During a walk in my local wildlife garden we noticed the first Cinnabar Moth (
Tyria jacobaeae) caterpillars of the year, and an adult also flew by. The caterpillars clustered at the flower heads of Ragwort (
Senecio jacobaea), which they prefer as food, and had defoliated the plants quite a bit. Many had already left in search for greener pastures and were around the ground or wandering over other plants. Cinnabar Moth caterpillars have a strong tendency to cluster together, an antipredator behaviour.
This behaviour , however, gradually dissapears as the larvae grow so that fully grown fifth instar larvae are actually agressive to each other and tend to be found spaced out in the plants.
Adults emerge in late spring from overwintering pupae and after mating, female lay eggs in the basal leaves of ragworts. Larvae hatch in about two weeks and their development takes about one month, after this, they pupate on the ground and remain in diapause until the following spring.
This moth is often active by day and therefore easy to spot due to its contrasting coloration, but they are most active at dawn and dusk. When they fly, their pink hindwings are quite apparent and they can be taken for some exotic bright pink butterfly. The bright colours of the moth and particularly, the striking yellow and black stripy pattern of the larvae are a warning sign of their distastefulness to vertebrate predators, a phenomenon called aposematism. The larvae ingests and stores in its tissues toxic alkaloids from the foodplant and the adult also synthesizes additional toxins itself. Surprisingly, many invertebrate predators feed on eggs and caterpillars of this moth.
Given that Ragwort is a toxic plant for horses and has been introduced in several countries where it has become an invasive weed, a lot of effort has been directed to study the ecology of the Cinnabar Moth as a way of controlling Ragwort.
More information
J.P. Dempster (1982). The Ecology of the Cinnabar Moth, Tyria jacobaeae L. (Lepidoptera: Arctiidae) Advances in Ecological Research, 12, 1-36 DOI: 10.1016/S0065-2504(08)60076-8
