Effect of herbivore density, timing of attack and plant community on performance of creeping thistle Cirsium arvense (L.) Scop. (Asteraceae)

Creeping thistle, Cirsium arvense (L.) Scop. (Asteraceae), is one of the mo st serious weeds in ecological compensation areas (within-field or border r efugia) in Europe. Since conventional weed control measures are restricted in compensation areas, augmenting indigenous agents for biological control of the weed may be a feasible alternative. In this paper, we studied the ef fect of density of the shield beetle, Cassida rubiginosa Muller (Coleoptera , Chrysomelidae), and two vegetation types typical for ecological compensat ion areas, on the performance of creeping thistle plants in an open field e xperiment. Early-season larval feeding had no measurable effect on creeping thistle growth, while late-season feeding significantly reduced shoot grow th. These findings were attributed to higher feeding rates of the herbivore s at higher ambient temperatures late in the season. Defoliation had a stro ng effect on the above-ground performance of C, arvense plants, but not on the below-ground performance. In contrast, the plant community affected all below-ground performance parameters measured, but only some of the above-g round performance parameters of creeping thistle. A combination of high lev els of plant competition and herbivory by C. rubiginosa larvae led to 50% m ortality in C. arvense plants during the growing season. We conclude that a ugmentation of indigenous herbivores of C. arvense in combination with brea king up the root system by tillage and the establishment of a highly compet itive plant community of beneficial herbs may be a feasible way to control this problematic weed in ecological compensation areas.