Monday, 27 July 2015

danger: do not eat

Ragwort is an attractive plant, with clusters of star-shaped yellow flowers at the top of each stem. It is also a noxious weed, and in the UK at least, it is an offence to allow it to grow on your land. There have been numerous cases of livestock being poisoned where the weed has sprung up on land given over to grazing.

Despite these strictures, and although it doesn’t grow in huge stands, like nettles and rose bay willow herb, it remains remarkably common. And although most animals that eat it will die, there are creatures that are happy to tuck in, including the larvae of the cinnabar moth. Cinnabar, a bright red sulphide of mercury, is the principal ore of this metal and the basis for the artists’ pigment vermilion. The moth is mainly black, but with vermilion edges to its wings.

I’ve provided these background details because a few days ago I was walking along a quiet lane near my house when I happened to notice a specimen of ragwort along the roadside that was infested with strikingly coloured caterpillars. The specimen pictured on the right is less than 1.5 metres away but is unaffected.

The first photo was taken immediately, and the second was taken by Paula a couple of days later. Although caterpillars can be seen on different stems, they are all part of the same plant. Somehow, the caterpillars are able to absorb the alkaloids in the ragwort without succumbing to the plant’s toxicity, although any bird that fancies a meal of such caterpillars will not be so lucky, because the alkaloids remain unaltered in the bodies of the caterpillars.

It is often said that vividly coloured creatures—reptiles, amphibians, insects—are the colour they are as a warning to potential predators, although I’m bound to ask how any predator would know that the bearers of such bright colours are lethal to eat. Nobody will have told them to keep away.

Another interesting point concerns the food supply. Caterpillars are no more than eating machines, and it is probably safe to assume that no individual leaves the plant on which they emerged from the egg, so if there are too many caterpillars on a single plant, the food supply is bound to run out before the caterpillars are ready to pupate. Cannibalism is therefore a possibility, so I will be keeping a close watch on this colony to see what happens as the caterpillars grow larger.


  1. A very interesting post Dennis, I enjoy reading about things like this. In London recently, there was a lot of media coverage regarding a plant called 'hogweed' which causes severe burns to anyone who dares to touch it. Those plants always look so beautiful and enticing but now everyone wants the local councils to get involved in removing these plants. People are beginning to find them in all different locations which can be very harmful.

    Love the colour of the caterpillars, they look so perfectly made but I'm sure they can quite easily change the look of any beautiful garden!

    1. Great to hear from you again Rum. I’ve read about problems this year with giant hogweed (common hogweed is harmless). Fortunately, I know what it looks like.

      Incidentally, these caterpillars won’t harm your garden, unless of course you’re growing ragwort (that’s the only thing they’ll eat).


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