If you are walking through our local grassland on a warm sunny day, especially along the coast path east and west of the town, you might be lucky enough to see what looks like a small dark butterfly flitting about. This could be an adult cinnabar moth, the one whose black and yellow striped caterpillars feed on ragwort.
If you are even luckier, one will land on a tall plant briefly and allow you a closer look. The closed upper wings are like dark chocolate to black velvet with a flash of vermilion along the leading edge and, if they open ready to fly, they reveal the flash of the vermilion underwings. The whole effect is somewhat like a noble wearing a velvet robe with a silk lining or a lady with a striking Victorian ballgown.
The name cinnabar refers to the colour flashes. Cinnabar is mercury sulphide, the main mineral for obtaining the quicksilver metal, and was used by artists as the pigment vermilion. The colour has had cultural significance for thousands of years in religion and art and was very expensive, but today most vermilion pigment is manufactured rather than from mined cinnabar.
Cinnabar moths are univoltine, they have a single brood in the year. Adults emerge from underground pupation cocoons in May. They mate and lay their eggs on the underside of ragwort leaves. The adults continue to fly until August and then die. This gives a local population one chance at surviving, if there is not enough ragwort in the area then this beautiful species will become locally extinct. They do not fly far and so it is unlikely that they will recolonise from other sites if the ragwort is brought back.
Ragwort is a toxic plant, it contains chemicals called alkaloids. The caterpillars have evolved an immunity to the poison and have turned it into a defence. They absorb the alkaloids and become poisonous themselves to birds and other predators. The caterpillars are yellow at first but soon develop their characteristic black and yellow warning stripes, a signal that says ‘don’t eat me’.
The warning is one that the caterpillars themselves can ignore and they will sometimes become cannibals and eat their brothers and sisters. Ragwort is rather like the dandelion, humans do not want it around. Unlike dandelions, it is actually a nuisance if you have grazing livestock and so it is actively removed from just the habitat that the cinnabar moth needs, grassland.
A single adult moth can lay up to 300 eggs. If there is not much ragwort then each plant will be stripped down to a skeleton by the caterpillars. In desperation they will eat each other, although many will just die of starvation.
Any caterpillars that do survive will burrow into the base of the grass in the autumn, spin a cocoon and pupate over the winter, to emerge in the following May. Let’s hope there will be food for them on Peak Hill this year.
The burnet moth is superficially similar to the cinnabar but it has spots rather than the coloured bar on the forewing and its caterpillars are less striking because they feed on bird’s-foot trefoil and so are not poisonous.
Ed Dolphin
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