If you were hoping that this winter's freezing temperatures had sent the hated stink bugs packing, you are out of luck. Turns out, they have the good sense to come in from the cold.
"I would be nice to think that winter killed them," said Stanton Gill of the University of Maryland Extension, where he specializes in integrated pest management. "But I doubt it. They are good at finding places to hunker down."
While one researcher recently found that nearly 98 percent of brown marmorated stink bugs died in the cold outside his lab, other experts expect to find no more than a 50 percent death rate over the winter. That is more than the normal 25 percent rate, but it won't rid the Mid-Atlantic of these annoying and destructive pests. They most likely snoozed through winter in warm spaces — like inside your house.
Other insects may not have been as fortunate. That includes bad guys like the insect that attacks Christmas trees and good guys like the Asian lady bug beetle, which eats aphids.
Gill began getting calls last month from people complaining that the brown marmorated stink bug, an invasive species that damages crops and drives homeowners crazy, was emerging from its hiding places.
"When it got really cold, people were turning up their heat and warming the inner core of their houses," said Gill. The stink bugs were feeling the chill up there in the attic or in the rafters of the garage and migrated toward the heat, he said.
But when Virginia Tech entomologist and Baltimore native Thomas Kuhar noticed that his stink bug population — 2,600 of them in 5-gallon buckets outside his lab — had been devastated by the cold, the rumor started that stink bugs might have met their match in the polar vortex. Between 95 and 98 percent died when temperatures in Blacksburg dropped to 5 below. Their tissues had frozen.
But stink bugs don't live in plastic buckets, exposed to the harshest temperatures. They hide under bark, in dead trees, in the gaps in siding. And in your attic.
"It was an anecdote," said Kuhar. "I simply noted that we usually have [an over-winter death rate] of about 25 percent, but this year was much higher."
"It was a single observation," agreed University of Maryland entomologist Michael Raupp. "And winters like this don't happen enough for us to know. We might have to wait 50 or 100 years to replicate these conditions."
There is some thought that the kill rate might have reached 50 percent this winter, but scientists won't know until the stink bug population emerges in the spring and becomes active. Even so, was it because of the cold? Or were other conditions responsible? Raupp asked.
"If you had to pick an insect that could survive a winter like this, it would be the stink bug," said Kuhar. "It has the behavior of seeking shelter."
The good news is, the winter cold did kill off many of the larvae of the emerald ash borer, which has been wiping out trees in Michigan and Minnesota — and threatening trees in several counties throughout Maryland. The U.S. Forest Service estimates that 80 percent of them died when the temperatures fell to minus 20 degrees. The bugs collected near the Twin Cities were found to be frozen stiff.
But Gill said temperatures did not get nearly cold enough in Maryland to destroy the invaders. "The trunks of trees provided some insulation so it is not likely our temperatures in Maryland reached low enough to significantly reduce [the emerald ash borer]," he said. "We need it to get colder."
Also, the hemlock woolly adelgis, which preys on Christmas trees in the northern Appalachians, appears to have suffered a 95 percent death rate in the cold. Also affected, according to a report written by Gill, was the Asian lady bird beetle – a "good guy" because it eats aphids, one of the most destructive insects in fields and gardens.
Raupp also reported that there is evidence that the mimosa webworm and the gypsy moth may have suffered higher kill rates in the cold.
The stink bug, which arrived from Asia and has no natural enemies in this country, was first noticed by homeowners near Allentown, Pa., in the mid-1990s. Since then, its numbers have exploded all over the Mid-Atlantic and it is found in 40 states, emerging now in the Pacific Northwest.
It is damaging fruit crops like apples and peaches and tomatoes, using its beak to pierce the soft skin and drink the juicy fluids inside. The holes that it leaves allow disease and other insects to destroy the fruit. But Raupp said scientists have also recorded damage on more than 150 varieties of trees and shrubs used in landscape plantings.
"The nuisance potential of [the stink bug] is almost without equal," Raupp writes in his blog, "Bug of the Week." In the fall, after damaging crops, it sinks its beak into tree bark and damages trees. Then it moves into houses and barns for the winter.
One homeowner in Western Maryland, Raupp reports, captured more than 26,000 between January and June of 2011 as they emerged from their winter slumber.
Meanwhile, Tracy Leskey, an entomologist with the U.S. Department of Agriculture, who keeps track of stink bug damage from her post in Kearneysville, W.Va., said she is seeing about a 50 percent kill rate from winter cold. There are places where it has been higher, but she said she can't be sure the cold is the reason.
"We are essentially 'CSI' for stink bugs," she said. "We are trying to find out what drives over-wintering mortality and what can we do to exploit it?