Hammer-Headed Flatworms Have Been Here A Long Time

By Doug Pifer

I was about to open the front gate when I spied a small worm crawling across my muddy tire track. Its golden color made me go back for a second look. Surprisingly, the front end of the worm resembled a hammerhead shark. As I returned with my cell phone to take a snapshot for identification purposes, the worm showed no alarm but seemed to flow over the ground like water. A quick search revealed it was exactly what it looked like, a hammerhead flatworm.

Hammerhead flatworms, recently touted as our latest alien threat, have been in the USA since 1891. It is believed they were introduced here accidentally through imported garden and landscaping materials from Southeast Asia. These creatures are possibly more common than they seem. The first and only one I saw was out and about in the early morning after a rain, which is when most of them are visible in the open. They would be difficult to find during dry spells. 

According to one source, at one time these worms were so plentiful in New Orleans they were used as demonstration specimens in biology classes. Hammerhead worms have become very prevalent in greenhouses throughout the country, so you may already have introduced them into your garden. Because they are voracious predators of earthworms, they are often a nuisance on earthworm farms. 

Gardeners might want to kill any hammerhead flatworms they see. But don’t squash or cut a flatworm into pieces with a shovel. The flatworm can reproduce by regeneration, which means the broken pieces are able to survive and become 
new worms. 

Sexual reproduction has not been observed, but these flatworms are known to lay eggs. Their typical way of reproducing is to regenerate themselves by fragmentation. A flatworm pinches or constricts the rear part of itself until it breaks off. The piece of worm left behind eventually develops a new head and lives on. This leads some people to call the animal “immortal,” which is not 
exactly true.

Here’s the preferred method of destroying a hammerhead flatworm. Wearing plastic or gardening gloves, place the creature in a sealable plastic bag and add table salt or 30 percent (cleaning grade) vinegar. Some sources additionally suggest placing the sealed plastic bag in a freezer for at least 24 hours before discarding to ensure the worm is dead.

Hammerhead flatworms are between four and eight inches long, sometimes longer. Unlike tomato worms which are immature insects, hammerheads are free-living flatworms, related to the parasitic tapeworms and liver flukes. The scientific genus name of this worm, Bipalium, means “two shovels, having a head shaped like a pickaxe.” Also known as broadhead planarians, they have become established in many tropical and subtropical parts of the world. Several species are now found throughout the United States, most commonly in the hotter, more humid regions of the southeast. 

The distinctive, half-moon shaped head contains sensory organs that help the worm locate its earthworm prey. Its method of feeding, rather gruesome, involves secreting a neurotoxin that immobilizes the earthworm. Then the flatworm digests the earthworm directly through the flatworm’s stomach, located on the underside of its body. Flatworms may not be permanent or abundant enough in an area to decimate earthworm populations. They also prey upon snails, slugs, and certain soft bodied insects. 

They also frequently eat each other. Otherwise, they have few natural enemies. The neurotoxin they secrete makes them distasteful or sickening to predators and is their 
only defense. 

The negative label “invasive” is often applied to species that have been introduced into this country. Often invasives out-compete native species, sometimes even diminishing entire populations. In the case of this flatworm, however, the earthworms it feeds upon are also invasive species. Most of the earthworms we see around here are species that have been brought in or accidentally introduced from other countries and have taken over the soil once populated by our native American earthworms.

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