nutrients up the tree before they burrow into the bark.

TOM TIDDENS: There's one right there.

SCHAPER: It's a small, white, worm-like creature about a half-an-inch long.

TIDDENS: Yeah, there's one right here, and let's see if I can kind of pull him out for you.

SCHAPER: Tiddens wants to see how this tiny but devastating insect has been faring through this week's bitter cold snap that sent temperatures here to 16 degrees below zero. Many arborists are hoping that these tree-killing beetles are all freezing and dying off.

TIDDENS: It turns out that that's not really the case. This insect, like a lot of other insects, actually have a strategy for over-wintering.

SCHAPER: He says the process called super-cooling begins in the fall.

TIDDENS: They'll cease feeding. It'll stay under the bark so it's protected there. It will actually purge all the stomach contents of its gut because that could freeze. And they actually fold themselves in half when they do that.

SCHAPER: And sure enough, many of the larvae we find are folded up to try to stay alive. Tiddens and other experts say temperatures inside the bark need to get to about 13 degrees below zero for the troublesome bugs to really begin dying off, and that means air temperatures need to be even colder than that, according to research biologist Rob Vinet(ph).

ROB VINET: So around minus-20, there can be as much as 50 percent mortality, and as temperatures approach minus-30, we can see nearly 100 percent mortality.

SCHAPER: Vinet works for the U.S. Forest Service in Minnesota.

VINET: I'm probably one of the few people that really root for an extremely cold day because I really do think it helps with some of the major insect problems that we have.

SCHAPER: Vinet says it got cold enough in parts of Minnesota to possibly wipe out up to 80 percent of the emerald ash borer population and significantly slow the advance of the invasive insect there. The cold may reduce the population by 10 to 20 percent in parts of southern Wisconsin, northern Illinois, Michigan, Indiana and Ohio, but Vinet cautions that the cold is just a temporary setback for the emerald ash borer.

VINET: We're simply not having the cold weather that we've had in previous years. So we can't count on the cold as being the solution to the problem.

SCHAPER: Vinet and other experts say this week's record-breaking cold might slow the spread of other invasive species, too, including the gypsy moth and the woolly adelgid, which has infested and killed hundreds of thousands of hemlock trees in the Northeast. And the numbers of some kinds of ticks should be reduced by the bitter cold, too. Another plus: biologist Greg Mueller at the Chicago Botanic Garden says the bitter cold won't hurt many beneficial species of insects, including pollinators such as honeybees.

GREG MUELLER: They actually kind of cluster together and form a bee ball around the queen and then vibrate their wings, and that keeps the whole nest up at warm, in the 90s.

SCHAPER: So even while it's howling outside, the bees are warm in their hives. So go ahead, join the entomologists and hope for another even colder polar vortex to come settle over the country again this winter. David Schaper, NPR News, Chicago.

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ROBERT SIEGEL, HOST: This is NPR News.