Successful Biological Control of Ragwort, Senecio Jacobaea, by Introduced Insects in Oregon

The purpose of our study was to estimate the variability in a biological control process on a regional scale, identify its causes, and quantitatively evaluate overall control success. We present evidence of the success of biological control of Senecio jacobaea (ragwort) in western Oregon following introduction of three natural enemies. First, observations from a single site showed that ragwort declined to <1% of its former abundance and has been replaced by a plant community composed predominantly of introduced perennial grasses. Second, a perturbation experiment showed that introduced insects, within one ragwort generation, can depress the density, biomass, and reproduction of ragwort to <1% of populations protected from natural enemies. Third, a 12-yr survey of 42 ragwort populations showed that strong and persistent depression of ragwort recurred at many sites and at different times. Three features of this case history may be useful in the development of ecological theory as an explanation and guide for biological control: (1) the impact of the natural enemies depends on the distribution of individual sizes and ages in the ragwort population; (2) the long-term dynamics of ragwort may be influenced by the presence of large persistent seed bank which is invulnerable to the natural enemies; and (3) the success of biological control of ragwort in western Oregon appears to be independent of variation in environmental conditions. Combining local, short-term experiments and regional long-term observations is a powerful method for demonstrating successful biological control.