Hammerhead Henry(etta) versus Jumping Jo(sephine)

Are hermaphroditic worms squaring off in your garden?

February garden

During a spell of unseasonably warm weather in February, I worked as much as I could to prepare the garden for spring. I happened to turn over a rock and discovered the worm shown in the photograph below. According to iNaturalist, this slimy yellow fellow is a planarian, probably Bipalium adventitum, or wandering hammerhead worm.

Wandering hammerhead worm

You might recall planarians from middle school science class where they are often used for student experiments due to their ability to regenerate from severed body parts. Species used for this purpose are usually freshwater planaria, whereas my garden buddy is a terrestrial flatworm. It too is probably able to undergo binary fission, meaning it can split into two parts, each of which will regrow the missing part. It's believed, however, that this particular worm usually reproduces sexually.

These hermaphroditic hammerhead worms, or landchovies as they are sometimes called, were discovered in the U.S. as early as the 1940s, and were probably brought in with plant material from Asia. They didn't receive much attention until the last few years, when news stories about them began to proliferate. Their creepy appearance and their ability to produce tetrodotoxin, the neurotoxin found in pufferfish and the blue-ringed octopus, makes for great copy. Also, populations seem to be expanding with warmer and wetter conditions, and more gardeners are encountering them.

Dormant flower bed harboring who knows what creepy crawlies

There are no doubt countless disgust-inspiring creatures in soil and mulch of healthy gardens, but most are too small to catch the casual eye, leaving us blissfully unaware. Planaria are easily viewed and, to be honest, look evil and out of place. A gardener could be forgiven if her first instinct was a killing spree to attempt to eradicate them, especially when encountering something like this:

Bipalium kewense, photo courtesy of Wikipedia

That's not my modus operandi, and I sought positive identification and more information before taking action. Though these worms produce a neurotoxin, there's little evidence that humans are at risk. It's advised to wear gloves when handling them and to refrain from eating them — in other words, just use common sense.

The most auspicious aspect of the presence of these worms is that their main foods are earthworms and slugs. Slugs are the bane of most gardeners, and though earthworms have traditionally been a welcome sight in soil, they are increasingly controversial. Native earthworms disappeared from much of the northeast U.S. during the last period of glaciation, and have been largely replaced by worms carried here by European settlers. Now these worms are in turn being usurped by Asian jumping worms. Both of these hermaphroditic foreigners process organic matter more quickly than than would occur in their absence, prompting fears of negative changes to forest ecosystems.

Peach buds

Boreal forests, stretching across much of Canada and Alaska, are believed to be particularly at risk. In these northern forests dominated by conifers, a thick layer of duff covers the ground and represents a large portion of the biomass. Earthworms consume this layer and speed up the cycle of decay, potentially resulting in ecosystem disruption, and northern forests becoming carbon emitters.

Far south of boreal regions in southern Ohio, the effect of non-native earthworms on our forests remains to be seen. I'm used to losing organic matter quickly in the garden due to our long, usually very wet, summers, so the thought isn't bothersome. European earthworms probably had a net positive effect in the garden. The jumping worms that have supplanted them tend to turn the top soil layer into hydrophobic granules, but my habit of building up soil constantly with mulch and compost means I haven't had observable detrimental effects.

Now enters the newest invader, the slug-slayer and worm-waster Bipalium adventitum. Will it slither in like a glistening knight-errant to purge the garden-kingdom of the pestilence of slugs and jumping worms, then fade away when the job is done? Or will it find its niche and eke out a living by snagging the occasional gastropod without significantly reducing populations? I cannot know for sure, so I will proceed by doing nothing.

Humble Hellebores

The internet is overflowing with advice to smash spotted lanternflies, crush the egg cases of Chinese mantids, gently dispatch brown marmorated stick bugs, and in general annihilate invading invertebrates. I ignore this advice — I'm inured to instructions to kill, kill, kill, and have no trouble resisting. After observing the same scenario play out many times without the world ending, I believe the onslaught is inevitable and efforts to avoid it are mostly wasted time. Better to put energy into strengthening ecosystems in general by increasing biomass, retaining water longer, and removing what is observed firsthand to be detrimental.

Quotes from an over-the-top article on killing invasive bugs

I'm especially reluctant to kill invaders if there is a chance they will kill each other. Though it will take place far from human sight in moist leaves and under rock and mulch, an epic battle between hermaphroditic worms is possible. As I imagine it, an army of landchovies will follow chemical trails to locate prey, which they will ensnare and coat with toxic slime. They will then suck out the insides of their victims, and poop the waste out their mouths.

View from the garden

The only part of this scenario that is fantasy is the scale — perhaps there will be no legion of Bipalium. However, individual planaria track and consume prey as described above, and while I probably won't ever observe it, I'll be on the lookout for telltale changes in the garden. I'll keep readers updated as the drama unfolds.