9 of the Worst Invasive Species in the U.S.

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Spotted lantern flies seen in Pennsylvania.

Humans are no strangers to changing the world around them, often to the detriment of plants and animals around them.

This can happen when we physically alter the landscape by mowing down forests to make room for buildings and parking lots. (Or creating fabric, for that matter.) But it can also happen when we purposefully—or unintentionally—introduce plant or animal species into a new area of the world, where they can upend the delicate balance of the ecosystems they invade.

Invasive species can wipe out native ones, destroy crops, and generally make a mess of things. The U.S. alone sees more than $120 billion in damage done by non-native intruders every year. Here are some of the most notorious invasive species that have made the U.S. their home or gained a toehold where they don’t belong thanks to people.

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Privets

Kudzu may be one of the most notorious invasive plants around, especially in the southern U.S., but its bark may actually be far worse than its bite. Privets are a lesser-known invasive plant that’s likely causing more harm in the South.

The shrubs belong to the genus Ligustrum. These innocuous-looking flowering plants were originally brought over as ornamental plants from Asia. But because the U.S. has so few native species of shrubs, privets were quickly able to establish themselves in the wild. Their explosive growth shrinks the population of other plants and also impacts native pollinators like butterflies and other insects. Perhaps the most established privet species, the Chinese privet (Ligustrum sinense), is described by the Department of Agriculture as “one of the worst invasive plants in the South.” If that’s not bad enough, the plants thrive with more carbon dioxide. With the way things are going, it’s likely to make privets even more of a nuisance in the future.

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Zebra Mussels

Zebra mussels (Dreissena polymorpha), named for the distinctive black stripes on their shells, were first discovered in the Great Lakes during the late 1980s, likely having arrived from the Caspian Sea region of Asia in the ballast water released by large ships. Scientists have desperately been trying to contain them ever since. These mussels can rapidly crowd out and devastate local populations of other mussel species once they reach a new source of freshwater. And they can grow so plentiful that they clog up intake pipes in water treatment and power plants.

So far, they’ve been limited to the eastern half of the U.S. But last year, wildlife officials made the alarming discovery that these mussels had managed to contaminate moss balls commonly sold and used in home aquariums across 21 states. The discovery prompted multiple agencies and the pet supply industry to urge customers who find these mussels in their moss balls to report their sightings and to safely dispose of them.

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Asian Longhorned Ticks

An underside view of an adult female Haemaphysalis longicornis tick. — An underside view of an adult female *Haemaphysalis longicornis* tick.

Ticks native to the U.S. are already a major public health menace, due to the many germs they can carry like those responsible for Lyme disease. So it was hardly good news when researchers in New Jersey announced that they had discovered a new bad tick in our backyard in 2017, the Asian longhorned tick (Haemaphysalis longicornis). The tick may have arrived on the backs of pets or other animals that traveled to the U.S. Since then, the tick has been spotted in 17 states, including New York, Georgia, and Tennessee.

The Asian longhorned tick is a threat to livestock, since it can quickly reproduce asexually and swarm the animals they feed off of in search of blood. But its level of harm to Americans is less clear for now. In its native areas, the ticks can cause and pass on germs that cause human illness, but early research has suggested that they won’t be a major cause of Lyme disease, and that they may not have that high a hankering for human skin. Still, their range is projected to eventually extend across half the U.S., and there’s other evidence indicating that they could spread at least some tickborne illnesses like Rocky Mountain spotted fever.

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Spotted Lanternflies

People are naturally frightened or disgusted by creepy crawlies, and with that emotional impulse often comes the desire to squish them dead. It’s rare that experts and wildlife officials wholeheartedly encourage that desire, but that’s exactly what they’re encouraging people to do with the spotted lanternfly (Lycorma delicatula).

Believed to have arrived in Pennsylvania around 2014 from their native home in parts of southern China, Taiwan, and Vietnam, the striking-looking insect has since spread throughout the Northeast. They’re not a danger to humans, but they can ravage fruit-bearing trees and other agricultural crops while also leaving behind a sticky residue called honeydew that further hampers plant growth. Their eggs can also latch onto surfaces, including our shoes and clothing, very easily, allowing them to spread further.

The situation has gotten so bad that last summer, officials in New York and Pennsylvania explicitly told residents to stamp out any spotted lanternflies they see on sight, and plenty of people did just that. More commonly, though, they’re controlled through chemical pesticides.

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Feral Hogs

Feral swine damaging pastureland in a 2013 image courtesy of the USDA.

