|Abstract or Summary
- The dynamics of tansy ragwort, Senecio jacobaea L. , and the cinnabar moth, Tyria jacobaeae (L.) were studied with the aid of a system simulation model. The model development and validation were based on knowledge of the plant and herbivore gathered from the literature, four years of data from previous studies in Oregon, and supplemental field studies conducted after early model development. Life table data for the herbivore, and growth and regrowth studies of the plant, provided information needed to estimate model parameters. Model-simulated annual changes in ragwort density and cinnabar pupae were within acceptable limits of field estimates made at a study site near Jordan, Oregon, during 1972-1977. After validation of the model, sensitivity analyses and simulation experiments were used to evaluate the potential of cinnabar moth as a biological control of tansy ragwort. Model simulations were used to suggest conditions under which cinnabar moth can reduce ragwort to low levels and maintain control, and to define when the herbivore cannot control the weed without the aid of other tactics. Maintenance of ragwort at low levels is associated with: (1) defoliation above 95%, (2) ragwort regrowth after defoliation of 90% or less, (3) seed germination rate of 1% or less if regrowth is good, (4) stage-specific herbivore mortalities of 25% or less, and (5) an extended period of moth emergence with oviposition of 40 eggs per plant or higher. Significant deviation from these levels by any one of these factors suppresses the ability of cinnabar moth to maintain low levels of the ragwort. The five conditions are interrelated such that a change in one factor can be compensated for by an opposing change in another. Other control tactics were investigated in model simulations to evaluate their compatibility with cinnabar moth. Simulation experiments suggested that cinnabar moth used with cultural control or the flea beetle, Longitarsus jacobaea L., can reduce ragwort levels below the level achieved by cinnabar moth alone. These results are qualified by assumptions concerning the activities of flea beetle and the effects of cultural practices. Whether cinnabar moth has high potential as a biological control of tansy ragwort depends upon site-specific environmental conditions and the desired level of control. In model simulations, ragwort biomass decreased sharply only when plant growth and regrowth was poor and defoliation level was high. With good regrowth and high defoliation, ragwort biomass stabilized at intermediate levels.