Arrow-Headed Flatworm

Subject: What kind of worm is this?
Location: southern California
December 21, 2013 8:46 pm
Hi, I live in southern California, and was outside getting some leaves out of my pool. It was about 55 degrees, rather typical for this time of year. I live in 91381. There was some leftover moisture from my sprinklers that had accumulated on the concrete hardscape. I noticed this weird looking, worm like bug. I have seen one like this last year. Sunny day, background is wet concrete.
The odd thing is that the color and markings resembled a baby snake, sort of. The head has an ever-changing shape that generally looks like a semi circle, or a fan shape. This bug moved like a worm. What is it? Is it dangerous? I have a Labrador Retriever that is outside, often and want to make sure there are no concerns for any of us.
Your help is greatly appreciated.
Signature: MPS

Dear MPS,
This oddity is an Arrow-Headed Flatworm, one of the Planaria, and we believe it is Bipalium kewensis.  According to Charles Hogue in his landmark book Insects of the Los Angeles Basin:  “the species was discovered in 1878 in the greenhouses of Kew Gardens near London, hence its scientific name.  It has a wide distribution in warm climates.  It needs a moist habitat and it is usually encountered near outdoor water faucets, where the soil often remains wet.  It original home is unknown but is possibly the Indo-Malayan region.  … These are benign creatures — they do not damage plants or cause any medical problems.”  We suspect that populations of this species get established in new locations when plants are purchased from nurseries.

Update:  Benign or Not???
Thanks to a comment from Barbara, we decided to do a bit more research and we found some interesting information.  The Dirt Doctor states:  “Rather than helping control termite larvae, grubs and other pests, etc. it seems that it is only a destructive pest that needs to be gotten rid of.  It only eats earthworms. The predatory land planarian is no friend of earthworms.  In fact, they are parasites that eat earthworms and can wipe out entire populations.”  Calling the Arrow-Headed Flatworm a parasite does not seem accurate to us.  A more correct term would be predator.  The Red Worm Composting website states:  “Land planarians can be a serious earthworm predator in certain parts of the world – generally they are more of a threat in warmer regions, but certain species are found in more temperate zones as well. They are particularly dangerous because they can reproduce incredibly quickly, and have been reported to wipe out an entire worm population (in a worm farm) in a matter of days.”  The two previous citations come from sites that recommend worm farming, and that is not necessarily a natural environment for the worms as they live in confinement.  The chances of a Land Planarian wiping out all the worms in a garden seem incredibly remote as the worms in a typical garden are not confined.  According to the Galveston County Master Gardeners Beneficials in the Garden page on Land Planaria:  “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 gardener’s perspective—but 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!”  The bottom line is that any species, however seemingly benign it might be, can negatively affect the natural ecosystem when it is introduced.  The Arrow-Headed Flatworm is an introduced species, so we will tag it as an Invasive Exotic species.  The larger issue here is how human behavior has irrevocably changed the ecology of the planet by introducing foreign plants and animals, either intentionally for food and decoration, or accidentally, and then how those introduced species interface with native plants and animals.  Once the factors of agriculture and animal husbandry are considered, the waters get very murky.  If a native meadow with native milkweed is destroyed to plant corn on many acres, and then some insect is introduced that decimates the corn crop, is the insect the invasive exotic or is the corn and the farmer who planted the corn to blame?  Sadly, that ship sailed long ago.