Defoliations by outbreak Lepidoptera are among the most large-scale biotic disturbances worldwide. The role of plant induced responses in outbreaks is, however, still unresolved. Ormiscodes amphimone (Oa) is a univoltine Saturniid moth native to southern South America that feeds from several plant species, but only occurs at outbreak densities in Nothofagus stands, where individuals suffer complete defoliations. Here we test the hypotheses that (1) outbreak susceptibility of Nothofagus species is related to inter and intra-specific differences in leaf palatability to Oa, (2) that the main host (Nothofagus pumilio; Np) compensates damage by producing herbivore-resistant new leaves after being totally defoliated, and (3) that evergreen Nothofagus are more herbivore-resistant than deciduous counterparts. We tested these hypotheses by collecting mature leaves from partially defoliated trees of two deciduous (Np and N. antarctica) and one evergreen species (N. betuloides) in an area experiencing an outbreak, new leaves formed after complete defoliation of Np, and mature leaves of the two deciduous species from undamaged populations. We estimated herbivore damage in the field and performed choice and non-choice feeding assays, measuring consumption rates as a proxy for herbivore resistance. We examined nitrogen, phosphorus, sugars, and polyphenol leaf-concentrations in order to account for differences in consumption.
We found that consumption rates (CR) were greatest in Np (deciduous), intermediate in N. antarctica (deciduous), and lowest in N. betuloides (evergreen). Similar patterns of damage were found in the field. CR were similar between damaged and undamaged populations of the two deciduous species. The best predictors of herbivory were leaf N and P concentrations in Np, which were higher than in the other species. Polyphenol concentrations, LMA and sugar concentrations differed between species and populations but held no apparent relation with CR. Complete defoliation of Np led to the production of new, N-rich leaves that were significantly less consumed than old leaves, albeit presenting similar polyphenol and P concentrations. Results support interspecific, but not intraspecific differences in resistance, show that new leaves formed after complete defoliation are more resistant than mature leaves, and agree with the hypothesis of higher resistance in evergreens than in deciduous species. As measured quantitative defenses did not translate into differences in CR, we suggest that feeding preferences of Oa respond more to nutritional (e.g. N, P) characteristics and to qualitative defenses arising due to a host phenological shift (e.g. leaf reflushing). Higher herbivore-resistance in new leaves may allow Np to compensate previous damage.