Caterpillars with yellow stripes and red heads are often the larvae of the cinnabar moth (Tyria jacobaeae). This striking caterpillar feeds on ragwort and other plants in the daisy family before transforming into a reddish-brown moth with black bands on its forewings.
Identification
The cinnabar moth caterpillar has a black head with red spots on top. Its body is black with bright yellow stripes running down the length of the back. The stripes may be solid or broken into dashes. This coloration serves as a warning to predators that the caterpillar is distasteful.
When fully grown, the caterpillar can reach 30-40mm in length before forming a pupa and undergoing metamorphosis into the adult moth. The transformation from caterpillar to moth takes around 4 weeks during the summer months.
Life Cycle
The life cycle of the cinnabar moth begins when a female lays clusters of eggs on the underside of ragwort leaves in late spring or early summer. After about a week, the eggs hatch into larvae that immediately begin feeding on the host plant.
The caterpillars have five instars, meaning they molt their exoskeleton five times as they grow, with the stripes becoming more pronounced after each molt. The mature fifth instar caterpillar stops eating and searches for a sheltered spot to form a pupa attached to a stem or on debris.
Inside the pupa, the caterpillar transforms into the adult moth over 1-2 weeks, emerging in mid to late summer. The adult moths live for around 2 weeks, feeding on nectar from flowers. Females lay several hundred eggs before dying off in late summer.
Habitat and Range
The cinnabar moth and its caterpillars live wherever their ragwort host plants grow. This includes grasslands, meadows, roadsides, railway embankments, and wasteland areas across Europe, Asia, and North Africa. The moth has also been introduced to North America and Australia to help control invasive ragwort plants.
In Britain, the cinnabar is widespread and frequently encountered from May to August in areas with ragwort growth. It is less common in Scotland.
Diet
Cinnabar moth caterpillars feed exclusively on ragwort (Jacobaea vulgaris) and related Senecio plants in the daisy family. They are specialist feeders and will not accept any other host plant. The caterpillars eat all parts of the ragwort, stripping the leaves first before moving on to flowers and stems.
| Host Plant | Regions |
|---|---|
| Common ragwort | Throughout natural range |
| Tansy ragwort | Introduced to North America |
| Marsh ragwort | Coastal regions |
The adult moths feed on nectar from various flowers, with a preference for plants like dandelions that open at night.
Behavior and Adaptations
The caterpillars are gregarious and feed together in groups for much of their development. The striking yellow and black coloration advertises that the caterpillar is unpalatable, a form of warning coloration known as aposematism. Studies have shown the cinnabar caterpillar contains high levels of bitter and toxic alkaloids sequestered from the ragwort host plant. This protects them from predation.
The moths are nocturnal and fly at night when their reddish-brown coloration helps camouflage them against the darkness as they seek nectar sources. During the day the moths rest motionless on plants and other surfaces. If disturbed they drop to the ground or fly short distances.
Natural Enemies
The cinnabar moth’s main defense against predators is its warning coloration and distasteful chemistry. However, some specialized predators have evolved to overcome these protections.
Insect predators like shield bugs will supplement their diet with cinnabar caterpillars despite the alkaloids. Some birds also learn to eat the caterpillars, starting with just a few to gauge their tolerance. Parasitoid wasps are not impacted by the toxins and will lay eggs on the caterpillars.
A species of parasitic fly (Blondelia nigripes) targets the pupa stage, laying eggs near sites where pupation occurs. When the eggs hatch the fly maggots burrow into the pupae and consume them.
Generalist insect predators typically reject cinnabar caterpillars after sampling their terrible taste. Spiders in particular will drop the caterpillar after a bite and not attempt to eat them. This reinforces the protective warning coloration.
Population Status
The cinnabar moth has a wide distribution and large total population across Europe and Asia. In most areas studied it is generally common wherever ragwort is present. The population trends appear stable with no evidence of significant decline.
Due to its association with ragwort, it has been accidentally introduced to parts of North and South America, Australia, and New Zealand over the past 150 years. In some regions it was deliberately released for biocontrol of invasive ragwort.
Relationship to Humans
Cinnabar moths are considered beneficial insects in regions where ragwort is a problematic weed, such as North America and Australia. As a biocontrol agent, the cinnabar moth helps suppress the spread of the noxious ragwort plants.
The large colorful caterpillars are popular among young children. But handlng should be avoided due to the alkaloids sequestered from the food plant that could cause skin irritation.
Conclusion
In summary, the yellow striped caterpillar with a red head is typically the larval form of the cinnabar moth. It feeds exclusively on ragwort plants and carries toxins from its host plant for defense. The striking colors warn predators of its bad taste. It eventually pupates and transforms into the reddish-brown adult moth. Cinnabar moth populations appear stable globally and they are considered beneficial where ragwort needs controlling.
