Tanacetum Vs. Jacobaea

Day 352: When people hear the word "tansy," they may be referring to plants of either genus, Tanacetum or Jacobaea. Both are toxic to livestock and contain volatile oils which may cause contact dermatitis in sensitive individuals, and both are considered invasive. The plants are easy to tell apart, especially when in bloom. Tanacetum vulgare (Common Tansy, top) has no ray flowers; Jacobaea vulgaris (aka Senecio jacobaea, Tansy Ragwort, bottom) does. The leaves are also distinctly different, although those of Tansy Ragwort can be highly variable, especially when young. The foliage of Common Tansy is fern-like; that of Tansy Ragwort is similar to that of radishes left too long in the garden, and forms a rosette. In Washington, the introduction of Cinnabar Moth (Tyria jacobaea) as a biological control has been fairly successful in reducing the occurrence of Tansy Ragwort, the caterpillars' primary food source. Once they have decimated an infestation of Ragwort, the caterpillars cease reproducing and therefore cannot become a secondary concern.

Biocontrol At Work

Day 262: Biocontrol of invasive species brings with it certain tactical issues and the need for long-sighted planning. If you introduce a predatory species to control the invasive, what is the possibility that the introduced species will develop into an unhealthy population? Will a species known to be attracted to the target invasive die out once its preferred host is gone, or will it shift to a new host and in doing so, become a problem in its own right? One of the best success stories I know is that of the Cinnabar Moth.

Cinnabar Moth larvae (Tyria jacobaeae caterpillars) are predatory on Tansy Ragwort (Jacobaea vulgaris, syn. Senecio jacobaea), rejecting most other plant species. They are hungry little buggers, and dozen of them can make short work of a three-foot tall specimen of Tansy in just a few hours. After feeding, they pupate and turn into adult Cinnabar Moths. The adults mate, but since they must lay their eggs on the host which the prior generation has consumed and that host is now absent, the critters have effectively eaten themselves out of house and home. It takes several years of the cycle repeating before the target is gone, but when there is no more food for the larvae, Cinnabar Moths disappear from the ecosystem as well. The larvae may even turn cannibalistic in the absence of the host, a self-limiting feature which makes up for the fact that both the caterpillars and the moths are distasteful to birds. A few may survive each year by shifting to a secondary host, but in the long term, they need a substantial volume of Tansy in order to reproduce effectively.

Allow Me To Introduce You

Day 300:  While we normally think of introduced species as a bad thing (case in point, the Tansy Ragwort shown in this photo), introduction of a species can sometimes be beneficial if managed wisely. The Cinnabar Moth (Tyria jacobaeae) was introduced to Washington in 1960 as a biological control for Tansy (Senecio jacobaea). You see, these little yellow-and-black buggers are very specialized. They are predatory on members of the Senecio family almost entirely to the exclusion of any other food. When that food source dries up (i.e., they eat themselves out of house and home), the caterpillars turn cannibalistic, thereby controlling their own population.