BIOLOGICAL CONTROL: EFFECTS OF TYRIA JACOBAEAE ON THE POPULATION DYNAMICS OF SENECIO JACOBAEA IN NORTHWEST MONTANA
Biological control, using introduced, specialist insects is a common strategy for controlling plant invasions. However, the efficacy of biological control agents in controlling their host plants is rarely quantified population level. I quantified the impact of a specialist biological control agent, the cinnabar moth (Tyria jacobaeae) on its host plant, tansy ragwort (Senecio jacobaea) in northwest Montana. Cinnabar moth damage and its effects on important plant vital rates were tested with and without specialist herbivores. The presence of moth larvae corresponded to a reduction in population growth rates to less than one, compared to herbivore-free controls, indicating the potential for successful biological control by this insect. However, delayed effects of cinnabar moth herbivory on tansy ragwort vital rates were realized during the year following moth herbivory, after the moths had disappeared from the system. Individual damage to flowering plants in 2005 led to increased survival of these plants in the following year compared to controls, by reverting back to a vegetative state. In addition, seed set was reduced in plants that were damaged as juvenile rosettes in 2005 that went on to flower in 2006. When these delayed effects were combined in matrix models, gains in adult survival did not outweigh the decreases in fecundity or transition rates in terms of population growth and our initial conclusions remained unchanged. However, further study revealed that moth larvae were more likely to be depredated by carpenter ants in xeric sites suggesting that moth populations may not be sustained in these areas. Cinnabar moth larvae can be effective in this system provided they consume a large number of seeds (>90%) in consecutive years, but requires that moth populations are established and sustained from year to year. While herbivores do show the ability to control an invasive plant species, this relationship is strongly contextual in this system. This work emphasizes the importance of recognizing the influence of habitat context on the outcome plant-herbivore interactions, specifically in invaded ecosystems.
biological control, cinnabar moth, invasive species, Senecio jacobaea, tansy ragwort, Tyria jacobaeae
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