Don't be fooled. Inside this thin coating of sweetness is a fiery core of total insanity.

Tuesday, July 24, 2018

Tansy Ragwort and Cinnabar Moth Caterpillars

A few weeks ago while taking a look around the long, dry, sunny bed that separates my front garden from my neighbor's on the right I discovered a squirmy, wormy mess of caterpillars. My first instinct was to squish them, but on second thought I decided to leave them be for the moment and Google them. I recognized the weed they were feasting on as something called tansy ragwort. It had sprung up in the bed a few years ago (during my summer of discontent), and because I just wasn't up to keeping that bed weeded and under control that summer, I let it flower and go to seed. I didn't notice seedlings last summer, but apparently I had them, because this year they grew tall and were flowering and now were covered with something voracious.

After some Googling I discovered they are the caterpillars of cinnabar moths. I remember seeing a few of these interesting red and black moths flitting around the garden earlier this spring. But of course I was never ready with my camera when I saw one. I think it's a pretty moth.

Photo of cinnabar moth taken from the OSU website

Although the adult moths drink nectar from whatever plants they can find it, they lay their eggs exclusively on tansy ragwort, much like monarch caterpillars which lay their eggs on varieties of milkweed, because it is all they eat. They're not as photogenic as monarch cats, but they do a great service to the county. 

Tansy Ragwort, you see, is a bad weed. If it gets into the hay supply, it can poison livestock, because it contains a liver toxin. Tansy ragwort (Senecio jacobaea) is considered a noxious weed in Washington state, and cinnabar moths (Tyria jacobaeae) were released specifically as a control agent.

For more info about the cinnabar moth, go here.

Cinnabar moth caterpillars devouring tansy ragwort in my garden

More cats in various instar phases

And now a few weeks later, what do the stems of the tansy ragwort look like? Well, that's it out of focus on the left in the picture below -- a bare stem with no leaves and no flowers. There was a single caterpillar left on it, but my phone camera wouldn't focus on it.

Once the cats reach a certain size, they fall to the ground and pupate over the winter, for several months, so I won't get to see them metamorphose out of a chrysalis like a monarch. In fact, this bed is one I'm planning to renovate this fall. It's behind the greenhouse and I seldom go there, but I've ignored it for too long and it needs some major work. I've been doing research lately on meadow and prairie style plantings, and that's what I want to do here. When I finally get around to digging everything out, I'll have to watch out for the pupae in the ground. If I see any, I'll set them aside to rebury, so hopefully they can wait out the winter months and survive to emerge next spring.

There's more info about tansy ragwort (which isn't the same as ragweed, which causes hay fever) here.


  1. So glad you Googled before you squished. That is one gorgeous looking moth, and as it turns out, this "pest" helps to keep a noxious weed under control. Great story.

  2. What a pretty moth! I'd no idea that they were released to control tansy ragwort and have never seen either the caterpillars or the moth. You find the coolest stuff in your garden!

  3. Thanks for the early morning education, and that's quite the striking moth!

  4. That's the prettiest moth I've ever seen! Infestations of caterpillars in operation are always a bit gross but, when they're consuming weeds, that's a valuable service. Next time, I'll research before I squish too.

  5. Finally, some caterpillars that attack a nasty weed! The moth is beautiful, so it is truly a win-win. I am in the midst of my summer of discontent, this year has been a doozy. Almost ready to throw in the trowel. :-)

  6. Very interesting. I know a bit about tansy and the cinnabar moth, but have not seen it in action.

  7. Very cool moths! The caterpillars not so much, but they have redeeming social value. Very interesting, thanks for sharing.


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