A brassicaceae’s life : the eco-evolutionary dynamics of induced defence to multi-herbivore attack

Plants harbour diverse insect communities, from mutualistic pollinators to antagonistic herbivores. To defend against herbivores, plants have evolved a myriad of defence strategies. How plants evolved these defence strategies against multiple types of herbivores that feed simultaneously or sequentially (i.e. cross-resistance) is poorly understood. This thesis aimed to bridge the gap between the ecology and evolution of cross-resistance to herbivores, to explain how plants evolved defence strategies against multi-herbivory. To achieve this, I assessed plant defence strategies and insect community assembly across the Brassicaceae plant family. Chapter 2 reviewed which ecological factors affect plant induced responses to sequential herbivory by performing a meta-analysis. The results revealed that sequential herbivory generally leads to induced resistance by reducing herbivore performance but not herbivore preference. Moreover, plants managed to reduce damage but did not reduce biomass loss effectively. The lack of studies reporting on plant reproductive output show a serious knowledge gap into the cost of induced responses to sequential herbivory and the consequences for plant defence evolution. Chapter 3 studied the evolution of induced responses to sequential herbivory. In 83 plant species of the Brassicaceae family, I assessed macroevolutionary patterns in constitutive, induced and cross-resistance by exposing the plants to either single or sequential attack by two herbivores (Myzus persicae and Pieris brassicae). Cross-resistance related positively to induced resistance, indicating that plants responding strongly to P. brassicae remained resistant even with previous herbivory by another species. This suggests that the variation in resistance to sequences of herbivores is mainly explained by the evolution of resistance to specific herbivore species. Chapter 4 assessed how induced responses to the early-season herbivore Pieris rapae can affect plant fitness through changes in the insect community and plant growth. I manipulated P. rapae presence by either excluding them or by manually infesting plants with P. rapae caterpillars. The legacy effects of early-season herbivory affected the subsequent insect community and plant growth, resulting in a combination of direct and indirect effects on plant fitness. These findings indicate that the herbivore community as a whole could shape the evolution of plant defences rather than each single herbivore alone. Chapter 5 assessed how the evolutionary history and mating system of a plant help to explain variation in the insect community and discuss how this may influence plant defence evolution. I performed a common garden experiment with 26 annual Brassicaceae species with varying resistance to common herbivores on which we monitored the herbivore and pollinator communities and recorded plant growth, leaf and flower traits. The results revealed that showed that evolutionary shifts in mating system not only affect inducibility but has consequences for the entire community. Chapter 6 discusses how the findings in the different chapters contribute to understanding the ecological interactions that have been shaped by plants’ adaptations against herbivores and how current interactions may shape future plant defence evolution. Finally, I provide an outlook on future research to further entangle the complexities of eco-evolutionary feedback between plants and insects.