FAYETTEVILLE, Ark. (KNWA/KFTA) — Hammerhead worms, a complex of species in several genera known as planarians or terrestrial flatworms, have been found in multiple Arkansas counties, including in Northwest Arkansas.
Though native to tropical and subtropical Southeast Asia, hammerhead worms have become invasive worldwide, and have been reported in Arkansas for at least a decade, according to the University of Arkansas Division of Agriculture. Jon Zawislak, extension apiarist for the University of Arkansas System Division of Agriculture, said hammerhead worms tend to prefer climates like those found in Arkansas.
“The worms are thought to have been accidentally transported in soil through the global horticultural trade,” Zawislak said. “These flatworms prefer warm climates and feel right at home in the southeastern United States.”
The most visually distinctive characteristic of these worms is their broad, spade‐shaped head. Bipalium kewense has a long flattened body that typically grows to eight to 12 inches, and sometimes even longer.
They are light-colored, with one to five dark, thin dorsal stripes. These worms are carnivorous and will prey on insect larvae, slugs, snails, and various earthworm species.
Hammerhead worms are also known to cannibalize each other. They can store food reserves in their bodies and survive several weeks without eating.
Many species of hammerhead worms contain a potent neurotoxin called tetrodotoxin, which they use to immobilize their prey and defend against predators. This is the same toxin found in pufferfish and a few other animals.
When they secrete this substance, it can irritate your skin if handled, and will sicken pets if eaten. As a precaution, gardeners should never handle these worms without gloves.
While generally found in warmer climates, hammerhead worms can survive cold conditions by seeking shelter. They tend to avoid the light and are typically found resting in cool, damp locations during the day, while they prefer to move and feed at night. They may be spotted under rocks, logs or thick vegetation.
Hammerhead worms are hermaphrodites and can mate with any member of their species, but more often reproduce asexually by fragmentation.
“Like many other planarians, if cut into pieces, these creatures can regenerate each part into a whole fully‐developed worm within a couple of weeks,” Zawislak said. “If injured, they can quickly regenerate damaged tissue. These worms regularly break off pieces of their tails as they move along, leaving a bit behind to become a new worm. This ability likely contributes to their success in colonizing new habitats.”
Hammerhead worms are considered highly invasive, Zawislak said, and a potential threat to local earthworm populations, as well as gastropods such as slugs and snails. Because of their cryptic habits, ability to reproduce by fragmenting and their lack of natural enemies in North America, hammerhead worms will likely prove impossible to control.
Zawislak said that if a gardener spots a hammerhead worm, he or she shouldn’t hesitate to kill it.
“But you don’t want to chop it in half with your garden trowel,” he said. “Using gloves, place it into a plastic bag or other container with salt and vinegar, then freeze it overnight before disposing of it.”
Arkansans can report any suspected invasive species to the Arkansas Department of Agriculture’s Cooperative Agriculture Pest Survey office at https://www.agriculture.arkansas.gov/plant-industries/regulatory-section/ag-pest-survey-program/.