Cinnabar Moth

Tyria jacobaeae

Cinnabar moths and their larvae are a unique example of a species that is not native to the area, but is still beneficial to the ecosystem they have been introduced to. These moths were intentionally released in North America to help control the invasive noxious weed, Tansy Ragwort. Cinnabar Moth larvae eat Tansy Ragwort almost exclusively, that is why this time of year you might notice tansy plants covered in these bright orange and black caterpillars. Some places on the Refuge have had single plants covered in 20 caterpillars or more!

Tansy ragwort is toxic and a threat to livestock and agriculture. All plant parts are toxic, with the highest amount of alkaloids in flowers, then leaves, roots, and stems. Toxic properties are a possible threat to humans through food chain contaminants. It may be confused with Common St. Johnswort and Common Tansy, both Class C noxious weeds. Common St. Johnswort flowers have 5 petals per flower (tansy ragwort commonly has 13 ‘petals’) and Common Tansy does not have any petals (ray flowers). If you need help with plant identification, contact your county noxious weed coordinator.

Both Tansy Ragwort and Cinnabar Moths are native to Asia and Europe, and all three are brightly colored summer staples around here. Tansy (and all the ones mistaken for it) is bright yellow, with those orange and black caterpillars all over it. Cinnabar moths, once the caterpillars go through their metamorphosis, are so named for their black and red wings. There is little variation in patterning, although on rare occasions the red markings may be replaced with yellow, or the forewing is entirely red with a black border, or the wings are completely black. Like many other brightly colored moths, it is unpalatable to would-be predators. The larvae absorb toxic and bitter-tasting alkaloid substances from the ragwort, and assimilate them, becoming inedible themselves. The bright colors of both the larvae and the moths act as warning signs, so they are seldom eaten by predators.

Females can lay up to 300 eggs, which they usually do in batches of 30 to 60 on the underside of ragwort leaves. When the caterpillars (larvae) hatch, they feed on and around the area of the hatched eggs- but as they get bigger and moult (instars), they mainly feed on the leaves and flowers of the plant, and can be seen out in the open during the day. Often, very few survive to the pupal stage, mainly due to them completely consuming their food source before reaching maturity; this could be a possible explanation for their tendency to engage in seemingly random cannibalistic behavior, as many will die from starvation. This behavior, and their particular taste for only certain types of plants, is what keeps them from becoming an invasive non-native.

Did you know? Tansy ragwort showed up in the Portland area in 1922, and spread throughout the state. Livestock and deer that grazed on tansy ragwort began to suffer fatal liver failure. Cinnabar moths were one of three natural enemies released to control the weed. They were first released in the 60’s, with additional releases thereafter.

-Samantha Zeiner, Friends of Ridgefield National Wildlife Refuge