Caterpillar’s revenge (redux)

I study caterpillars.  But really whatever you study, if you are into it enough, becomes part of you.  And eventually, those targets of our attention and love (and well, okay, our obsession), get their revenge.  I’ve spent a lot of time on Monarchs and Milkweed, studying and contemplating their toxicity.  Milkweeds make poisons, monarchs eat the milkweed (and eat nothing else), eventually making the monarchs themselves toxic too. But milkweed’s toxins can be tamed, and as long as you treat it right, both milkweed and the monarch can be eaten (for details you’ll have to read my book). But as I have outlined in previous blogs, the milkweed fauna is full of interesting critters.  For today, I want to focus on the other Lepidopterans that eat milkweed, the tiger moths or Arctiinae (a sub-family with 11,000 species, with perhaps 100 worldwide that eat milkweeds — just a guess) Incidentally, there are more arctiid species than all of the birds on the planet. I’ve been interested in the Arctiids for quite a while, as they are spectacular moths, often toxic, hairy, colorful, and with decidedly interesting dietary choices.  This story begins a decade ago, when after sorting through the pupae of many Euchaetes egle for a research project, a week later my fingers crumbled with contact dermatitis like I had never seen before:

The pussy rash that emerged after handling Euchaetes egle pupae. The caterpillars themselves are perfectly fine to touch (please don’t be afraid!), but extensive handling of their hairy pupae (cocoons) was an itchy and painful mistake.
a2 Euchaetes egle wiki
An adult Euchaetes egle (image from Wikipedia) mis-named the “milkweed tussock moth”.  This is a misnomer because tussocks are a family of moths distinct from the tigers.
Starting at the beginning, Euchaetes eggs are typically laid late in summer on the undersides of milkweed leaves, usually in large clutches of up to several hundred eggs.
Newly hatched caterpillars feed gregariously (in large groups), and seem to somehow avoid poisoning by milkweed latex. See the little drops?
c euchaetes larvae, by ellen woods
As caterpillars growth, they can defoliate the milkweed (photo by Ellen Woods)
c more euchaetes larvae, by ellen woods
And as they growth more, they transform into orange, white and black fuzzies that continue to defoliate the milkweed (photo by Ellen Woods)
Euchaetes egle, the milkweed tiger moth, showing off the same colors as adult monarchs!
Sticky toxic milk (latex) is a problem for all of the chewing herbivores of milkweed.
Euchaetes copes with latex by patiently cutting the main veins and the feeding on distal (on the other side) tissues that have been drained of their latex.
g euchaetes pupae bugguide.netnodeview70710
A Euchaetes pupae/cocoon showing the offending hairs. A hard-shelled pupal case in under this bed of hairs.  Saliva and other allergens are likely mixed in with the hairs as a protective shield against would-be predators (mice?).  Photo from bug guide.
The result was dramatic when I sorted through many pupae, naive to the effects of these pupal hairs.  They got their revenge and I wouldn’t soon forget. But it was a decade before they struck again.
j Pyrrharctia isabella, the isabella tiger moth
Fall 2018. The isabella tiger moth (Pyrrharctia isabella) a remarkably common sight towards the end of summer.  This “toxic plant generalist” does not eat milkweed, but it is a tiger moth.  Kids pick them up, we all love the woolly bear, and they are harmless in this state!  But as I went for a jog one Saturday, I felt something squishy in my sneaker.  No bother, I kept running.  But that night, my foot felt itchy.
A few days later when my foot broke out in an incredible pussy rash, I rememberred the squish in my shoe.  Upon inspection, a macerated woolly bear caterpillar was in my right shoe.  The itch was fiery. But it subsided in a week or so.
Little did I know that my favorite alpaca running socks were seemingly poisoned forever.  After the rash reemerged a few weeks later when I wore the socks again, I had to get rid of them.  Lesson learned?  Well, I still love caterpillars. Yes, they can be toxic and rashy.  But they are just protecting themselves.  These tiger moths are safe to handle as caterpillars, just don’t eat or puree.
m anurag with karina boege
Thanks for reading.  Here I am with itch-free recovered feet and fingers, along with my friend and colleague Karina Boege.

12 Replies to “Caterpillar’s revenge (redux)”

  1. I’m perplexed by the woolly bear encounter.

    You squished the critter under your shoe, and the toxin went through the top of your shoe, and the alpaca socks, to poison the top of your toes.

    How does that happen? I’m having trouble visualizing the trajectory of the chemical.

    • Dear Tim, good point! The Woolly bear had taken up habitation inside my shoe (left outside to dry from the last run). Around that time of fall, a month ago, they are typically looking for places to spend the winter (no migration!). Unfortunately, the squishiness was inside my shoe.

  2. Great article, and I was happy to learn that the milkweed “tussock” moths are really a kind of tiger moth! Where do the milkweed tussock moths typically pupate? Our milkweeds – especially A. tuberosa – frequently get defoliated by these guys. Given their numbers in our garden I wonder why I haven’t stumbled across their pupae before..

  3. Last year I had tons of the Tiger Moth cats but I didn’t know what they were. All I knew is they were completely taking over my Milkweed. Not knowing what they were, I “got rid” of them by hand with no gloves. I finally just out of curiosity saved a few on milkweed and they made the exact type chrysalis in your photo. I kept them around for quite a while in my garage and nothing ever happened so I threw them out. I’m just glad to know what they were and lucky enough to not get the blisters/rashes that you got. Thanks for the education! Will be more careful now. 🙂

  4. Hi,
    I have recently had a reaction on which looks exactly like your fingers. Can I ask how you were treated?
    Many thanks

  5. Hey there, I put a woolly bear (Isabella tiger moth as your article seems to clarify) into a container with goldenrod and milkweed. They seem to be happily munching on the milkweed and I’m confused as to whether or not that’s a bad thing.

    • Thanks for writing! This seems just fine to me… they often like to eat wild plantain… Frankly, I am surprised that they eat milkweed, my guess is that they would not eat intact plants (with latex flow). Isabella is a generalist, so I suppose it is happy to eat cut milkweed leaves. Very interesting!

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