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 investigate this relationship and found that the solitary parasitoid, <i>Hyposoter ebeninus</i>, which reduces the feeding behaviour of its host <i>Pieris brassicae</i>, positively impacts the seed production of wild <i>Brassica nigra</i> plants 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. Our results suggest that plant-plant competition is a critical factor that interacts with effectiveness of indirect defence and should be considered in future studies on the evolution of plant defence strategies.