Herbivore defense syndromes differ between native and invasive species

1. In their non-native ranges, invasive plant species may escape specialist herbivores and thus experience exclusive pressure from generalists. This is predicted to shape the defense strategies of successful non-native invaders, as proposed by the Shifting Defense Hypothesis. However, most research to date has quantified only limited sets of defense traits of invasive species, and thus our understanding of their defense syndromes, which encompass suites of both physical and chemical traits, remains poorly understood. We hypothesized that invasive species will exhibit defense syndromes tuned more to qualitative mechanisms that repel generalists, and less to quantitative mechanisms that deter specialists.2. We evaluated a comprehensive suite of leaf physicochemical traits, including quantitative structural defenses, qualitative chemical defenses, and nutritional properties, for 15 phylogenetically matched pairs of non-native invasive species and native congeners co-occurring in abandoned farmland in China. We coupled this trait screening with assessments of herbivore damage in a common garden and laboratory larval growth assays using a generalist herbivore (<i>Spodoptera littoralis</i>; Lepidoptera).3. Invasive species experienced 50.8% less herbivory and reduced larval growth rate by 25.7%, relative to native congeners. This increase in resistance corresponded with a distinct defense syndrome: invaders allocated more to broad spectrum, apparently qualitative, chemical defenses, exhibiting 43.1% higher phenolic concentrations, but less to quantitative structural defenses than natives. Hierarchical partitioning revealed that phenolics were the primary predictor of lower <i>Spodoptera</i> growth on invasives, whereas leaf C content was the best predictor for lower <i>Spodoptera</i> growth on natives.4. We found difference in defense syndromes between non-native invasive species and natives, and identified the defense strategies of both, and provide evidence that is consistent with predictions of the Shifting Defense Hypothesis.