Abstract:
To assess the effect of summer moisture stress on the capacity of tansy ragwort (Senecio jacobaea L.) to compensate for defoliation by the cinnabar moth (Tyria jacobaeae LT7), the performance of plants grown under different irrigation regimes was measured. The proportion of plants producing
new leaves following defoliation, the number of nodes at which these leaves were produced, the biomass of the new leaves, the proportion of plants producing secondary flowers, the number of secondary capitula produced, and the rate at which mature fruits were produced all increased with increasing frequency of irrigation. A comparison of natural ragwort populations at a site with a relatively high amount of summer rainfall with a site with little summer rainfall and a comparison of populations during wet and dry years at the same site indicated that the relationship observed between moisture stress and compensation capacity under experimental conditions exists in natural populations.
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.
