From induced resistance to defence in plant-insect interactions

Plant-insect interactions typically take place in complex settings of interactions among multiple trophic levels as well as multiple species in each trophic level. The complex interaction network may strongly impact on extrapolations of resistance traits to have a defensive function. For example, the induced response plants express to their current attacker often enhances resistance to that attacker, but may make a plant more susceptible to attack by another herbivore. Hence, the defensive function or plant fitness benefit of the response to a single attacker may be misinterpreted from pairwise interactions. Moreover, plant physiological responses to a first stress by herbivory may hamper the response to a second stress and lead to conclusions of maladaptation in plant defence responses. In light of the entire community of attackers and beneficial organisms the plant interacts with, the susceptibility to some attackers may be a consequence of adaptations that reduce fitness costs of herbivory when considering the full sweep of species that affect plant fitness. A similar argumentation may apply for indirect resistance in which predators or parasitoids dampen the effect of herbivores on plants. Plant volatiles that attract third trophic level organisms such as parasitoids may at the same time attract enemies of the parasitoids in the fourth trophic level, hyperparasitoids, which again dampen the effect of parasitoids on herbivores. In addition, the effectiveness of predators and parasitoids may be dependent on habitat complexity. Here, I plea for studies on the full plant-associated community to understand the fitness outcome of an (induced) plant trait and hence coin it induced direct or indirect plant defence.