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Thanks to the half-moon shape of their heads, hammerhead worms are known by many names, including hammerhead flatworm, arrowhead flatworm, and broadhead planarian (a category of flathead worms). Dubbed a “toxic terrestrial flatworm,” the hammerhead worm is a carnivorous land planarian known to prey on earthworms, slugs, insect larvae, and other hammerhead worms. Because they feed voraciously on earthworms, which are critical for their role in aerating and fertilizing soil, hammerhead worms are considered a threatening invasive species.
Native to Southeast Asia, hammerhead worms have been found in numerous parts of the United States since the early 1900s and have become invasive around the world. In fact, they’ve recently been spotted invading the D.C. area and New York state. However, their need for high humidity restricts them mostly to tropical and subtropical regions. They prefer cool, damp habitats, and because they are light-sensitive, they feed at night.
According to Michael Skvarla, Ph.D., assistant research professor of arthropod identification at Pennsylvania State University, “Bipalium adventitium has the widest range and occurs as far north as Ontario. Bipalium vagum is found primarily in the Southeast and only occurs in greenhouses and other protected structures in the Northeast, as they can’t survive outdoor winter temperatures.”
Like the hammerhead shark, the hammerhead worm gets its moniker from its distinctive spade-shaped head. It has a long, flattened body that ranges from 8 to 15 inches, and it can be found in shades of gray, brown, gold, and green. The worm secretes a potent neurotoxin, tetrodotoxin, which it uses to immobilize its prey and deter predators. The noxious chemicals it releases can cause skin irritation in people and trigger nausea in animals who consume the worms, although that’s highly unlikely, according to Skvarla (more on this below).
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Before You Begin
Because hammerhead worms consume beneficial worms, secrete poisonous toxins, and transmit harmful nematode parasites, they should be removed whenever found. Care should be taken to avoid touching them.
What risks do hammerhead worms pose?
While hammerhead worms may seem like vicious creatures with their unique appearance, if they start making themselves at home on your property, there’s no need to panic just yet.
According to Skvarla, “the concern about hammerhead worm tetrodotoxin is exaggerated. Yes, some species have been reported to have tetrodotoxin, which is an extremely potent poison. But they are incapable of injecting it into people and are extremely distasteful, so pets and small children are unlikely to eat them.”
In fact, according to Skvarla, there have been no reported cases of people or pets being sickened through contact or ingestion of these worms. “The only advice I give people is that you should either handle them with gloves or wash your hands before touching your eyes or mouth if you handle them without gloves,” Skvarla says.
Ridding Your Property of Hammerhead Worms
Step 1: Conduct an inspection.
Like the earthworms they devour, “hammerhead worms are typically found in damp environments: along stream banks, under rocks, stones, logs, or bushes,” Skvarla explains. Like their prey, they often appear on top of the soil after a rain.
“They don’t burrow per se, but will wiggle through loose soil,” Skvarla says. The worm’s inconspicuous eggs hide in plants or debris and are encased in a cocoon. These then are unknowingly distributed by landscapers, garden centers, and other retailers. With no known natural predators, the worms spread quickly.
Step 2: See a worm? Snap a photo.
You may wonder, “Should I kill hammerhead worms?” Yes, but first take a photo and send it to your local cooperative extension service or, if you live in Texas, to the Texas Invasive Species Institute. There is no formal reporting system in most areas, but these groups, some state agricultural departments, and the National Invasive Species Information Center provide resources and study and track the unwanted worm. The worms can potentially threaten crops and ecosystems.
Step 3: Collect worms in a sealable container.
If you find a hammerhead worm in your garden, capture it in a sealable plastic bag or some other sealable container like a glass jar with a lid. Most sources advise tossing the sealed container into the trash once you’ve captured the worm. Placing the worms in a container ensures they won’t be able to get away and makes it easier to apply a solution to kill them.
“They’re generally slow and will ball up if disturbed,” says Skvarla. “You can just pick them up,” he says. Don’t touch a hammerhead worm with your hands, however; use a stick, gloves, or paper towel to place it into the container. If you do touch it, be sure to wash and disinfect your hands immediately afterward.
Step 4: Add salt or vinegar to the container.
Apply salt and/or grain vinegar concentrate to the hammerhead worms in the bag, seal the bag, and place it in the freezer for 48 hours to ensure that the worm has dissolved. Soapy water, neem oil, citrus oil, boric acid, or pesticides may also work.
Do not cut the worm into pieces. Each section can regenerate into a fully developed worm within a few weeks. One of the ways some reproduce is by asexual fragmentation, so you could compound the problem if you cut up the worm in an attempt to kill it.
Step 5: Discard the dissolved worms.
After a hammerhead worm has been sealed in a plastic bag, treated with salt (or a combination of salt and vinegar), and placed in the freezer for 48 hours, the Texas Invasive Species Institute recommends discarding the still-sealed baggie in the trash.
Step 6: Seek out additional worms, and repeat Steps 3 through 5 as needed.
If you have found one hammerhead worm, there are probably more, so remain vigilant. Examine your garden, particularly in the early morning after a rain when they’re likely to be easily found on the surface.
Be sure to take a photo to send along to the appropriate local reporting group, then collect, kill, and freeze the worms before you dispose of them as described above. Because they secrete toxins, it’s imperative that you handle the worms properly, and since some can reproduce by division, ensure that they are completely dead before discarding them. Keep them away by regularly looking for and managing the worms throughout the spring and summer.
Final Thoughts
It’s important to keep an eye out for these invasive, toxic worms. The slimy predators threaten earthworms, which are vital to our ecosystem because they help decompose organic matter and incorporate soil amendments. Knowing how to identify and correctly kill and dispose of these nuisance worms will help keep your garden and local wildlife safe and healthy.