Interactions between plant competition and herbivory on the growth of Hypericum species: a comparison of glasshouse and field results

The combined effects of interspecific plant competition and herbivory by a mite, Aculus hyperici Lire, on the growth of two Hypericum species were com pared in separate glasshouse and field experiments. The impact of mites on H. perforatum L. was slightly greater than their effect on H. gramineum For st. In both the glasshouse and the field, competition affected Hypericum gr owth more adversely than herbivory. There was little evidence that combinat ions of competition and herbivory caused complex synergistic reductions in plant productivity. In combination, herbivory and competition caused propor tional 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 experime nt. The results are discussed in relation to the biological control of H. p erforatum by A. hyperici, and the impact of this arthropod on the growth of H. gramineum, a nontarget native species.