Horsehair Worms in the Garden

Horsehair worm in pool
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A reader recently submitted a question via the All About Worms Facebook page about a worm “coming out of the ground” in his garden. The most distinguishing characteristic of the worm is that it is “very thin, almost like nylon fish line,” and it moves in a “swaying motion.” The reader only asked if what he found is a type of worm, but we’ll do him one better by identifying the exact worm we think he found: a horsehair worm (occasionally spelled “horse hair worm”). Since horsehair worms are nematodes (roundworms), they are actually worms, not just creatures that everyday language, with its understandable but often frustrating contempt for scientific precision, has wrangled into the “worm category.”

More often than we’d like, we have to answer reader-submitted questions tentatively – we won’t have enough information to work with, or the facts we are given lead to two or more plausible identifications. In this instance, though, we are quite confident our reader found a horsehair worm because we don’t know of any other worm that can be described as “like nylon fish line” apart from horsehair worms, whose hairlike bodies look like, well, fishing line. We have also used the description “discarded dental floss” to describe their remarkable body dimensions, and long ago people thought they were actually the hairs from a horse’s tail that had somehow managed to come alive. So, we can’t be absolutely certain of our identification, and a picture would be helpful to confirm our suspicions, but we think there is a very good chance our reader found a horsehair worm based on its physical description.

At this point, we have answered our reader’s question, and in fact have gone beyond it, but we’ll add a couple more notes before concluding. First, the reader mentioned that he found the worm in his garden after it had rained, and this lends further credence to our identification because horsehair worms can only survive in water, which is why people will occasionally find them in strange places, like the toilet. Ultimately, a horsehair worm needs a long-lasting body of water to live out its days, but they will occasionally come out when they are near any water/moisture, like a garden after it rains.

This last issue is connected to another point of clarification, which is that horsehair worms don’t simply come out from under the ground, as our last sentence might seem to imply, and as our reader thought he observed. Rather, they emerge out of the bodies of insects like crickets. We explain this in greater depth elsewhere – see the article about horsehair worms coming out after it rains, for example – but basically horsehair worms are parasitic creatures that spend most of their life growing inside a host insect. When they are finally mature, they will exit the host, but only if it is near a body of water. So, our reader evidently saw a horsehair worm after it had exited whatever creature it had infected, and since the worm was swaying around, this perhaps made it look like the worm was coming out of the ground, as if it were an earthworm coming up during a rainstorm.

Again, we don’t want to imply that we have perfect confidence that our reader found a horsehair worm, but it certainly seems highly probable given all that we know about our reader’s situation.


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Horsehair Worms in the Garden
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Horsehair Worms in the Garden
The most distinguishing characteristic of the worm is that it is "very thin, almost like nylon fish line," and it moves in a "swaying motion."

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