Mediation of herbivore attack and induced resistance by plant vigor and ontogeny

A large number of insect galls induced by Contarinia sp. (Cecidomyiidae) on cashew plants, Anacardium occidentale L. (Anacardiaceae), and induced resistance (hypersensitivity) against galling were observed in five restored different-aged stands in the Amazonian tropical rain forest. We tested three hypotheses: (1) the effect of age-dependent changes on the attack by Contarinia sp. and on induced resistance of A. occidentale to herbivory (plant ontogeny – herbivory hypothesis); (2) the effect of leaf size on the oviposition preference by the gall-midge (plant vigor hypothesis), and (3) whether past attack could influence future attack and induced resistance (attack prediction hypothesis). Tree age positively influenced attack levels and gall density. The leaves of older trees experienced four-fold greater attack and supported two-fold more galls. Hypersensitive response was also positively affected by tree age. This induced resistance was six-fold higher on older trees. Therefore, we suggest that induced resistance in A. occidentale was age-dependent, hence supporting the plant ontogeny – herbivory hypothesis. Higher preference of Contarinia sp. on larger sized leaves of A. occidentale was only observed in old stands, hence providing support for the plant vigor hypothesis. The same trend was observed in hypersensitive response. Only two older plots (5–7-year-old) were better predictors of current attack and resistance of A. occidentale, hence supporting the attack prediction hypothesis. Our results suggest that plant development is an important factor that contributes to the structuring of interactions between host plant and insect herbivores. However, more information about ontogenetic changes and regeneration processes is needed to understand plant–herbivore interactions in the Amazonian forest.