An increasing number of studies on insect-plant relationships are providing a large volume of information indicating the variability and flexibility of species interactions that are important for improvement of biodiversity. Recent studies on plant-mediated species interactions of herbivorous insects have revealed that interspecific interactions are often indirect, asymmetrical and subtle, and that morphological, phenological, and chemical changes in the host plant alter the success of predation or parasitism by natural enemies. Even insects feeding at different times or on different parts of a plant may have a substantial effect on the quality or quantity of resources available to one another. Three-trophic-level interactions have recently received much attention in insect-plant interactions, addressing a significant role of natural enemies as part of a plant's battery of defenses against herbivores. It is also recognized that changes in host plant quality affect the efficacy of parasitoids or predators directly or indirectly by altering insect host location or vulnerability. Interactions between temporally separated guilds may play a critical role in distribution and survivorship, and thus the population dynamics of insect herbivores. There is evidence to support the view that one species attacking a host plant early in the season can change the performance or abundance of another species attacking late in the season, through changes in host quality. Similarly, this notion is applicable to the situation of spatially separated guilds that utilize different parts of the shared host plant in very different manners ; for example one attacking leaves and the other roots. Such interactions among temporally or spatially separated guild members sharing the same host plants are often asymmetrical.