Feral hogs (Sus scrofa) were brought to the U.S. by humans during two major migrations in the 1500s and 1900s, first as a source of food and later as hunting stock. But while their fondness for backyards have become the stuff of memes, they’ve really become a serious threat to ecosystems across the country. In the U.S., they’re wreaking havoc on the Southeast’s salt marshes, due to their ravenous hunger for mussels in the area. Worldwide, they’re a surprisingly large contributor to greenhouse gas emissions, since they disturb soil, an act that releases carbon dioxide into the atmosphere. Rarely, they can even spread dangerous brain infections to the hunters who feed on them.

Feral hogs may be the internet’s favorite animals outside of cats and dogs, but their real-life presence is anything but.

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Burmese Pythons

A Burmese Python being held by wildlife experts during a press conference in the Florida Everglades about the non-native species on January 29, 2015 in Miami, Florida.

Alligators may be the most well known fearsome creature in the Florida Everglades, but it’s the growing invasive population of non-native Burmese pythons that are a real threat to this precious ecosystem.

The snakes were likely introduced to the region by the intentional or accidental release of captive pets. Regardless of how they ended up in the wilds of South Florida, they’ve become a huge issue. Wherever these slithery serpents have become established in South Florida, the local population of animals they feed on has dwindled. Populations of raccoons, opossums, and bobcats have declined around 90% or more since 1997 in areas where the pythons have lived longest. Other animals like marsh rabbits, cottontail rabbits, and foxes have essentially vanished. Last year, the pythons were spotted in the Northern Everglades Wildlife Refuge for the first time, a worrying sign that their presence is only growing.

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Asian Carp

Carp fish, which originally hail from Europe and Asia, have been a nuisance since they were first introduced to the U.S. roughly 100 years ago. But newer introductions of bighead carp, black carp, grass carp, and silver carp more recently have only added to the headache.

These fish are commonly imported to the U.S. to be farmed in agricultural ponds. But through flooding and/or accidental release, they made it into the wild. The hungry, hungry invaders outcompete other native species for resources and leave—in the words of the USDA—a “trail of environmental destruction in their wake.” Right now, their impact has been mostly limited to the Mississippi River, but since the system connects to so many other sources of water, their impacts may not stop there.

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Barred Owls

A barred owl fledgling soars through the trees at Muir Woods in Mill Valley, California.

Barred owls are an interesting example of an invasive species. They’re actually native to the eastern half of North America. But it’s strongly suspected that human activity around the turn of the 20th century changed the surrounding landscape dramatically enough that it allowed the owls to expand westward. And once they did, they started outcompeting the already endangered northern spotted owls living in the Pacific Northwest and, more recently, California.

In recent years, Oregon has started programs testing whether removing barred owls can help northern spotted owls rebound. So far, the results have been encouraging, with the removals seemingly stopping the long-term population decline of native owls in that area. Spotted owls have other problems to deal with, such as general habitat loss, but humans just might be able to fix one of the many problems we’ve created for the endangered owls.

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Asian Giant Hornets

An Asian Giant Hornet up close and personal.

The U.S. has plenty of invasive species, but few have as evocative a nickname as the murder hornet, more formally known as the Asian giant hornet (Vespa mandarinia).

These giant buggers likely first arrived in Canada underneath our nose in cargo containers. While they pose a threat to humans, their murderous reputation is more chalked up to the bees they prey on. A few hornets alone can wipe out an entire nest within days, often with plenty of decapitation involved. If that wasn’t enough, their stings reportedly feel like being “stabbed by a red-hot needle.” Bees living in murder hornets’ native range have evolved defenses, but the bees in the U.S. have no such luck, raising concerns that these hornets would further disturb the already declining bee population.

So far, despite plenty of anecdotal reports, their sightings have been limited to Washington state. But while bug scientists have been able to effectively eradicate local nests they’ve discovered so far, it may only be a matter of time before they spread further into the U.S.

9 of the Worst Invasive Species in the U.S.

From nightmare shrubs to not-so-funny feral hogs, invasive species are disturbing the delicate balance of native ecosystems across the country.

Privets

Privets

Zebra Mussels

Zebra Mussels

Asian Longhorned Ticks

Asian Longhorned Ticks

Spotted Lanternflies

Spotted Lanternflies

Feral Hogs

Feral Hogs

Burmese Pythons

Burmese Pythons

Asian Carp

Asian Carp

Barred Owls

Barred Owls

Asian Giant Hornets

Asian Giant Hornets