Invasive and toxic hammerhead worm is popping up in Ontario, GTA. What you need to know

An invasive and toxic worm that can grow up to three feet long has made its way to Ontario, and with sightings across the GTA becoming more prevalent, gardeners and pet owners are being warned about its presence.

Since 2019, the broadhead planarian, more commonly known as the hammerhead worm, has continued to be observed in the province. This invasive species from Asia has been spotted plenty in the U.S. and Quebec. It looks a lot like an earthworm but gets its nickname from the flat shape of its head.

But perhaps more notable is the worm’s toxicity, a paralyzing toxin made of the same molecule as that of a puffer fish—one of the most poisonous in the world.

“It isn’t really harmful to adults. It might create a rash and reaction in really young children, but they can be a bit of a problem,” said John Reynolds, a laboratory biologist. “But they certainly aren’t fatal.”

Hammerhead worms are not known to possess biting mouthparts or any structures designed to bite humans or other animals.

How did hammerhead worms get here?

While they might be intriguing from a biological standpoint, hammerhead worms are considered invasive in certain regions where they’ve been introduced. They can disrupt local ecosystems by preying on native earthworms and altering soil composition.

Cathy Kavassalis, a member of the Halton Region group Master Gardeners of Ontario (MGOI), says the worms are likely coming into the province from the nursery trade. As for the possibility of the hammerhead worm showing up in your garden?

“Potentially. Particularly if you have moist soil that is really rich in earthworms,” Kavassalis said. “Don’t touch it with bare hands. Wear gloves. We give this advice to many things in the garden. I’d be more concerned with the damage they’d cause to ecosystems than people.”

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Hammerhead worms are predators that primarily feed on earthworms and other soil invertebrates.

They also have a unique hunting method in which they extend their pharynx out of their mouth to engulf prey.

“Earthworms are an invasive species themselves, so the ones that you see in your garden are actually problematic,” Kavassalis said. “They harm fields of Trillium.”

What to do if you see one

If you notice a hammerhead worm, it’s best to observe it from a safe distance without disturbing it. If you find one in an area where it is not native or where it may cause ecological disruption, avoid relocating it to a new environment.

Ontario’s Ministry of Natural Resources and Forestry tells CityNews since hammerhead worms are not regulated under the Invasive Species Act, the ministry is not actively collecting data on the province’s land planarians.

“We do encourage the public to report sightings via the Early Detection and Distribution Mapping System (EDDMapS), which is an online tracking system operated by the Ontario Federation of Anglers and Hunters, with funding from the Ministry of Natural Resources and Forestry.”

In the case of a person or a pet ingesting one of the invasive worms, you’re asked to seek emergency attention immediately.

Kavassalis noted that there is more than one species of hammerhead worm, and depending on which version takes hold in Ontario, their diets will be an important factor. Some prefer earthworms, snails, and slugs, while others feed on the larvae of grounding pollinators.

“That will be a problem,” she said. “We don’t have in place a system to look at how things are spread, like the worms in these soils.”