Oranges and apples? Plant responses to insect herbivores of distinct feeding guilds

Plants face a diversity of biotic challenges in the field. To understand plant responses to multiple herbivory challenges, chemical ecologists generally employ arbitrary herbivore loads in factorial presence-absence treatments of two species, or additive density designs. These experiments do not disentangle herbivore species-specific responses of plants from responses to the experienced herbivory pressures.

We suggest a novel conceptual approach for determining functionally equivalent loads of each herbivore species, exerting on the plant similar herbivore pressures. We calibrated equivalent loads of caterpillars and aphids for similar damage to cabbage seedlings, coupled with the quantification of plant chemical responses, including plant hormones, volatile organic compound profiles, and glucosinolates.

Vegetative growth decreased linearly with the loads of each herbivore species, producing a one-to-one mapping between equivalent herbivore loads. Consequently, differences between plant chemical responses to the herbivores could be attributed to the identities of the herbivore species, therefore removing the common confound of unstandardized herbivore pressures in multiple herbivory experiments.