The invaders came every fall, but would still manage to catch the Benzine family by surprise. The children shrieked as a great swarm of insects would abruptly rise in a black cloud and fall upon the family, crawling inside their clothing, biting them, and driving them inside for weeks. Tens of thousands of the bugs managed to squeeze inside the Benzine’s central Wisconsin home every year, flying around the house and lingering all winter until they finally died.

And that’s when things would get worse. The tiny dead beetles would accumulate in vast piles, their carcasses leaking a reeking red fluid. “I used to clean up six times a day, cleaning hundreds of them at a time,” said Gail Benzine, as she remembered the invasions that struck her home during the 1990s.

Meet Harmonia axyridis, a.k.a. the Asian Lady beetle, a pea-sized nuisance native to Asia. Its aliases also include harlequin ladybird and Halloween lady beetle, since they all invade homes in October in preparation for overwintering. Although the population has stabilized and the enormous swarms of the 1990s are rarer, Harmonia has settled into the ecosystem as an annual annoyance for hundreds of thousands of Midwestern homeowners, causing discomfort, property damage and even allergic reactions. There is no feasible plan to eliminate the beetle, so instead scientists have been putting it to work to control other agricultural pests.

“There’s pretty much no way we could get rid of them at this point,” said Robert Koch, an entomologist at the University of Minnesota. “They are established. They are here to stay.”

Yet how Harmonia’s population exploded remains a mystery. As early as 1916, the U.S. Department of Agriculture raised thousands of the insects in a lab and released them in Georgia to control aphids that were damaging pecan crops. The USDA similarly released them in Louisiana and Mississippi.

Scientists don’t know if one of the released populations somehow got out of hand, or if a few insects carried into the States in shipping containers from Asia pioneered a separate invasion into the Midwest that expanded across the continent — or maybe both. A genetic analysis of the population in the United States points to two separate initial populations that subsequently exploded, which could support either theory.

Whatever its roots, by the mid-1990s the beetle had become a serious problem for U.S. homeowners. Harmonia had gone rogue after many decades in small, isolated pest control populations.

“My phone rang so constantly I couldn’t put it down long enough to recharge the battery,” said Phillip Pellitteri, an emeritus entomologist at the University of Wisconsin’s insect diagnostic lab. “I remember walking out and looking up as the beetles were swarming. It was mindboggling.”

At the invasion’s peak in the 1990s, the lady beetles would pile up to six inches deep in farm silos, barns and homes. Since then, the infestations have eased somewhat, a trend Pellitteri says is common in invasive insects as native species adapt to snack on the invaders. But Harmonia remains a nuisance across the Upper Midwest — especially when the weather turns chilly.

The insects have a behavioral quirk with an evolutionary origin suited to its native Asian habitat. In preparation for the winter, Harmonia beetles are attracted to any prominent shape on the horizon and fly toward it for shelter. In Asia, these would be mountains and cliff faces on which the insect could nestle itself into a crevice for protection against winter storms. In the Midwest, it’s barns and homes on hills.

Preparing for the winter also means gorging on fruit juices to build up nutrition stores, especially after the insect runs out of aphids to snack on. The beetle is particularly fond of grapes, and is nearly impossible to remove from bunches of the fruit. In fact, the chemicals in one lady beetle can spoil an entire batch of wine, which is why vintners in Harmonia-infested regions across the U.S. must use extra pesticides to protect their vineyards.

After the first fall freeze, beetles begin piling up against Midwest homes. Thousands can sneak in at a time through holes or cracks and will spend the winter scuttling around in the walls. In Polonia, Wisconsin, homeowner Todd Cook peeled off the old vinyl siding on his home to replace it, but found instead the corpses of hundreds of thousands of Asian beetles packed from the foundation to the eaves.

The ones that make it inside are even worse. They pester homeowners throughout the year, scavenging for food, biting people and otherwise getting too close for comfort.

When frightened or attacked, Harmonia bleeds from its leg joints as a defense mechanism, ejecting a viscous red-orange fluid that stains anything it touches. Attempts to move or kill the beetle leave behind permanent splotches on clothing, floors and drywall.

“I can’t hang laundry, because it comes out dirtier than it went in,” said Pat Onan who lives in Nelsonville, Wisconsin. The fluid doesn’t just stain, it reeks. Cook described the stench as “burnt rubber,” while others described it as “musty” and “acrid,” lingering for days after contact.

Harmonia can even cause severe allergic reactions. Benzine has ended up in the emergency room twice, just from touching a place where Asian lady beetles had crawled that she hadn’t thoroughly cleansed. Allergists verified Harmonia as the culprit. “Even the smell will trigger my eyes to water,” she said.

Infested homeowners don’t have any recourse beyond hiring an exterminator to try to beetle-proof their homes. Darrell Werner, who runs a Green Bay-based pest extermination company, offers a twice-yearly chemical spray at $200 a pop, or a diligent examination of the building to thoroughly seal all entry points. It’s almost permanent, but costs up to $5,000.

“People have accepted the fact they are going to have to put up with these things,” said Minnesota’s Koch.

Even as homeowners complain about the invasion, however, farmers have reason to celebrate. As it turns out, Harmonia is really, really good at what it was initially brought here to do: eat aphids.

“We intentionally introduced this effective predator, and it’s doing what we want it to do,” said Kaitlin Whitney, who studies natural pest control at the University of Wisconsin. Harmonia munches on all sorts of aphids, the little insects that suck nutrients out of plants and leave them to decay, and isn’t picky about its meals. That means it can rotate around a farmer’s plots over the course of a season, controlling the pests of different crops.

Yet Harmonia is especially talented at devouring soy aphids. When soy was brought to the U.S. from Asia, the soy aphid hitched a ride, with thousands of bugs sometimes infesting one plant. Soy is the second-most planted crop in the US, driving a $40 billion industry, but the soy aphid can reduce crop yields by up to 40 percent. Harmonia’s natural prey is the soy aphid, and having the lady beetle in and around cropland prevents damage and reduces the need for expensive and environmentally taxing pesticides.

Lady beetles native to the U.S., are commonly known as ladybugs and vary in color from greenish to cream and take on a range of shapes. They also prey on aphids, but are usually smaller and pickier than Harmonia. In fact, Harmonia not only outcompetes other lady beetles, it even eats them. There is some evidence that native lady beetle numbers are declining due to Harmonia’s invasion, Whitney said.

That’s why scientists like Whitney are working with farmers to preserve native lady beetle species while maximizing Harmonia’s pest control abilities. They have found that preserving trees or original grassland between crop fields, as well as reducing chemical pesticides like neonicotinoids, can protect native beetles.

While it might be tempting to let the native beetles die since Harmonia is better at controlling pests, Whitney thinks that’s a bad idea. “At this point it would be dangerous to rely on Harmonia for all of our pest control,” she said. If Harmonia took over, eliminating the native species, and a disease swept through the population, there would be no lady beetles at all to control the aphids.

In the meantime, Harmonia marches on, and is in the process of colonizing parts of Europe and South America, where a different set of environmental and economic complications await — and where homeowners are likely to face the same stinking mess.

*Correction, July 30, 2015:

A previous version of this story incorrectly stated that the above photo was of Harmonia axyridis, when in fact it’s of a different, but similar ladybug species.