Meet the hammerhead worm: The invasive species coming to a backyard near you

May 28, 2023

Fun fact: This invasive species can grow well over a foot long and secretes a toxin to “digest” their prey outside the body before they eat it.

The latest invasive creature appearing in backyards across the United States is a strange-looking worm. One that can reproduce by splitting itself in two and secretes the same toxin found in pufferfish.

“It’s very visceral for people to hear about hammerhead worms and then see one in [their] yard,” says entomologist Matt Bertone, director of the Plant Disease and Insect Clinic at North Carolina State University. “Hammerhead worms are particularly abundant and common throughout the U.S., but they’re so foreign-looking that people really react to them.”

Hammerhead worms have spread to about 30 states. They’re found largely east of the Mississippi, on the west coast from California to the Pacific Northwest, and even Hawaii, says Michael Raupp, professor emeritus of entomology at the University of Maryland.

These flatworms can grow well over a foot long. They eat other fauna on the ground, including roundworms, by excreting a paralyzing toxin.

“It’s an outside-of-their-body digestion,” Raupp says. “Then they slurp up the tissues and bring them into the digestive tracts.”

The toxin won’t affect humans though. “The toxin itself does not go through [our] skin, but can be harmful if ingested or gets inside the bloodstream somehow,” Bertone says. “By my calculations you'd have to eat several of them to be poisoned.”

Bertone says to consider that “fear of hammerhead worms is much, much worse than the actual animals themselves.” Here’s what you need to know about these slithering critters, when they pose a threat, and how to safely get rid of them.

Blake Saengerhausen is used to encountering creepy crawlies like scorpions, tarantulas and centipedes. But during a routine house visit in June, the co-owner of Deep Six Pest Control in Seguin, Texas, encountered something he hadn’t seen before in his 13 years in the business.

He found hammerhead worms, members of the Bipalium genus, and a lot of them. They can be identified by their shovel-like head and yellowish-brown, striped body.

“We started pulling everything out of the greenhouse and finding them everywhere,” he says. “We salted them and scooped them up with spoons, but I’m pretty sure we didn’t get them all.”

While these worms are relatively new arrivals on U.S. soil, they’ve still been here since the early 1900s and are just one group of many flatworms living in our backyards.

“Hammerhead worms are native to the old world tropics, including Africa and Asia,” says Bertone. “Flatworms need moisture to survive…They are often found under rocks and logs where humidity is relatively high, and will die if the habitat is too dry.”

Hammerhead worms can lay eggs but also have the ability to reproduce through a process called asexual fissioning or fracturing, Raupp says. “They can break apart and the two parts will regenerate and create new body parts,” meaning slicing them in two will only multiply them.

The way hammerhead worms attack their prey—typically earthworms, snails and slugs—is also pretty gruesome, he says. “Once they’ve captured their prey, they immobilize it with a very sticky mucus or slime.” They use a potent neurotoxin called tetrodotoxin (the same toxin found in pufferfish) that paralyzes their prey.

He adds that this is the first record of land-dwelling invertebrates producing and secreting this particular neurotoxin.

Though we can’t be sure how these worms are establishing in new areas, they are commonly found in the soil of potted plants, Bertone says. These contaminated plants are shipped to new areas with hammerhead worms hiding in a moist soil that they love.

“As we move products around the world, we are also relocating creatures, plants and animals and microbes to different places on the planet, and to new places all the time,” Raupp says. “It’s one of those undeniable and unfortunate truths, like global climate change. And the movement of non-indigenous species to new places does create havoc on ecosystems around the world.”

There’s a “biological tit for tat,” too, he says. Invasive species in the U.S. like hammerhead worms, spotted lanternflies, murder hornets, and the brown marmorated stink bug, are all from Asia.

Likewise, American species are wreaking havoc abroad, including the fall webworm, which devastates thousands of acres of forest in China each year. There’s also the sycamore lace bug in Europe, which has been blamed for feasting on the elegant plane trees lining Paris’ famous Champs-Élysées.

But what are the hammerhead worms doing to our ecosystems here?

“That’s largely a mystery,” Raupp says. “Any time you’re upsetting the natural equality of ecosystems–where you have things [eating] earthworms that aerate and add nutrients to the soil, and slugs, and snails turning over vegetation–we really don’t know what impact they can have with these soil communities.”

Bertone is less worried about the long term impact of hammerhead worms in the U.S.

“These are not plant pests and not a concern for humans,” he says. “They’ve gotten into the public vocabulary because they freak people out by how they look. They’ve been here so long and have probably caused all the damage they’re going to cause.”

While the U.S. government is readily allocating resources to crop and forest-destroying pests–including the spotted lanternfly, emerald ash borer beetle and Asian long-horned beetle–the same focus is not being put on the hammerhead worm, Bertone says.

According to a United States Department of Agriculture (USDA) spokesperson, “hammerhead worms do not feed on plants and therefore are not classified as ‘plant pests.’ Accordingly, USDA does not regulate hammerhead worms.”

“Flatworms aren’t directly dangerous to people,” Bertone says. That said, he adds their populations should be monitored so that they don’t go out of control in certain areas with vulnerable local fauna, like islands.

It should go without saying–you should never eat the worms. And you’ll want to make sure your pets don’t either, as the side effects can include nausea, Raupp says.

Don’t pick up a hammerhead worm with your bare hands. Their slime can stick to you and make them difficult to pick up, and they can wriggle away and split themselves to reproduce, he adds.

You can pour table salt on them, and dispose of them with gloves or scoop them away with a shovel. You can also let them soak in a container of 1:1 mix of bleach and water. Toss them out with household trash, and don’t put them in compost, where their eggs could live on.

To avoid introducing hammerhead worms and any other invasive species to new areas, the USDA recommends cleaning your outdoor gear, shoes, and tires before traveling to avoid spreading them to new locations.

Likewise, avoid moving potted plants and planting soil from your garden anywhere else. And if you need to move soil between states or within your own state, contact your local USDA State Plant Health Director for advice.

Bertone’s approach to dealing with hammerhead worms is a more merciful one. “Your first reaction is to kill them,” he says. “But to me, I’m just like, leave it alone…They’re interesting creatures, but they’re not an imminent threat kind of thing.”