They're described as "aliens" and "invaders" intent on annihilating everything around them, but this is no Hollywood horror film.

The interlopers are worms, those squishy wrigglers we've all been taught to welcome to the woods and garden for the good work they do. You know, they aerate the soil for better drainage; their castings - the polite term for excrement - are a nutrient-rich fertilizer; and their slimy little selves are buffet grub for birds, snakes, and other creatures.

So, what, earthworms are bad guys now?

In reasonable quantities, no. If there are too many of them, yes.

"They're taking over," said Martha Moore, youth education coordinator at Tyler Arboretum in Media, where the advancing hordes - Asian Amynthas hilgendorfi and agrestis - have become teaching tools. Children delight in their nine-inch-long, iridescent bodies, which are way grosser than the boring, five-inch European worms other generations grew up with.

At this point, those five-inchers are vastly outnumbered by their longer, fatter, more aggressive counterparts, which have settled by the zillions into Canada and the northern tier of the United States, wreaking havoc with forest soils and upsetting complex ecosystems.

The highest earthworm concentrations in the Mid-Atlantic region are in urban areas such as Philadelphia, according to Anne Bower, associate biology professor at Philadelphia University, who has studied invasive earthworms at the 340-acre Schuylkill Center for Environmental Education in upper Roxborough.

Fall, before freezing temperatures become the norm, is a great time to see what the hubbub is about. Until they die from the cold, earthworms should be visible at the Schuylkill Center, the Wissahickon, at the John Heinz National Wildlife Refuge at Tinicum, or the more than 40 gardens and other horticultural destinations in Southeastern Pennsylvania, South Jersey, and northern Delaware.

Until relatively recently, when scientists began studying the phenomenon and raising the alarm, no one paid much attention. It's harder to ignore now.

"One time I was walking on a dirt trail at Jenkins and I stepped on something and kind of slipped. It was a mound of worms!" said Sharon Torello of Paoli, who volunteers at the arboretum in Devon.

Virtually all the earthworms in this part of the country hail from Europe or Asia, filling a void created thousands of years ago when native North American worms were wiped out by glaciers.

European settlers brought worms with them, and, starting in the 1700s, so did ships bearing exotic plants and carrying soil as ballast to American ports and cities from Philadelphia to the Great Lakes to Minnesota to California.

Fishermen, too, have abetted the problem by dumping unused bait worms at water's edge, where they can take to the woods and reproduce at will.

The invaders are destroying what's called the leaf litter or duff layer of the forest, a vital blanket of decomposed leaves and other organic material that composes nature's perfect fertilizer. This layer nurtures trees, young seedlings, and woodland creatures.

Without it, trees get stressed, making them more susceptible to disease and damage from temperature extremes.

In public gardens, home plots, and golf courses, too, worms are eating their way through leaves and mulch, while depositing piles of droppings that look like BB pellets.

Travis Beck, horticulture director at the 600-acre Mount Cuba Center in Hockessin, Del., describes this indelicate calling card: "You know the dribble castles you make at the beach in summer? Instead of sand dribbles, it's worm poop with a really black, dribbled look to it."

There's another aesthetic issue.

"As the worms move in and started feeding on mulch, overnight raccoons and other critters come and dig up the worms," Beck said. "We arrive in the morning and the path looks like strafing has run over it. It's been pockmarked with little holes all over."

Those nitrogen-rich castings - historically coveted for gardens - can be a problem, too. A surfeit of worms produces more nitrogen than native plants can use, resulting in nitrogen runoff, invasive plants moving in, more bacteria, and fewer beneficial fungi to help trees take up water and nutrients.

By gorging on the organic matter, the worms also expose bare patches on the forest floor, causing compaction, erosion, and root damage.

The destruction takes virtually no time at all.

According to Bower, leaves that drop this fall at the Schuylkill Center should remain in the duff layer for years. But "we expect by March the main layer will be gone," she said.

Created on land that was a working farm until the mid-1960s, the Schuylkill Center is particularly vulnerable to invasion by worms, which seem to gravitate to "disturbed" or urban areas.

"Our forest is a young forest," said Melissa Nase, the center's land stewardship manager, with many tulip poplar trees as opposed to old-forest hickories and oaks. Poplar leaves are easily digestible, compared to the fibrous hickory and oak leaves.

So what can be done?

During the Schuylkill Center research, conducted roughly from 2003 to 2009, Bower and her students tested 26 earthworm-control treatments over three years. They found that adding granulated sulfur to the soil, or a combination of sulfur and eight or nine inches of oak leaf litter, created a more acidic, worm-unfriendly environment, significantly lowering the earthworm population.

But is that practical? Most gardens and habitats have a range of plants with different soil requirements.

Many balk at the idea of synthetic pesticides because of the size of the area to be treated and fear of collateral damage. Also, more products now are geared toward killing a specific pest, and, so far, earthworms are not among them.

Peter Landschoot, turfgrass science professor at Pennsylvania State University, cites a new fertilizer called Early Bird, made from tea seed meal, that has shown promise in eliminating earthworms on North American golf courses.

Landschoot's colleague Sjoerd Duiker, an associate professor of soil management and applied soil physics, has another idea: Introduce a predator. "Perhaps you have to keep chickens," he said.

The Schuylkill Center is opting instead to restore the 10 acres Bower studied. Maybe by replacing the invasive Japanese stilt grass and garlic mustard with small native trees and shrubs that foster a healthy ecosystem, Nase said, "eventually, we'll get it to the point where we can outcompete the worms or at least make the problem more manageable.

"It's such an overwhelming problem," she said. "For now, we live with it."