Compensation and Regrowth in Ragwort (Senecio Jacobaea) Attacked by Cinnabar Moth (Tyria Jacobaeae)

Abstract

Cinnabar moth larvae each eat abnout 0.5 g dry matter of ragwort during the course of development and plants are often stripped of all their leaves and flower heads by caterpillars. The ability of ragwort to compensate for cinnabar moth feeding is affected by the size of the plant, but the timing of attack and the number of feeding larvae have little effect. Small plants (< 20 cm rosette diameter) produce about 2500 seeds after being completely stripped of their initial set of flower heads; defoliated large plants produce over 10,000 seeds. Compensation in seed production is due mainy to the production of new capitula on regrowth shoots from the root crown or from surviving parts of the primary shoot. The number of seeds per capitulum is lower under some defoliation regimes than in undefoliated plants. Seeds produced on regrowth shoots are substantially smaller than seeds produced on primary shoots. Root weight is not reduced by defoliation. Large plants (> 30 cm rosette diameter) allocate proportionately more of their biomass to reproductive tissues than do small plants, and undefoliated plants allocate more than defoliated individuals. Plants do not necessarily die after flowering; they retain a food reserve in the rootstock for regrowth the following spring irrespective of their defoliation history. The overwinter death rate of plants which have flowered is only 25%. Ragwort and cinnabar moth provide what is probably the best example of non-reciprocal population regulation; the herbivore is food limited but the plant is not herbivore limited.