Last week, I wrote an article about a type of invasive worm that is becoming more visible in Ontario: The Asian jumping worm which can destroy soil structure.

There is another species of earthworm that is also becoming an invasive species.

Hammerhead worms (genus Bipalium) are invasive terrestrial flatworms originally from Southeast Asia, particularly countries like China, Japan and Korea. They also have made their way to North America mainly through the trade of plants and soil.

They have a long, flat, ribbon-like body. Their length ranges from a few inches to over a foot long — yikes!

As their name suggests, hammerhead worms have a broad, flattened head that is shaped like a hammer or anvil. The worms are typically brown, grey or yellowish in colour with darker stripes running the length of the body. They move using cilia on their underside and by having muscular contractions.

Hammerhead worms are usually found in moist, shady environments such as under rocks, logs, leaf litter and garden soil. They are nocturnal, meaning that they are more active at night or in moist environments.

Why are they considered invasive or dangerous? I am glad you asked.

Hammerhead worms prey on common earthworms and other invertebrates, which are crucial for soil health. They digest earthworms externally by secreting a mucus that contains enzymes and neurotoxins to immobilize and liquefy their prey.

The neurotoxin is called tetrodotoxin, the same toxin found in pufferfish. While the amount is not usually lethal to humans, it can irritate skin if handled without gloves, and it’s potentially harmful if ingested by pets.

The hammerhead worm reproduces mainly asexually, meaning that it can reproduce by fragmentation. If you were to cut a hammerhead worm in half (or even smaller pieces), the worm can regenerate into two new worms. Some species can reproduce sexually, but asexual reproduction is more common in invasive species.

As we know, regular earthworms help aerate soil and recycle organic matter. Finding earthworms in the soil while gardening is always a good sign that you have healthy soil.

A decline in earthworm populations due to hammerhead worms can negatively affect agriculture, gardens and ecosystems. We need to be diligent in controlling this invasive species if we want to keep it at bay.

Here are some ways to control them:

Use gloves or tools in the garden. Never touch them barehanded due to their toxins (tetrodotoxin).

Place them in a sealed plastic bag with vinegar, salt or citrus oil to kill them. Dispose of the sealed bag in the trash.

Do not cut them — each piece can regenerate into a new worm.

If you have found this worm in your garden, do not share any plants with neighbours and friends.

Visit EDDMapS Ontario to report sightings with precise location information and photos. This is managed by the Ontario Federation of Anglers and Hunters and is supported by the Ministry of Natural Resources and Forestry. Reporting any sightings will help efforts to control the spread of the hammerhead worms.

Standard pesticides are ineffective and can harm beneficial soil organisms more than the worms themselves.

Joanne Young is a Niagara-on-the-Lake garden expert and coach. See her website at joanneyoung.ca

The Niagara-on-the-Lake Horticultural Society is pleased to be hosting a series of Saturday morning gardening classes, available to the public. They will be facilitated by Joanne Young on Saturday mornings, and they will run until May 31 at the NOTL Community Centre.

May 17 – Creative Containers
May 24 – Pruning Evergreens
May 31 – Tough Plants for Tough Places

Join us for the classes that interest you. For all the seminar details and to pre-register for the classes, visit notlhortsociety.com/classes.