INTERACTIONS BETWEEN INVERTEBRATE HERBIVORES AND PLANT COMPETITION

1. The combined effects of different types of invertebrate herbivory and competition on plant performance were studied with a manipulative field experiment involving chemical exclusion and hand weeding, using four species of annual crucifer. Two of the species, Raphanus raphanistrum and Capsella bursa-pastoris, were locally abundant at the study site, whereas the other species, Sinapis arvensis and Sisymbrium officinalis, were uncommon. 2. Plant masses were log-normally distributed at harvest. There was a non-linear relationship between plant mass and siliqua production in all four species. 3. Weeding resulted in a threefold increase in plant size and a 2.6-fold increase in siliqua production. 4. Molluscicide application resulted in a 37% increase in plant size and siliqua production. 5. The effects of competition and mollusc herbivory on plant mass and siliqua production were independent of plant species (i.e. the species-competition and species-molluscicide interactions were not significant). 6. The effect of mollusc herbivory was independent of the weeding regime, indicating that there was no synergism between herbivory and competition. 7. The application of above-ground and below-ground insecticide resulted in a decrease in the proportion of leaves damaged, although neither affected plant size or siliqua production.