Fall is in full swing across North America. Leaves are dropping, temperatures are dipping, daylight is dwindling — and for a growing number of Americans, stink bugs are swarming.

The U.S. has several native stink bugs, but they're increasingly overshadowed by a larger relative from Asia: the brown marmorated stink bug. More than a decade after its debut in southern Pennsylvania, the species now breeds in 15 states and exists in at least 25 more.

"This is one of the worst invasive pests we've ever had in California," says Chuck Ingels of the Cooperative Extension office in Sacramento County, where a major outbreak was recently discovered. "These bugs aggregate in such numbers that there are reports of people using manure shovels and five-gallon buckets to dispose of them. The strong, unpleasant odor the insects emit when disturbed makes cleanup still more daunting."

The swarms and stench are only part of the problem. Brown marmorated stink bugs are a crop pest in Asia, and they're especially destructive in North America due to a lack of natural predators. They spend summer ruining commercial crops like tree fruit, corn and soybeans, then flock into people's homes by mid-October to ride out the winter.

Their most destructive year so far was 2010, when they caused $37 million in damage to Mid-Atlantic apple farms alone. But 2013 has seen a similar stink-bug surge, following a one-year population boom of 60 percent that was reported in March. While the invasion is still worst in Mid-Atlantic states, it has also spread west; scientists are expecting "buckets full" of the stink bugs in Wisconsin, for example, and Sacramento is now home to what may be the first reproducing population in California outside Los Angeles County.

brown marmorated stink bug

The back of an adult brown marmorated stink bug. (Photo: Sam Droege/USGS)

Not only do brown marmorated stink bugs have bigger bodies than their native American cousins, but they also tend to congregate in much bigger crowds, according to a fact sheet from Penn State University's entomology department. This can be alarming when they're discovered inside a house, often prompting homeowners to take abrupt lethal action. Scaring or squashing the insects triggers their namesake defense mechanism, however, and a disturbed swarm can reportedly stink up a house for days at a time.

The good news is stink bugs don't reproduce indoors, and they don't cause any direct harm to people. The skunky smell can be unsettling, but beauty is ultimately in the nose of the beholder. "Some people find the smell bothersome," writes Penn State entomologist John Tooker, "whereas others claim it is somewhat pleasant, smelling like cilantro."

Their appetite is a different story, though, especially since brown marmorated stink bugs eat so many different types of crops and can often bypass conventional pesticides.

"Chewing insects are easier to kill because they can consume large amounts of plant tissue that has been dosed with insecticide," Tooker explains. "Stink bugs insert their mouthparts into fruits or plant stems and bypass most of the insecticide residue on the plant surface. Thus, residual activity of insecticides against stink bugs tends to be weak, and adult bug populations may reinvade fields following treatment."

brown marmorated stink bug nymph

A brown marmorated stink bug nymph. (Photo: photochem_PA/Flickr)

The U.S. Department of Agriculture has deemed brown marmorated stink bugs its top "invasive insect of interest," and is now mulling whether to introduce a parasitic wasp called Trissolcus halyomorphae, which naturally preys on the bugs in Asia. The wasp doesn't sting humans, but it does lay its eggs inside stink-bug eggs, letting its offspring devour their hosts once they hatch. Still, since the wasp would also become established, scientists are conducting lab trials to make sure it won't decimate native species.

The USDA also recently organized an online census called the "Great Stink Bug Count," hoping to capitalize on the popularity and efficiency of citizen science to monitor the stink bug's spread. But according to Consumer Reports, that effort — like many scientific endeavors across the country — is now sidelined by the federal government shutdown.

[Update, Oct. 15: The Great Stink Bug Count is back on, the Washington Post reports, thanks to university researchers who are stepping in until the shutdown ends.]

For more information about brown marmorated stink bugs, including tips for identifying them, check out this video (and others) from the USDA Specialty Crop Research Initiative:

Related invasive species stories on MNN:

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Invasive stink bugs swarm across the U.S.
Brown marmorated stink bugs are wreaking havoc — and just reeking — as they spread throughout the country.