Direct and Indirect Effects of above- and Below-Ground Insect Herbivory on Plant Density and Performance of Tripleurospermum perforatum during Early Plant Succession

Abstract

The effects of above- and below-ground insect herbivory on scentless chamomile, Tripleurospermum perforatum (MWrat) Wagenitz, were evaluated in a natural plant community by manipulative field experiments in 1988 and 1990. Foliar and soil insecticides were applied singly and in combination during the early colonisation of bare ground. The effects on plant density and performance were assessed and related to the occurrence of damage and of insect herbivores. The efficacy of the insecticide treatment was evidenced by a reduction in visible damage and in the number of stems and flower heads infested. A number of insect species were found by dissection of stems and flower heads. The density of plants and their cover abundance (a reflection of the number of stems) were increased by a reduction in herbivory above or below ground, while plants in the combined insecticide treatment were taller and thinner. There was no significant interaction between the effects of the two insecticides. The study highlights the importance of separating direct effects of herbivore reduction in a given plant species from indirect effects resulting from the modification of the competitive balance between plant species.