A day after I roto-tilled the garden we had a warm spring rain, after which I saw many long, thin worms on the loose soil. Each was waving its body in the air in a circular motion. What surprised me was not the worms’ behavior, but the number of them — this small garden plot contained hundreds of horsehair worms.
My earliest encounters with horsehair worms involved finding them in livestock troughs at my uncle’s farm. My uncle explained that these worms originate when hair from horses’ manes or tails drops into a trough and then turns into a living worm. I began to doubt this alleged “natural history” in fourth grade when I noticed horsehair worms exiting the bodies of drowned grasshoppers in the troughs.
The most common horsehair worm in Humboldt is easiest to find after those warm spring rains and can be seen waving around on bare soil or on plants. Whether a single worm or a group in a Gordian knot, they are in constant motion. They are very long and wire-thin, indeed looking very much like a 3- to 6-inch piece of wire. Horsehair worms lack eyes, a functional gut and excretory, respiratory and circulatory systems. How do they survive? They are internal parasitoids of invertebrates such as insects and spiders. Like many internal parasites, they have evolved simplified bodies but a complex natural history. Larval horsehair worms live in the host’s body cavity and absorb nutrients directly through their skin. In some species, the infection drives the host to seek out water. The host then drowns — hence the grasshoppers in my uncle’s water troughs — and the adult worm leaves the body to seek a mate. They lay eggs in the water and once the larvae hatch, they find an unlucky invertebrate host and repeat the cycle. But be not concerned: Horsehair worms cannot infect vertebrates like humans and their pets.
Nor can fungi in the genus Cordyceps, despite what you may have seen on the post-apocalyptic drama The Last of Us. The series is set decades after a fungal infection has caused a pandemic and the collapse of society. Yes, another zombie apocalypse.
There are many species of Cordyceps, and at least one of them occurs here in Humboldt. In the 1980s, the Nature Conservancy asked me to do a survey of invertebrates at Lanphere Dunes. During the years it took to complete the survey, I would occasionally find mummified beetle larvae bearing what looked like antlers. I was pretty sure this wasn’t an experiment gone awry from the Cal Poly Genetics Department but it definitely required more research.
The larvae I found were very large, and I knew that one of the biggest insects living in the dunes was the ten-lined June beetle (Polyphylla decemlineata), which can grow to more than 1.5 inches in length. Using the beetle’s scientific name, I searched through the college library (time consuming — there was no internet in the 1980s), looking for fungal diseases of insects. I finally found Cordyceps. Interestingly, most information about the fungus related to its medicinal qualities, with very little about its natural history. Some species of Cordyceps have been used in traditional Chinese medicine for approximately 1,500 years. I was not able to identify the local Cordyceps fungus to species, but it is not the same as the medicinal Cordyceps.
The antlers growing out of the beetles’ head and other parts of the body are actually reproductive structures that produce spores. The fungus consumes the beetle larva from the inside out and the reproductive structures jut out of the larva’s body to spread spores over a wide area. Like horsehair worms, some species of Cordyceps also practice “mind control” on their hosts, compelling them to climb up plant stalks so the spores can rain down on hapless potential victims below.
I hope you’ve found the natural history of these two organisms fascinating. There are many stories like this waiting to be found in your backyard or on nature walks. Look closely and see what you find.
Pete Haggard (he/him) and Jane Monroe (she/her) are the coauthors of Rewilding: Native Gardening for the Pacific Northwest and North Coast, available now from The Press at Cal Poly Humboldt and in local bookstores and nurseries.
This article appears in Duane Flatmo Wants to Wow.