Effect of Summer Moisture Stress on the Capacity of Tansy Ragwort (Senecio jacobaea) to Compensate for Defoliation by Cinnabar Moth (Tyria jacobaeae)

SUMMARY (1) Tansy ragwort plants were grown under different irrigation regimes to assess the effect of summer moisture stress on their capacity to compensate for defoliation by larvae of the cinnabar moth. Three measures of compensation capacity increased with increasing frequency of irrigation: the amount of growth produced following defoliation, the number of nodes which produced this growth, and the number of flower heads (capitula) produced following defoliation. Thus moisture availability limited plant compensation under these experimental conditions. (2) Compensation capacity was positively correlated with moisture availability in (a) comparisons of populations at a high summer rainfall site and a low summer rainfall site and (b) comparisons of populations during wet and dry years. Thus moisture availability appeared to limit plant compensation under natural as well as experimental conditions. (3) These results indicate that the full potential of the cinnabar moth as a biological control agent of tansy ragwort will be apparent in years with below average summer rainfall. In wet areas the introduction of additional biological control agents should be considered. Investigation of the influences of abiotic conditions on the interactions between plants and their herbivores may be necessary in other studies concerned with predicting the outcome of this interaction.