Introduction

Much of earth's biodiversity is composed of species that feed upon plants, and in turn these herbivores are the prey base for carnivorous species. Food webs have been characteristically used to describe and examine the direct links among these trophic levels. As a consequence of feeding, however, herbivores cause many morphological and chemical changes in their host plants (Karban and Baldwin 1997, Agrawal et al. 1999). The induced changes that herbivores create indirectly provide new links among trophic levels. These indirect links add an additional level of complexity in understanding not only trait evolution within trophic levels, but also the evolutionary connections among levels within plant–herbivore–carnivore communities. In ecological time, we know that herbivore-induced changes in plants can facilitate indirect interactions such as those between plants and carnivores (Botrell et al. 1998). Parasitoid and predator species modify their searching behavior in response to these changes, and can learn to associate plant changes with herbivore location (Turlings et al. 1993, Vet et al. 1995). The use of herbivore-induced changes has been demonstrated for a number of parasitoid and predator species that attack insect pests of agricultural systems. For example, the aphid parasitoid Aphidius ervi is more attracted to broad bean plants that are infested with the aphid Acyrthosiphon pisum. Naïve female parasitoids are attracted to plant volatiles that are released as a consequence of aphid feeding (Du et al. 1998).