Should you kill hammerhead worms? What to know about the invasive creatures.

Once again, hammerhead flatworms are wriggling into the spotlight — this time in Canada.

The toxic and invasive pests, which originate from Southeast Asia, have the ability to regenerate even if chopped into pieces. They have long been spotted across the United States and in other countries, including Canada. On iNaturalist, an invasive species reporting platform, there is evidence of Canadians mentioning the worms since at least 2019, with the most recent report logged as of Tuesday.

But sightings are increasing across Ontario, where the species is making itself “at home,” and some Canadians are wondering what to do if they spot a giant toxic worm on their land and if they should kill it.

“People are surprised to see them because they are very unusual,” John Reynolds, a laboratory biologist and worm expert, told CTV News Toronto.

What do hammerhead flatworms look like?

Hammerhead flatworms are, you guessed it, flat in appearance.

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The group of worms is usually referred to as the broadhead planarians and contains more than 200 different species, experts said.

They have broad, shovel-shaped heads and “are usually shades of tan or brown with various patterns of stripes on their backs,” Theresa Dellinger, an entomologist with the Insect Identification Lab at Virginia Tech, wrote in an email.

She said some of the worms are “long and skinny,” while other species “are shorter and broader.” They range in length from about 1½ inches to about a foot.

Where are hammerhead flatworms found?

The worms are spread through the horticultural trade and the movement of infested plants and soil, according to the U.S. Agriculture Department. Experts say their egg cases are tough, hard to detect and able to endure weeks in transport.

Dellinger said that hammerhead worms probably entered North America “in the soil of plants imported from Asia” and that once they arrived “they were spread in a similar manner through the movement of potted plants and perhaps mulch and similar materials.”

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The planarians are usually found in dark and moist conditions such as under rocks, in leaf litter or in flower beds, Dellinger said, adding that “they are not typically found in hot, dry environments.”

In the more northern parts of the United States and the southern parts of Canada, the most common species is Bipalium adventitium, called the wandering broadhead planarian, said Peter Ducey, a professor in biological sciences at the State University of New York at Cortland, in an email.

“It has been in North America for probably more than 100 years, but the northern extent of its range may be expanding as the climate warms,” Ducey said.

Last year, The Washington Post reported that hammerhead worms were invading the D.C. area, much to the concern of residents. “These worms are one of the craziest creatures I have ever seen and are pretty darned creepy,” entomologist Michael Raupp said in July.

In the United Kingdom, Buglife, a nature conservation charity, said it is “concerned” about an invasion of nonnative flatworms. “Once introduced these flatworms can reproduce rapidly, cannot be eradicated, and pose a risk to native soil invertebrates such as earthworms by feeding on them,” the website said.

Are hammerhead flatworms dangerous?

Some species of broadhead planarians, such as Bipalium adventitium and Bipalium kewense, secrete a neurotoxin, which is found in the worm’s mucus, Dellinger said. It is believed to be used to subdue prey, though the worms are unable to bite or inject the toxins into humans or animals, she said.

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The neurotoxin called tetrodotoxin also is found in puffer fish and certain salamanders, Ducey said.

“I don’t know of any cases of humans or their pets being seriously harmed by handling broadhead planarians,” but “It is wise not to handle the flatworms, and certainly not a good idea to lick them,” Ducey said.

Seek veterinary advice if your pet eats a hammerhead flatworm, and wear gloves if you handle the worms, experts said.

When asked if Canadians should be worried about recent reports, Ducey said that “this is not a ‘sky is falling’ situation,” though “any spreading, nonnative species should be monitored carefully to see whether they are having negative impacts on the native ecosystems.”

The flatworms are considered a possible threat to the environment, however, as they feed off earthworms, slugs and snails and may disrupt soil ecosystems. The predators “could impact agricultural, horticultural, and natural ecosystems,” according to the U.S. Agriculture Department.

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While flatworms are not generally considered a health risk to humans, Ducey urged people to be vigilant in reporting sightings of hammerhead flatworms to government agencies and researchers.

Should you kill hammerhead flatworms?

“Hammerhead worms found in the United States are not native to North America,” Dellinger said. “It’s okay to kill any that you find around your home as these species are considered invasive.”

She said you can kill the hammerhead flatworm by dropping it into a container and using one of these methods:

• Keeping the container in the direct sun for several hours.
• Sprinkling some table salt into the container.
• Squirting some hand sanitizer into the container.
• Placing the container in a freezer.
• Adding soapy water into the container.

Dellinger warned against preventively scattering salt in areas where hammerhead worms might be found, as the salt will harm plants and beneficial invertebrates.

She also cautioned against using pesticides as a preventive measure “as this will likely kill more beneficial species of animals than just the occasional hammerhead worm.”

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Experts also warned against trying to destroy hammerhead worms by chopping them up, as this will produce more hammerhead worms.

“Hammerhead worms can regenerate from fragments of their body, which is why we suggest you don’t cut them in half to kill them,” Dellinger said.

Ducey said the group of worms was “incredibly fascinating biologically,” and their “special powers of regeneration” are being researched for possible medical implications.

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