NOTE: Please do not send photos from outside Iowa as they will not reach the proper authorities if your state is tracking hammerhead worm.
The appropriately named hammerhead worm (Bipalium sp.), also called the shovel-headed worm and broadhead planarian, is a large predatory flatworm drawing media and public attention in states surrounding Iowa. Hammerhead worms are non-native and of concern as an invasive species.
It appears there are at least four species (if not “several”) of non-native bipalid flatworms established in the U.S. Some articles report that hammerhead worms have been in the U.S. for nearly a century, though most reports are very recent. Hammerhead worms are thought to be native to Southeast Asia and were “probably introduced to North America accidentally during the early 1900s in soil on the roots of horticultural plants.” (Wisconsin Department of Natural Resources Miscellaneous Publication PUB-SS-1041 2008).
The head of hammerhead worms has a distinctive fan, shovel, or half-moon-shape. They can be gray, brown, or green (“earth-colored”) and may have distinctive stripe patterns. Hammerhead worms have a long, thin, flat, unsegmented body. The commonly reported species, B. adventitium, has a single dark stripe on the top of the body. Length can vary from 1.5 to 3 inches and up to 12 inches for some species.
Hammerhead worms crawl using a “highly ciliated creeping sole,” a region of microscopic, har-like projections on the underside of the body. When they crawl, the head and part of the neck are held above the surface and moved from side to side.
Hammerhead worms spend most of their time hiding under leaf litter and mulch. They primarily move and feed at night, though, as with garden earthworms, hammerhead worms may be on sidewalks and driveways after heavy rain.
Hammerhead worms are predators. They feed on earthworms, insects and insect larvae, and young snails and slugs. Because they feed on earthworms that are beneficial to soils, the hammerhead worms could be a threat to agriculture and horticulture. However, the impact has not been determined. Hammerhead worms do not feed on plants.
Some hammerhead worms produce a potent neurotoxin that helps capture prey and deter predators. They may be poisonous if consumed (don’t eat them!). Use caution because the toxin could be dangerous if absorbed through the skin.
Parting Thought
Hammerhead worms are hermaphrodites; each one contains male and female organs, just like in earthworms. Sexual reproduction is possible, but it appears reproduction by fragmentation is also possible. A hammerhead worm cut in half may grow into two worms.
Our records indicate a suspect hammerhead worm from Des Moines, Iowa (2019), but we have had no other reports from Iowa. Handle worms with gloves or paper toweling and desiccate specimens with solarization. Do not chop hammerhead worms into little pieces!
Resources:
- American Scientist Magazine: https://www.americanscientist.org/article/invasion-of-the-flatworms
- University of Florida: http://blogs.ifas.ufl.edu/wakullaco/2019/01/31/hammerhead-worms-are-an-invasive-species/
- Wisconsin DNR: https://dnr.wisconsin.gov/
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on December 11, 2020. The information contained within may not be the most current and accurate depending on when it is accessed.