Combined effects of arthropod herbivores and phytopathogens on plant performance

Summary Many plants are simultaneously attacked by arthropod herbivores and phytopathogens. These may affect each other directly and indirectly, enhancing or reducing the amount of plant resources they each consume. Ultimately, this may reduce or enhance plant performance relative to what should be expected from the added impacts of herbivore and pathogen when they attack alone. Previous studies have suggested synergistic and antagonistic impacts on plant performance from certain combinations of arthropods and pathogens, for example, synergistic impacts from necrotrophic pathogens together with wounding arthropods because of facilitated infection and antagonistic impacts from induction of pathogen resistance by sucking herbivores. We compiled published studies on the impact of plant–herbivore–pathogen interactions on plant performance and used meta‐analysis to search for consistent patterns of impacts among plant, herbivore and pathogen characteristics and experimental conditions, and to test the suggested hypotheses on synergistic or antagonistic impacts. None of the hypotheses based on proximate interactions between arthropods and pathogens were supported by our analysis; in contrast, the patterns we found were related to plant traits and experimental conditions. Our results suggest that immediate loss of resources from interactions between arthropod herbivores and pathogens is generally moderated by compensation to an extent where there are no interactive effects on plant performance. However, as interactive impacts also differed among environments and parasite manipulation methods, this suggests that the ability of plants to compensate such losses may depend on environmental conditions and probably also overall infection load.