The Tale of the Denki Mushi

It begins in 1906, when the unique cocoons of the denki mushi were first spotted on ornamental trees in Dorchester, Massachusetts. Denki mushi means “electric bug” in Japanese, a common name owed to the venomous spines of this vibrant slug caterpillar native to much of eastern Asia. It likely made its voyage to the US in the early 1900s as stowaway cocoons on trees imported to plant nurseries around Boston.

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With no native predators willing to take on the electric bug, it established itself as an abundant defoliating pest on a wide variety of (mostly ornamental) trees, albeit in a very restricted range around Boston. In 1930, the USDA intervened by introducing a parasitoid tachinid fly to control the denki mushi. Unlike most biocontrol attempts during the early 1900s, this introduction was a success - so successful infact, that the species was practically wiped off the map, and the story of the denki mushi was all but forgotten about for nearly 100 years.

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In 2018, the denki mushi quietly showed its face again - on an island in the Boston Harbor, an iNaturalist.org observation was made of the seemingly long lost slug caterpillar. About a year later, a young entomologist, Luke Aengenheyster, discovered a denki mushi caterpillar around Ipswich, Massachusetts. Luke shared his exciting discovery with The Caterpillar Lab helped Sam Jaffe bring the story of this once invasive, now rare slug caterpillar back into the light.

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In 2023, I (Jack) was in the middle of my second summer working for the lab when Sam regaled the story of the denki mushi to me. Being that it had rarely been seen since Luke’s discovery in 2019, Sam challenged me to find the denki mushi once more - a quest that felt fitting given that most of the limited sightings had been around my hometown of Ipswich.

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Over the last two years, I've spent many days and late nights searching for the elusive denki mushi, most of which turned up unsuccessful. My first year of searching yielded absolutely nothing. It wasn't until last September, whilst searching for the caterpillar, that I found my first evidence of the denki mushi in the form of its cocoon - a stunningly artistic and durable construction reinforced by crystalized calcium. About eight months later, a gorgeously golden denki mushi moth emerged.

This past winter, I’ve uncovered over thirty cocoons around Ipswich, all of which were already empty and likely many years old - evidence of a humble and secretive denki mushi population that has seemingly evaded detection for decades. Moving into the Spring, my fascination and determination to find the denki mushi slug caterpillar had boiled over into near obsession, and it had become one of my favorite stories to share during our educational programs.

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In May, Katie Hone The Monarch Gardener, a fellow naturalist and native Ipswichite found an enclosed cocoon of Monema flavescens whilst out in the woods of Ipswich. Just a few weeks before, Katie attended a talk I helped give for the lab - so she knew just the denki mushi fanatic to reach out to. In July, Katie found another cocoon, and this time it emerged as a moth over night. Suddenly I felt a sense of urgency that I might miss the flight of the denki mushi moths.

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With Katie’s help, we organized an impromptu moth lighting event with the Ipswich River Watershed Association in order to attract some of the moths, and hopefully acquire eggs. Despite no denki mushi moths coming to our sheet, I witnessed so much excitement and enthusiasm from everyone that attended the event that night, and that gave me all the motivation I needed to continue searching for the caterpillars as slug caterpillar season approached.

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That brings us to just two weeks ago. Whilst searching through iNaturalist once again, I came across a fairly recent observation of several empty Monema flavescens cocoons on Deer Island in the Boston Harbor. On somewhat of a whim, I ventured out to Deer Island in search of more cocoons. Eventually I stumbled across a red oak tree that was, much to my excitement, covered in them. As I looked for more and more of the empty cocoons, I suddenly spotted little dew-drop covered spines peaking out from under a leaf. In that exact moment, I knew I had found what I was looking for. When I finally turned the leaf over, I was in pure awe of the vibrant colors of the denki mushi slug caterpillar.

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Following the excitement of this find, I’ve found it difficult to talk about anything other than the denki mushi. Already, we’ve seen so much fascination and curiosity from sharing the bizarre and unique story of this species at our shows. As far as next steps, there may be some very interesting research projects that come about regarding the denki mushi in New England - one of which will be a risk assessment of whether or not the denki mushi poses a potential of returning as an invasive species in the US. If my obsessive searching for the past two years tells us anything, it’s that the denki mushi is still quite obscure, and does not appear to be making a come back as a problematic species - so for now, we can simply rejoice in the incredible life history of our secret denki mushi!

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All images presented here are taken by myself, Jack Forrester of The Caterpillar Lab, over the last two years. Denk you all very mushi.

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- Jack