Cinnabar caterpillars

26 July 2018 | Posted in Charlotte Owen
Cinnabar caterpillars
© Alan Price, Gatehouse Studios

By Charlotte Owen

WildCall Officer

It seems to have been a good summer for cinnabar moth caterpillars.  They can be found in their hundreds munching away on yellow-flowered ragwort, and their bold black-and-gold stripes make them easy to identify.  As well as being the cinnabar caterpillar’s main food plant, ragwort supports more than 40 other insect species and is an important source of nectar.  It is also toxic and well known for its potential to poison horses and other livestock if eaten, particularly via contaminated hay.  The toxins within the growing plant make it so bitter and unpalatable that it is usually avoided but the cinnabar caterpillars feast upon ragwort without ill effect.  They actually benefit from its toxicity by eating enough of it to become toxic themselves, and their colourful stripes are a warning to predators: I’m poisonous and taste terrible, don’t try to eat me.


Newly-hatched caterpillars are vulnerable at first so will cluster together, starting at the base of the plant and working their way up.  As they grow bigger and develop their toxic defences they start to spread out, and with hundreds of hungry mouths their host plant can be rapidly reduced to a ragged, leafless stem.  Large populations can easily decimate an entire patch of ragwort and many caterpillars will starve if their food runs out before they are fully grown, so as competition intensifies the caterpillars become more aggressive and may even turn to cannibalism in their quest for calories.  It takes about a month for them to develop fully, at which point they will descend to ground level ready to pupate.  They will remain here all winter, safe inside their cocoons, and complete their metamorphosis the following spring to emerge as beautiful black and red adults.


It is thanks to this vibrant red colouration that the moth earned its unusual name, inspired by the red mineral cinnabar - a toxic mercury ore that was once widely used as an artist’s pigment.  The adult moths fly during the day and could easily be mistaken for exotic butterflies.  They are brightly coloured for the same reason as the caterpillars, since the ragwort’s toxins remain in their bodies and still provide an effective defence against predators. 

Cinnabar adult

Cinnabar moth © Alan Price

Comments

  • Jane Coward:

    27 Jul 2018 05:36:00

    Really interesting, I knew a certain amount about ragwort and its relation with the cinnabar moth but you have added a lot more to it, thank you!

  • Sue Miles:

    28 Jul 2018 06:16:00

    Instead of pulling it out, my daughter left a clump of ragwort growing in her garden. Not only are the flowers very pretty, but she now has lots of cinnabar caterpillars happily munching away on it.

  • Lisa Morgan-Anstee:

    15 Jul 2019 20:41:00

    I’m really happy I left some Ragwort around my garden…I have hundreds of these caterpillars and its nice to know I’m doing my bit for wildlife.

  • Dave cook:

    16 Jul 2019 10:37:00

    First time ive noticed cinabar caterpillers on ragwort and lots this year. I used to see hundreds when a kid back in the 70s in the hay meadows. But all gone now thanks to intensive farming. Hope they come again next year and munch every last ragwort to the root on the farm !

  • Anne:

    22 Jul 2019 07:59:00

    I have seen cinnabar moths in my garden and there is some ragwort in next door garden. Yesterday I had my staircarpet (made of wool not synthetics) out on the path to shampoo. Very quickly there were several cinnabar caterpillars crawling on it. The stone wall boundary of my garden is a metre from the path and there is a couple more metres of open rough ground between my garden and the ragwort next door. That seems to suggest they must be feeding on something in my garden. What else are they known to eat and what were they finding attractive on my carpet?

  • Adele:

    24 Jul 2019 09:28:00

    I have seen a few Cinnabars and my grandson currently has one in a tub. I don’t have ragwort in my garden so like Anne I am interested whether they have another plant habitat as my grandson wants to feed him….

  • 25 Jul 2019 11:41:04

    @Adele & @Anne: Common Ragwort is the best bet but they will also eat groundsels. Probably best not to keep them if you don’t have a good supply of ragwort.

  • Sophie howard:

    30 Jul 2019 22:48:00

    We have a ragwort plant in our garden and we’ve had about 20 cinnabar catapilers on it. We noticed them today. Just wanted to ask how are people getting rid of these in their gardens? and what are they doing about their animals as we have a dog?

  • Sara:

    01 Aug 2019 07:58:00

    Are cinnabar caterpillars dangerous to dogs? They seem to be all over the weeds in my garden. The weeds do not look like ragwort. How can I get rid of them if they are harmful to my dog? Thanks

  • 01 Aug 2019 08:14:39

    @Sara & @Sophie: Cinnabar moth caterpillars are not dangerous to dogs. They mainly eat Ragwort but will also eat Groundsels.

  • Denise:

    01 Aug 2019 09:25:00

    I live in a village on the Isle of Wight and I have hundreds of these caterpillars all over the garden seem to be eating everything .

  • Andrew Baker:

    04 Aug 2019 06:30:00

    Just noticed about 20 or so caterpillars on our sole ragwort plant (which I hadn’t known that we had in the garden) – they have managed to strip it bare very quickly so now I shall have to watch them to see if they start eating each other or some other flowers/plants/vegetables

  • Suzy Davies:

    05 Aug 2019 14:42:00

    Your article gave me just the right answers. Having decided to leave the ragwort in my garden, as it had about a dozen Cinnabar Moth Caterpillars on it, I wondered where they had all gone to. Surely they haven’t been predated I thought, given their toxicity. Now I know, they have descended underground to pupate. Thank You.
    Suzy Davies, Crawley.

  • Sylvia Westrup:

    11 Aug 2019 11:39:00

    Thanks very informative. Recently found them on my allotment in Tunbridge Wells. I don’t have ragwort so hope they find enough to eat. Will look out for the moth.

  • janet cooke:

    13 Aug 2019 05:40:00

    Fascinating. they appeared like magic, watched them with interest then one day they’d disappeared so now I know through your information where too. Look forward to next year so can watch with a bit of knowledge……..

  • Jasen:

    18 Aug 2019 10:31:00

    Sad to hear that so many people talk of ‘getting rid’ of these wonderful creatures. I have noticed great bio diversity in my garden since I stopped getting rid of things that people feel are scary looking or could be poisonous. Everything works as one and leaves me alone. 😊

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