The impact of root herbivory as a function of plant density and competition: survival, growth and fecundity of Centaurea maculosa in field plots.
Two interrelated experiments at Basel on the effect of root herbivory by the moth Agapeta zoegana on C. maculosa, under various competitive environments, showed that competition with grass (Festuca pratensis cv. Cosmos) was the most important single factor and reduced survival of rosettes to 17% of that of the control plants. This strongly limited plant growth, biomass and fecundity of survivors, leading to a 94% reduction in biomass and a 83% reduction in seed output/area. Increased C. maculosa density only slightly reduced plant height, survival and fecundity under the conditions studied, but reduced biomass and shoot numbers/plant. However, plants at high densities produced slightly increased biomass and significantly more seeds/area than controls. Intermediate levels of herbivory had no significant impact on biomass or shoot number, either per plant or per area, but decreased plant height and reproductive mass, as well as survival of young rosettes. The intraspecific gradient of plant reponses to root herbivores was found to be influenced by the competitive environment and the degree of herbivory. In nutrient-rich soils, plant reactions were well buffered from losses from intermediate levels of herbivory when plant density was low. The herbivore effect increased in plots with high density and was most detrimental when additional grass competition was present. Under competition with F. pratensis survival, shoot number and fecundity decreased linearly with increasing numbers of herbivores. That low levels of herbivory increased survival, shoot number and biomass/area as compared with the control plots, indicating that the relation between the applied stress and the plant's response to it was not a simple one. The effect of these root herbivores as biological control agents will therefore be determined mainly by soil conditions and the competitive status of the weed.