Are associational effects driven by insect herbivore abundance or foraging behavior? A test of foraging decisions by grasshoppers (Order: Orthoptera)

The importance of associational effects—where the amount of insect herbivore damage received by a focal plant depends on the identity of its neighbors—is being increasingly recognized in many ecosystems. We tested four alternative hypotheses regarding how neighboring plants affect herbivory by grasshoppers on a focal plant: H1a: the herbivore repellent hypothesis, H1b: the selective grazing hypothesis, H1c: the decoy/dilution hypothesis, and H1d: the herbivore spillover hypothesis. We created experimental plant neighborhoods (10 m²) by manipulating plant density (three levels) and palatability (two levels) around a focal (palatable) plant species within two habitat types: post-agricultural and remnant longleaf pine habitats (n = 16 study sites). Grasshoppers (Order: Orthoptera, Family: Acrididae) are the dominant insect herbivore in this system and grasshopper abundance varies widely among our sites, allowing us examine the role of grasshopper abundance on damage to focal plants. We also employed foraging experiments to determine feeding decisions by the grasshopper Melanoplus angustipennis. By coupling experimental plant neighborhoods with grasshopper foraging experiments, this research provides insight into associational plant defense theory involving generalist herbivores by providing a predictive framework for when and where associational defenses are likely to depend on insect herbivore abundance or foraging behavior. Understanding associational defenses that are driven by foraging behavior rather than abundance, as is hypothesized to be occurring in the study system, may be particularly important because herbivores effects could be disproportionate to their abundance, which has clear implications for basic and applied ecology.