| Interactions between species of the same trophic level have long been studied in a direct context, addressing competition for simultaneously shared resources. More recently, competition between insect herbivores is studied in a new context that of temporal plant-mediated competition due to induced plant defence. Early-season herbivores were identified to alter plant quality in such a way that this affects performance and ultimately the fitness of herbivores that colonize these plants later in the season. Herbivore-induced plant defences in which plants mobilize their defences in response to herbivore attack are now recognized as a dominant factor affecting interactions between herbivore community members. However, their role in spatial and temporal competition between organisms at the third trophic level has received little attention so far. The fitness of insect parasitoids, that use herbivore larvae as a host for their offspring, is directly affected by host quality. Induced plant defences that affect the quality of the herbivore host will affect parasitoid performance. Parasitized herbivores may, however, elicit different responses in plants than unparasitized herbivores. Thus, parasitoids may affect plant quality mediated through their herbivorous host. Consequently, they can affect parasitoid larvae that occupy other herbivores on the same plant or those that are even temporally separated. As a result, competition between parasitoid species that use the same herbivorous host may be shaped by plant-mediated effects, similar to plant-mediated herbivore competition. This, however, has hardly received attention in the literature to date. To determine to what extent these mechanisms affect parasitoid competition, we propose to study competition between parasitoids in a multidisciplinary approach by integrating molecular and chemical profiling of plant responses with assessments of performance and behaviour of parasitoid wasps under laboratory and field conditions. |
