Invasions are driven at the level of populations. Biological control using specialist herbivores known to impact life stages influential to population growth are commonly introduced, but assessment of such programs is sorely lacking. In this study we tested the potential for which an invasive plant (Senecio jacobaea L., tansy ragwort) could be controlled by a specialist biological control agent, (Tyria jacobaea, cinnabar moth) using an experimental demographic approach. We used matrix population models and experimental moth larvae additions to explore demographic responses of tansy ragwort to herbivory in two different environments. In both xeric and mesic sites, we hypothesized that tansy ragwort population growth rates (l) would be greater than one in the absence of herbivory, as expected for an invasive species.

Results/Conclusions

In the presence of moth larvae, population growth rates were less than one, indicating the potential for successful biological control by this insect. Our results suggest that if herbivory is extremely high, tansy ragwort can be controlled in this system, and cinnabar moths might be effective in other areas where tansy ragwort is invasive, provided that cinnabar moth populations persist at levels high enough to destroy ~95-99% of seeds. However, lower rates of herbivory may not be effective in reducing the population growth of tansy ragwort, and, as noted in previous studies, juvenile rosettes, the most important life stage for persistence of tansy ragwort, remain largely unaffected by herbivory from moth larvae.