There has been a lot of talk lately about a toxic, cannibalistic land-shark that is invading your garden and sucking out all of the worms, leaving death behind. Calm down, it will all be okay… Hammerhead Worms are a subfamily of Land Planarians found originally in Madagascar and southern Asia. They prowl the forest floor for soft-bodied invertebrates such as slugs and earthworms. These critters have found their way onto several continents including, North America. How you ask? Simply through the exotic plant trade. This is not unusual, as hundreds of animals such as Greenhouse Frogs, Tropical House Geckos, and Brahminy Blindsnakes have used these methods to stake claim to new lands. It is actually the most common ways we spread nonnative species. These worms arrived to the US in the soil holding these plants.
The other day, Evan and I were herping near Lexington, when we flipped a strange creature. It was about 2 cm long, and looked like something one might find in a discarded tissue. I knew right away it was a species of Hammerhead Worm. I collected it (and it is now living in a petri dish on my desk) to remove it from the environment, which was pristine. The worm, after warming up in my pocket, then stretched-out, and was about 3 inches long. After a minute of looking, I knew it was Wandering Broadhead Planarian (Bipalium adventitium). It was not until I showed the worm to Lindsey, that I found out this worm was all over social media. So, if you are out herping and encounter one of these worms, what are you supposed to do? Well, lets start by clarifying some rumors flying around. My first step was messaging Leigh Winsor (PhD). He is an Adjunct Senior Research Fellow at James Cook University, and currently the lead investigator in undertaking a risk assessment of alien invasive flatworm species for the European Union. Dr. Winsor sent me a plethora of materials to read over, so now I can give you the lowdown on the Hammerhead Worms. Let us dive into these rumors about these Planarians.
Worm Invasion: These worms are often described as an “invasive species”, but really, they are not. Not all introduced species are considered “invasive”, as invasive really means they have a negative impact to the ecosystem, and are breeding out of control. If a species is doing just that, then US or state governments will list these animals as invasive. For example, did you know the Western Honeybee, the most common in the US, is actually native to Europe. Other examples are the introduced Common Earthworm, Common Roly-poly, and fescue. These beneficial species were never found in North America until Europeans settled these lands. Currently, Hammerhead Worms are not listed in Virginia as invasive, and are “introduced” or “alien” species, but we do not know the impact these animals will have yet to assign them a status.
“New Invader”: I just covered why they are not “invasive”, but despite all the sudden outrage, they are not new. The first Hammerhead Worm was found US soil in 1901, and they spread shortly after. All the outrage is only as the climate has become wetter, and this invertebrate has become more common. Not to mention more people are gardening after COVID.
Toxic: Yes, these worms carry tetrodotoxins. These toxins are what make Pufferfish toxic. It is thought to use these to paralyze its prey, and deter predators. The main difference is that Pufferfish have highly concentrated tetrodotoxins, while the Hammerhead Worm, in the US at least, are very weak. In other words, do not eat it or allow your pets to, wash your hands after handling one, but it is not going to kill everything that touches it.
Earthworm Terminator: These worms are predators that will eat earthworms, slugs, and even other Hammerhead Worms. These animals are probably not going to hunt earthworms to extinction though. Most biologist say, it is likely they will impact earthworm populations, but not with the extremity of reports.
The “Hydra Worm”: This one is 100% true. This species can reproduce with fission. This means if you chop it in two pieces, it will regenerate into two worms. Earthworms can do this as well.
Did you know there are actually four species of Hammerhead Worm in Virginia? These are the Wandering Broadhead Planarian (Bipalium adventitium), Three-lined Planarian (Bipalium pennsylvanicum), Shovel-headed Garden Worm (Bipalium kewense), and Diversibipalium multilineatum which lacks a common name. Let us discuss these species a bit more.
Wandering Broadhead Planarian (Bipalium adventitium) – 2-4 inches: This smaller worm is one of the more common Hammerheads. It is a snot yellow worm, with a brown vein down its back. It has a small, gray-brown head. These are found in the western half of the state and NOVA. This species is thought to be from Japan, and showed up in the US shortly after WWII.
Three-lined Planarian (Bipalium pennsylvanicum) – 3 inches: This is another small worm, but it this one is less wide spread. They too are a gross yellow with a brown vein down their back, but they also have two gray lines on either side of that vein. The Three-lined Planarian’s head is more crescent shaped than the Wandering Broadhead Planarian. This worm is currently contained to NOVA.
Shovel-headed Garden Worm (Bipalium kewense) – 8-11 inches: This is the beast fit for the news headlines. This large worm has that same brown vein, but it has two stripes on either side of this vein. This species also has a dark, broken “collar” around its “neck”. This seems to be a species that sticks to urban areas around the state and is mostly found along the lower James River, Virginia Beach, and NOVA. They breed very slowly, so they will probably never become a problem.
Diversibipalium multilineatum – 6-8 inches: This is another large worm, but this one is much rarer. The brown vein present in the other species, is hidden under a brown or black dorsal stripe. They have two clear stripes on either side of this mid-dorsal stripe as well. They lack any kind of collar. This species has only been found a few times in NOVA.
For more ID info, check out this PDF by University of Georgia.
So, calm down. These worms are not going to destroy your garden or the environment. Several Amphibia species have been found to eat these, as well as larger Hammerhead Worms. If you feel like you must kill them, you can pour boiling water on them (which will kill any plants the water contacts), or squash them under your foot, twisting and grinding them into the ground. They have been here for over 100 years, and have not bothered anything yet. Only time will tell how they will find their niche in our ecosystem.