A hammerhead flatworm known as a Wandering Broadhead Planarium
From a distance these flatworms look just like any other worm, but a closer examination will show a broadened head that gives them their name. Unlike earthworms that eat plant debris in the soil, hammerhead flatworms eat other invertebrates like snails, slugs, and earthworms. Some species of hammerhead flatworm have a very pronounced hammer-shaped head, but the species we are finding here in Ohio, the Wandering Broadhead Planarian (Bipalium adventitium) and Three-Lined Land Planarian (Bipalium pennsylvanicum), have heads that look slightly flattened. Another distinguishing feature of these flatworms are the black lines running down their back. From the pictures you can see the single line of the Bipalium adventitium and the multiple lines of the Bipalium pennsylvanicum.
Hammerhead flatworms consume their prey by secreting a toxin in their mucous that dissolves their food so they can consume it in liquid form. Gross right? Well, this is where much of the hysteria surrounding these worms comes into play. That toxin they secrete has been known to give some people skin irritation if they are sensitive to it, so it is best to handle these worms with gloves. There are similarities in their toxin to that of the pufferfish, a tetrodotoxin, but there is little known about how this toxin affects the environment where hammerhead worms are found. Unless consumed in large quantities, there is likely little to be concerned about with your family and pets.
We mentioned that these species have been re-discovered– this is because hammerhead flatworms have been present in Ohio since at least 2004, but anecdotal encounters go back to the 1980’s. These invasive flatworms have been here for many years, but they have gained more attention recently. If you do encounter one, you can carefully kill it with salt, rubbing alcohol, or vinegar applied directly to the flatworm. Use these methods carefully as they are more likely to damage your plants than the flatworm itself. There are no products or home remedies that you can apply to your entire yard to kill them all that won’t kill everything else, so our advice is to not worry about them until you see one. When that happens, then take action with the aforementioned methods and report sightings through the Great Lakes Early Detection Network App, available at https://apps.bugwood.org/apps/gledn/