Abstract

The combined effects of interspecific plant competition and herbivory by a mite, Aculus hyperici Liro, on the growth of two Hypericum species were compared in separate glasshouse and field experiments. The impact of mites on H. perforatum L. was slightly greater than their effect on H. gramineum Forst. In both the glasshouse and the field, competition affected Hypericum growth more adversely than herbivory. There was little evidence that combinations of competition and herbivory caused complex synergistic reductions in plant productivity. In combination, herbivory and competition caused proportional reductions in growth, approximately equivalent to the product of the proportional growth under competition and herbivory individually. Broadly similar results were achieved in both the glasshouse and the field experiment. The results are discussed in relation to the biological control of H. perforatum by A. hyperici, and the impact of this arthropod on the growth of H. gramineum, a non-target native species.