Just when you thought it was safe to put away the bug spray and open the windows, a new home invader has appeared to infest your autumn nest: the overwintering pest.

The culprits: According to the experts at Terminix, the two most common perpetrators are the stinkbug and the box-elder bug.

More than 200 species of stinkbugs live in the United States and Canada. But the brown marmorated stinkbug (Halyomorpha halys), a nonnative species, was first discovered in this region in Allentown in the late 1990s. According to a fact sheet prepared by Steve Jacobs, a senior associate at Penn State's Extension Service, the bug belongs to the insect family Pentatomidae and is known as an agricultural pest in its region of origin - China, Japan, Korea and Taiwan. Its introduction here was, the entomologists say, "accidental."

So far, it hasn't bothered agriculture here, but when this variety of stinkbug feeds on fruit trees in Asia, it leaves a characteristic distortion referred to as "cat facing" that renders the fruit unmarketable.

According to experts at United Exterminating in Cherry Hill, the box-elder bug (Boisea trivittatis) is part of the order Hemiptera, which means "half-winged." Most Hemiptera are good bugs - that is, they eat things that would otherwise damage trees, shrubs and other plants. Box-elder bugs "are sort of in the middle" - they do little damage to the trees they attack but can become a nuisance when they come indoors.

Modus operandi: With cooler temperatures, these pests beat a path to your door, seeking warmth and shelter till spring. They head to the walls of your house with western and southern exposures, always the sunniest spots, but especially after the autumnal equinox.

Inside job: Basking in the warmth on your house's exterior isn't enough. These insects also find small voids in the wall, where they can ride out the winter. And once inside, they proliferate, wandering at will.

Mob behavior: On warm winter days, these pests also may awaken from a hibernation-like state, prompting a migration indoors - often in large numbers.

Easy pickings: The cleanliness of your house - or the lack of it - has no bearing on whether these bugs will invade. Either way, they'll infiltrate structures with exterior cracks or openings. Once indoors, the box-elder bug can stain countertops, walls, floors and other surfaces with its excretions. The stinkbug can spray a fluid with an unpleasant odor several inches - hence the name.

Repeat offenders: If the conditions that originally attract, and then support, these overwintering pests aren't addressed, it is likely that they will be a problem year after year.

Corrective measures: Preventive steps work best. According to Penn State's Jacobs, exterior applications of insecticides may offer some relief. A synthetic pyrethroid should be applied by a licensed expert before the bugs appear in the fall. Because sunlight breaks down insecticides, the effectiveness may not last more than several days or a week.

Once the pests are within a wall, there are few things that can be done to eliminate them. So before they get in, Terminix suggests that you seal all exterior cracks, especially on the southern and western walls of your house.

Make sure windows and doors are tight, and install insect screening on foundation and attic vents. If these overwintering menaces appear indoors, try to trap them with a vacuum cleaner. Discard the bag once you've completed the job.