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.