Plant defence strategies to multi-herbivore attack

In terrestrial ecosystems, plants mediate many direct and indirect interactions between organisms. Interactions with insect herbivores are not only very common, but also represent a strong selection force on plant traits. There is extensive knowledge on plant defence against single or dual attack. However, how plants defend themselves against the attack of multiple insects is poorly understood. Therefore, the aim of my PhD project, was to explore how Brassica nigra plants deal with multi-herbivore attack by its most common herbivores.

The diversity of insects is reflected in the array of strategies with which they exploit plants. Studies on dual herbivore attack identified that defence to one attacker may cause energetic and physiological constraints to deal with a second attacker. How these constraints shape plant plasticity in defence to multi-herbivore attack and what defence strategies plants may deploy to their full community of attackers is a major knowledge gap in plant science. In chapter 2, I provide a framework for plant defence to multi-herbivore attack by defining the repertoire of plastic defence strategies that may allow plants to optimize defence against a multitude of stressors.

Plants have evolved plastic defence strategies to deal with uncertainty of when, by which species and in which order attack by herbivores will take place. However, due to antagonistic cross-talk between phytohormones, induced plant responses to current herbivore attack may come with a cost of compromising resistance to other, later arriving herbivores. The feeding guild of the initial herbivore is considered to be the primary factor determining whether resistance to subsequent attack is compromised. In line with the current paradigm of phytohormonal regulation of plant defence to insect herbivory, in chapter 3 I show that plant responses to chewers and phloem feeders are regulated via the SA and JA pathways, respectively. However, also in chapter 3, by investigating 90 pair-wise insect-herbivore interactions among ten different herbivore species, I show that resistance of the annual plant B. nigra to a later arriving herbivore species is not explained by feeding guild of the initial attacker. Instead, the prevalence of herbivore species that arrive on induced plants based on three years of season-long insect community assessments in the field explained cross-resistance. Therefore, plants tailor induced defence strategies to deal with common patterns of sequential herbivore attack and anticipate arrival of the most prevalent herbivores.

In chapter 4, I went one step further towards more natural conditions, and challenged plants with simultaneous attack by multiple herbivores. I subjected B. nigra plants to 51 treatments representing attack by an increasing species richness (1, 2 or 4 species) of either phloem feeders, leaf chewers, or a mix of both feeding guilds when keeping total density of attackers constant and studied how this affects plant resistance to subsequent attack by caterpillars of the diamondback moth (Plutella xylostella). I show that increased richness in phloem-feeding attackers compromised resistance to P. xylostella. In contrast, leaf chewers induced a stronger resistance to subsequent attack by caterpillars of P. xylostella while species richness did not play a significant role for chewing herbivore induced responses. Plant responses to all the chewers and to all the combination of chewers were regulated via the JA-pathway. The expression of the SA-responsive gene (PR1) was not induced by any of the phloem feeders, compared to expression levels of these genes in undamaged plants.

In chapter 5, I studied how insect herbivore species richness and trait composition of these species affect plant interactions with other insect community members (i.e. a herbivore, its parasitoids and pollinators) and whether these interactions affect plant fitness. I show that herbivore richness, its composition of functional traits and specific herbivore species affected the abundance of P. xylostella, the parasitism rate of P. xylostella and pollinator visitation to B. nigra plants. The composition of the introduced herbivore community affected plant fitness directly via reducing plant biomass that predicted resource investment into reproductive organs and these effects were stronger than fitness consequences of herbivore-induced plant-mediated interactions with other community members. All of these interactions involved effects of herbivores on pollinator visitation that closely correlated with the total number of seeds produced by a plant.

In chapter 4 and in chapter 5 I thus show that the richness of herbivores simultaneously attacking a plant influences a plant’s ability to defend itself against a subsequent herbivore and to maintain interactions with mutualistic members of the community. However, the question of how plants defend against the attack of multiple herbivores that arrive sequentially, remained unanswered. In chapter 6 I investigated whether B. nigra plants are able to defend themselves against caterpillars of the late arriving herbivore P. xylostella, when plants had been previously exposed to sequential attack by four other herbivores. I manipulated the order of arrival and the history of attack by four herbivores to investigate which patterns in sequential herbivory determine resistance against the fifth attacker. A sequence of attack with four episodes of attack by P. xylostella induced resistance to larvae of P. xylostella. The number of times the plant was attacked by herbivores of the same feeding guild, the identity of the first attacker, the identity and the guild of the last attacker as well as the order of attackers within the sequence of multiple herbivores influenced plant resistance to subsequent herbivory.

In conclusion, my thesis provides answers to important questions on plant responses to multi-herbivore attack. I show that plants tailor induced defence strategies to deal with common patterns of sequential herbivore attack and anticipate arrival of the most prevalent herbivores. Additionally, I show that B. nigra plants channel their defence responses stronger towards a feeding-guild specific response when under multi-species attack by herbivores of the same feeding guild, but integrate responses when simultaneously confronted with a mix of herbivores from different feeding guilds. Importantly, under field conditions, I show that herbivore richness, functional trait and herbivore identity affect plant fitness and plant interactions with a herbivore, parasitoids and pollinators. Finally, I show that history of sequential attack is an important factor determining plant resistance to herbivores.