We agree with our reader, this thin, white worm is probably a horsehair worm! Horsehair worms are threadlike worms named for their resemblance to the hair of a horse’s mane. As adults, horsehair worms are free living. However, they develop as parasites in the bodies of arthropods such as beetles, crickets, cockroaches, and some grasshoppers. Eventually the horsehair worm will be mature enough to lay eggs and will be ready to leaves the host’s body. The worm must wait for the host to go near a body of water, and eject itself from the host, killing the host in the process. While horsehair worms are parasitic to arthropods, they are harmless to humans and pets.
The reason we are hesitant to say this worm is definitely a horsehair worm is the location in which it was discovered. Horsehair worms are usually found in bodies of water, like swimming troughs, ponds, and pools. We aren’t sure why this horsehair worm was hanging out in a garden on a cucumber plant. If any of our readers are familiar with a worm that resembles a horsehair worm but is more commonly discovered near and on plants, we invite them to share their knowledge on this post in a comment!
To sum up, a reader found a thin white worm on her cucumber plant. We think this creature is probably a horsehair worm, but we aren’t sure why it was hanging out in a garden and not in a body of water.
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Researchers at U.C. Davis have been very helpful. They say behavior looks like a mermithid nematode as opposed to a horsehair worm, because the mermithids climb up plants to lay their eggs. Here are two links. I am taking a specimen to them and if anything changes I’ll post update.
http://oregonstate.edu/dept/nurspest/mermithid_worm.html
https://entnemdept.ifas.ufl.edu/creatures/beneficial/misc/mermis_nigrescens.htm
I found a clear worm that looks the same waving from the end if a branch approx 6′ high in my lemon tree. I am in Vallejo Ca and it has been raining steadily for 16 hours. The only body of water on my yard is a small fountain on my patio on a different side of the house. It is about 3″ long. I will send pics to author