The effectiveness of indirect plant defence is dependent on plant competition

The net benefit of indirect defence strategies to plant fitness in natural conditions is likely dependent on the ecological context. Little attention has been given to the relationship between plant–plant competition and the effect of herbivore natural enemies on plant fitness. We conducted a field experiment to compare the effects of unparasitized Pieris brassicae and caterpillars parasitized in the laboratory by the solitary parasitoid Hyposoter ebeninus on the seed production of wild Brassica nigra plants grown in different densities. We found that H. ebeninus, which reduces the feeding behaviour of its host, increased plant seed 0000-0003-4220-9075 production compared to unparasitized herbivory, but only in high plant density when competing plants are damaged by unparasitized herbivores. To further investigate this relationship, we used a three-dimensional plant model to simulate different levels of herbivory, parasitism and plant density. Synthesis. Our results suggest that plant–plant competition is a critical factor that interacts with the effectiveness of indirect defence and should be considered in future studies on the evolution of plant defence strategies.