Abstract
Herbivory‐induced responses in plants can both negatively affect subsequently colonizing herbivores and mitigate the effect of herbivory on the host. However, it is still less known whether plants exhibit specific responses to specialist and generalist herbivores in non‐secondary metabolite traits and how specificity to specialists and generalists differs between invasive and native plant populations. We exposed an invasive plant, Alternanthera philoxeroides, to Agasicles hygrophila (Coleoptera, Chrysomelidae; specialist), Spodoptera litura (Lepidoptera, Noctuidae; generalist), manual clipping, or application of exogenous jasmonic acid and examined both the specificity of elicitation in traits of fitness (e.g., aboveground biomass), morphology (e.g., root:shoot ratio), and chemistry (e.g., C/N ratio and lignin), and specificity of effect on the subsequent performance of A. hygrophila and S. litura. Then, we assessed variation of the specificity between invasive and native populations (USA and Argentina, respectively). The results showed S. litura induced higher branching intensity and specific leaf area but lower C/N ratio than A. hygrophila, whereas A. hygrophila induced higher trichome density than S. litura. The negative effect of induction on subsequent larval growth was greater for S. litura than for A. hygrophila. Invasive populations had a weaker response to S. litura than to A. hygrophila in triterpenoid saponins and C/N ratio, while native populations responded similarly to these two herbivores. The specific effect on the two herbivores feeding on induced plants did not vary between invasive and native populations. Overall, we demonstrate specificity of elicitation to specialist and generalist herbivores in non‐secondary metabolite traits, and that the generalist is more susceptible to induction than the specialist. Furthermore, chemical responses specific to specialist and generalist herbivores only exist in the invasive populations, consistent with an evolutionary change in specificity in the invasive populations.
Highlights
Perceiving and responding to attack by herbivores is an important trait of many plants (Karban & Baldwin, 1997)
Compared to the two abiotic stimuli, plants induced by the two herbivores did not differ from the clipped plants in total triterpenoid saponins concentrations, but plants induced by the S. litura and A. hygrophila exhibited triterpenoid saponins concentrations that were 30.0% and 22.0% higher than the jasmonic acid (JA) treatment on average, respectively (Figure 4a)
Invasive populations damaged by A. hygrophila had higher total triterpenoid saponins than either of the two abiotic treatments (38% for clipping and 62% for JA on average, respectively; Figure 6a), while there were no significant differences between S. litura and the two abiotic treatments
Summary
Perceiving and responding to attack by herbivores is an important trait of many plants (Karban & Baldwin, 1997). It is necessary to assess specificity of elicitation to herbivores with different diet breadths in a wide range of plant traits, especially in traits other than secondary metabolites. Some recent studies have indicated that invasive populations of exotic plants may have different specificity of induced response to specialist and generalist herbivores relative to native populations (Huang et al, 2010; Wang et al, 2013). We assessed specificity of induced response in a wide range of plant traits (fitness, morphology, and chemistry) and the potential differences of populations in the alligator weed, A. philoxeroides, which is native to South America, but has colonized large regions of the United States, Australia, and China (Julien, Skarratt, & Maywald, 1995). What is the difference in specificity of induced response between invasive and native populations of A. philoxeroides?
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