The density and composition of insects on a given plant species can vary greatly among individuals of that species. Understanding factors causing this variability can help us to predict the composition of insect communities on plants and their responses to environmental changes. The main aim of this thesis was to elucidate factors that structure the insect community associated to individual plants.
Plant quality and diversity are increasingly recognized as important determinants of the composition and abundance of terrestrial insects. Hence, I specifically examined how individual variation in plant quality and local variation in the diversity of the plant community determine the performance and abundance of insects on individual plants. As a model system I used aboveground and belowground communities associated to ragwort (Jacobaea vulgaris Gaertner synonym Senecio vulgaris), an outbreak plant species native to the Netherlands.
My PhD research provides new insights into the role of individual plant quality and the surrounding plant community for insect communities associated to individual plants. One of the new insights/interactions I discovered is that herbivores can cause herbivore-induced soil legacies. These legacies potentially are an important mechanism for the spatiotemporal dynamics of plant-insect interactions in natural communities. My research also shows that there is high intraspecific variation in plant quality among individual plants in the field. However, it remains difficult to disentangle the role of this intraspecific variation in plant quality in structuring insect communities in natural settings where the surrounding plant community may have a greater effect on the colonizing aboveground communities associated to individual plants than the host plant itself.