The Land Planarian (Bipalium kewense) a most unusual and most intriguing creature. It is also known as the Arrowhead Flatworm.
Originally thought to be from Indo-China, the Land Panarian was discovered living in the United States over 100 years ago. The widespread occurrence of Land Panarians is a result of horticultural practices and dispersion of potted plants in commerce.
Fully mature Bipalium kewense can be 10+ inches in length and 0.2-0.5 inches wide. Land planarians in our area are gray to brown. They have two distinctive dorsal stripes that run the length of the body. A sticky mucus membrane on the lower side of the body secretes a slime helps them to move, much like a slug.
The head is usually shaped like a half-moon or arrowhead. There may even be eyespots present but the land planarian does not have actual eyes. Its mouth is located mid-way down the body (on its lower or ventral side) and its mouth also serves as its anus.
Land Planarians are not segmented like earthworms, but they do reproduce by fragmentation at the posterior end. The tip will attach itself to something in the soil, and the parent worm will pull away. The fragment is able to move immediately and will develop a head within 10 days.
Fragmentation is the primary form of reproduction, but the flatworm will also lay eggs in a cocoon. At first the cocoon is bright red, but within 24 hours it turns black. Eggs will hatch about 21 days later (depending on temperature and moisture conditions).
Now the good news . . . Land Panarians are effective predators as they will eat slugs and many types of harmful insect larvae. The thought of having a beneficial that preys on slugs should be encouraging!
But now the not-so-good news . . . while all of this sounds rather benign, the land planarian is not necessarily without flaws (at least from a gardenerís perspectiveóbut Mother Nature does not operate in such black-and-white perspectives). Like an earthworm, it burrows in moist soil, but it can exhibit much more sinister epicurean habits. Although it will eat slugs and harmful insect larvae, the Land Planarian will also dine on earthworms!
It does so by lying atop the earthworm (the sticky mucus holds the earthworm to the soil. The land planarian then protrudes its pharynx out of its mouth and into the earthworm to suck out bodily fluids of an earthworm.
Land Planarians are reported to be cannibalistic when food resources are low. This may help to keep the population down, since the mucus membrane of the worm deters most other pests.
While they pose no danger to humans or plants, Land Planarians have been labeled a nuisance in the southern United States in particular, and have been known to decimate earthworm populations in farms and earthworm rearing beds.
I have collected several specimens in my landscape over the years yet I still have a plentiful supply of earthworms. Dr. William Johnson (our Galveston County Extension Horticulture Agent) has noted that over the years, he has occasionally seen Land Planarians and local gardeners have also submitted samples to the Extension Office. Yet (in our area at least), earthworm populations have not been devastated as some might proclaim. So, should we be alarmed that the Land Planarian will decimate our earthworm populations? Probably not, based on anecdotal evidence.
Beneficials in the Garden & Landscape is an Earth-KindTM program coordinated through Extension Horticulture at Texas A&M University. Earth-Kind uses research-proven techniques to provide maximum gardening and landscape enjoyment while preserving and protecting our environment.
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