Guess who’s coming for a visit. Or, more precisely, what. Here’s a hint: It has six skinny legs, never knocks and kind of stinks.

As the fall air turns cold, swarms of brown marmorated stink bugs are ready to crawl from woods and fields — especially in the Mid-Atlantic region, including Maryland, Virginia, West Virginia and Delaware — in search of a warm spot to spend the winter, and any house will do.

So get ready. After last year’s steep decline, favorable conditions this year have apparently resulted in a population boom.

“The numbers have just been way up all summer long heading into August,” said Mike Raupp, a professor of entomology at the University of Maryland. “The data coming in says numbers are pretty high.”

The federal government reiterated that message. “There are certainly plenty out there,” said Tracy Leskey, a research entomologist for the U.S. Department of Agriculture. “In some locations, we’ve seen high trap counts.”

Brown marmorated stink bugs like to cluster together as they go dormant over the winter. (Katherine Frey/The Washington Post)

By “some locations,” Leskey means the Mid-Atlantic, where the aforementioned states make up the heart of the region. It also includes Pennsylvania, where invasive stink bugs native to China and the Koreas first emerged about 15 years ago, probably after crawling out of a cargo ship.

Leskey, Raupp and other scientists said it’s too early to tell how bad the home invasions will be. But Leskey is hoping some homeowners will help.

“We have a project called the Great Stink Bug Count, where
homeowners as far north as Ohio and south in Georgia are counting stink bugs,” she said.

By the end of October, scientists expect to have the raw numbers they will need to start compiling data. They plan to analyze the colors of homes, their sizes, location, elevation and surrounding vegetation to see what attracts the bugs.

They’re trying to figure out why some people end up with thousands of bugs and neighbors less than a mile away hardly get any.

Raupp knows that homeowners don’t care about any of that, so he has provided a how-to video on getting rid of stink bugs. If the recent past is any guide, more than a few will need it.

Step one, Raupp said, is to seal any cracks around the windows and doors with caulk. Get weather stripping, and patch even the tiniest sliver in the wall. Grab a can of foam spray to block holes around outdoor electrical outlets.

But don’t flush stink bugs down the toilet, Raupp said. That will only waste water and drive up your water bill. “That’s just not very green,” he said.

Step two: Make a bug trap out of a small plastic water bottle. Cut off the top, invert the nozzle into the bottle by sticking it upside down to create a funnel. Brush the bugs in and they will easily slide down, he said.

And step three: Fill a bucket with soapy water and toss the stink bugs in after nudging them into the bottle, sucking them up in a hand-vac or snatching them up in a napkin. “They’re just not great swimmers,” Raupp said.

Take good notes, because Asian stink bugs are apparently here to stay. They’ve been observed in 41 states — most recently in Hawaii — up from 33 last year, making big gains and staking a sustained presence in the United States.

But in their drive to stop them, scientists have made significant gains of their own, such as copying a pheromone emitted by feeding stink-bug males that attract females and nymphs meals. They are using their lab-created scent to lure and trap bugs, which is one of the reasons why they are confident of their population estimates.

The triangle-shaped traps, or little temples of doom, have been scattered across the country, luring the creatures into grooves. Instead of food, a pest strip and vapor puts them to sleep, said Christopher Bergh, a professor of entomology at Virginia Tech.

Like they do with other flying insects, scientists rigged a few captive stink bugs to a mill that records their stamina as they fly in a loop. Most of the bugs logged a couple of miles per day, but one mighty outlier went for 75 miles in 24 hours, which could explain why the bugs have spread so quickly.

In parts of Maryland, West Virginia and Delaware, scientists have been astonished at the high numbers of bugs found, piled six inches deep in some traps. But in southern Virginia, relatively few stink bugs were trapped, said Ames Herbert, an extension entomologist for Virginia Tech.

“We’re down here also surveying a large area. We haven’t seen as many as they have in the more mid-Atlantic area,” he said.

Scientists near Virginia’s southern border recently found a way to thwart the bug, Herbert said. They treated the edge of soybean fields with insecticide, rather than spraying the crops, and stink bugs stayed away.

“Last year and in 2011 we saw plenty of activity in the north central area, Orange, Culpeper, Madison and Spotsylvania counties,” Herbert said. “We kind of expected it might be a little worse . . . but we didn’t see them in new areas this year.”

Even in the bug-infested Mid-Atlantic, this year’s explosion won’t come close to rivaling the epic year of 2010. That’s when a stink-bug swarm seemed to appear out of nowhere, eating through farmers’ fields and causing $37 million in damage to the apple crop alone. Their annoying modus operandi has been closely watched every year since.