Kelly’s Field Notes
Common Name: Cinnabar Moth
In caterpillar form, the cinnabar moth is hard to miss! It is about 10 cm (4 in) in length, and striped yellow and black. As a moth it is equally beautiful, with red and black forewings and red hindwings. Their wingspan is 3 to 4 cm (1.3 to 1.7 in). The cinnabar moth is named for the mineral cinnabar or cinnabarite, which was a source of red pigment called vermillion. It is an ore of the metal Mercury and highly toxic. Sometimes the reddish markings can be yellow but this is rare.
Female cinnabar moths lay up to 300 eggs, in batches of 30 to 60. Their preferred vegetation is ragwort. The eggs will take about 2 to 3 weeks to hatch. First instar larvae, which are grey-green, will hang together on the leaf they hatched on then move after their first molt to the flowering shoots to feed on flowers. At this stage they are the vibrant black and yellow described above. They build up poisons, in the form of alkaloids, from ingesting ragwort sap, so their bright coloring isn’t a bluff! Some species of cuckoo, however, do eat poisonous caterpillars and the cinnabar caterpillar is on the menu. Ants, parasitic wasps, terrestrial isopods, and rodents are their primary predators.
It will take a little over a month for all five instars, culminating in pupation in the soil. The pupae will then hang out until the following May, then emerge as an adult. The adults will live about two weeks, they do not eat during this time. Their goal is to find a mate and lay eggs. While the caterpillars primarily feed on Common Ragwort (Jacobaea vulgaris, syn. Senecio jacobaea ), they are sometimes found on other species of ragwort and groundcels in their invasive range. Because of this plant-loyalty (host fidelity), they are easily found in open grassy habitat. They are also active day and night, frequently misidentified as butterflies.
Cinnabar and Ragwort:
The cinnabar moth is native to Europe and Asia, but was introduced to North America, Australia, and New Zealand to control ragwort. They are found in Nova Scotia and small pockets along the east coast of the U.S., but are mainly in the Pacific Northwest. The cinnabar moth was first introduced to the U.S. by way of Oregon beginning in 1960 and continuing for roughly two decades. Common ragwort is native to Europe and western Asia, but has become a problem for ranchers. If ingested, cows and horses develop liver disease and can die within a few weeks to a few months. Sheep, however, are not affected and are often used to graze land, reducing ragwort populations, before allowing cattle to graze. While herbicides or physical removal do work, the cinnabar moth is an impressive biocontrol. One study showed a reduction of 50%-70% in ragwort populations due to cinnabar caterpillars.
The cinnabar moth wasn’t the only insect released to control ragwort. The tansy ragwort flea beetle (Longitarsus jacobaeae) and the ragwort seed head fly (Botanophila seneciella) are the other two biocontrol agents. Each insect attacks the weed at a different time and location. While cinnabar caterpillars attack the leaves, buds, and flowers of the plant in mid-summer, the ragwort flea beetle feeds on the roots (as larva) and on leaves (as adults) in fall. The larva of the seed head flies eat the seeds as they develop in spring, reducing the spread of the ragwort.
Oregon and Washington used to supply cinnabar moth larvae to landowners but that practice has since stopped. Landowners in Washington, however, can get the flea beetle and California landowners can get the cinnabar moth larvae.
Poison Powers - caterpillars ingest poisonous ragweed, giving them protection from predators.
Flight - cinnabar moths are very active, day and night.
Massive Appetite - few caterpillars make it to the pupal stage because they are so hungry, they clear out the ragweed before hitting that stage of their development. Hungry hungry caterpillars indeed! Because of this they sometimes engage in cannibalism.
Team Player - Cinnabar moth was one of three insects (ragwort flea beetle, Longitarsus jacobaeae and ragwort seed head fly, Botanophila seneciella syn. Pegohylemia seneciella) released to control tansy ragwort and they’ve been successful at keeping the weed at a manageable level. Cattle deaths from ragwort poisoning reduced by 90% by the 1980s. Our best moms, the earwigs, also feed on ragwort flowers.
Cinnabar Moths in Culture:
The cinnabar moth was first described by none other than Carl Linnaeus himself! In 1758 in the 10th edition of Systema Naturae.
What’s in a name? The cinnabar moth was named for its beautiful colors. The pseudo-Greek word Tyria is a reference to the city of Tyre in Lebanon, known for its purple dies. Its species name, jacobaeae, refers to its food source in its home range, the tansy ragweed (Jacobaea vulgaris).
Cinnabar caterpillars are an incredibly efficient form of biocontrol! They protect cows and horses from ingesting toxic ragwort.
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