POPULATION DYNAMICS OF CINNABAR MOTH AND RAGWORT IN GRASSLAND

Population densities of Senecio jacobaea (Compositae) and Tyria jacobaeae (Arctiidae) were studied in mesic grasslands at Silwood Park, Berkshire, 1981-1988. The host plant fluctuated 13-fold in population density; the insect fluctuated 8-fold in adult female density and 9-fold in egg batch density. Realized fecundity of cinnabar moth was density-independent at 3.3 egg batches per female, <67 of potential fecundity based on pupal dissection. Larval survival depended on the ragwort shoot biomass available at oviposition and the mean size and spacing of plants; emigration began earlier from small plants, and density-dependent mortality occurred during dispersal. Survival was low when spring was late and plants were small at oviposition, when egg batch density was high and fewer plants were available during larval dispersal, and after years of high flowering when the larger, flowering rosettes died back. Plant recruitment was not related to flower production or moth abundance in the previous year, and may be microsite limited. Weather and rabbit Oryctolagus cuniculus disturbance influenced plant recruitment from seed and from perennating rootstocks of large plants. Defoliation did not increase plant death rate. Seedling abundance did not peak following years of peak ragwort seed production. The key factors of ragwort-cinnabar moth interaction appeared to be autumn and spring rainfall and the timing of spring growth. Autumn and spring rains encouraged ragwort growth, and caused insect abundance to peak 1 yr after host biomass. Early spring growth, and large plants at egg-laying, reduced larval competition. The interaction between the plant and herbivore is asymmetric. As in other plant communities, cinnabar moth is food-limited, but in mesic grasslands ragwort biomass is not herbivore-limited, and the plant behaves as an iteroparous perennial. -from Authors