Plant diversity effects on insect herbivores and their natural enemies: current thinking, recent findings, and future directions

• The magnitude of variation in plant traits, as well as the combined influence of key plant and consumer traits determines plant diversity effects on associated insects.
• Understanding the influence of diversity on higher trophic levels will require determining how plant-based variation modifies the strength of trophic interactions.
• Plant diversity effects on higher trophic levels in many cases involve feedbacks mediated by direct and indirect effects.

A rich body of theory has been developed to predict the effects of plant diversity on communities at higher trophic levels and the mechanisms underpinning such effects. However, there are currently a number of key gaps in knowledge that have hindered the development of a predictive framework of plant diversity effects on consumers. For instance, we still know very little about how the magnitude of plant trait variation (e.g. intra-specific vs. inter-specific), as well as the identity and combined effects of plant, herbivore and natural enemy traits, mediate plant diversity effects on consumers. Moreover, the fine-scale mechanisms (e.g. changes in consumer behaviour or recruitment responses) underlying such diversity effects in many cases remain elusive or have been overlooked. In addition, most studies of plant diversity effects on associated consumers have been developed under a static, unidirectional (bottom-up) framework of effects on herbivores and predators without taking into account the potential for dynamic feedbacks across trophic levels. Here we seek to address these key gaps in knowledge as well as to capitalize on recent advances and emerging frameworks in plant biodiversity research. In doing so, we provide new insights as well as recommendations which will stimulate new research and advance this field of study.