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 Fahrenheit 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, straw-like mouths were stuck to the pine needles from which they suck nectar.
“If you poke them with a stick, they’ll normally move their little microscopic legs,” said Patrick Tobin, a research entomologist for the Forest Service in Morgantown, W.Va. But not this winter.
Based on surveillance, researchers believe more than 95 percent of hemlock adelgids were killed in the northern Appalachians and at 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. Three generations or more will spring to life between March and October.
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.
“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 if 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, where they have feasted on farm crops for years.
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.
The conclusion? “There should be significant mortality of [stink bugs] and many other overwinter insects this year,” the professor, Thomas Kuhar, told The Washington Post Capital Weather Gang last month.
But Raupp, who called Kuhar “a top-notch researcher,” is more guarded. Blacksburg had a few days at 4 degrees below zero, but Maryland did not.
A research entomologist for the Agriculture Department also has low expectations. When researchers for the USDA 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,” said the entomologist, Tracey Leskey.
Stink bugs are from China, where temperatures often plummet. Even if they did die in bunches, they enter “winter with an enormous population,” so plenty of survivors rush out in spring to multiply.
A large group of stink bugs cozied up in the warm homes of residents who couldn’t seal enough cracks to prevent them from slipping through.
“Every day I’ve had a stink bug wandering across my desk,” Raupp said. “They’re doing fine in my house.”
It might seem that frequent snowfall would help kill the bugs, entomologists said, but instead it insulates and protects them.
As if stink bugs weren’t bad enough, another invasive bug from Asia has made its way to Maryland. Detected in Georgia in 2009, the soybean-loving kudzu bug has since decamped toward the north.
This winter’s polar shock might be that insect’s Waterloo. University of Georgia entomologist Wayne Gardner said kudzu bugs are slowed by just a layer of frost. Maryland’s temperatures dipped to 5 degrees.
The corn earworm that devastates that crop prefers tropical climates, and typically heads south when temperatures cool. The vortex that hit 49 states with extreme cold and snow offered few safe havens. And gypsy moths were found frozen and flat on their backs near trees they infect.
But the cruel winter apparently had little effect on ticks. In New Hampshire, they continue to weaken adult moose and kill calves. Tick populations are booming because cold weather now arrives in mid-October, too late to kill them in the brush where they wait to hitch a ride on moose until May.
“We’ve had a higher tick load on moose this fall than we’ve ever seen, except for one year previously,” said Kristine Rines, a wildlife biologist and moose project leader for New Hampshire’s Fish and Game Department.
To disrupt the tick reproduction cycle, winter would have to arrive “at its regularly scheduled time” in New Hampshire, around late September, Rines said. “We’ve already seen some . . . calves dying.”