Context-dependency of plant-herbivore interactions

One key factor that may contribute to the context dependency of plant-insect interactions is associational effects—indirect plant-plant interactions mediated by herbivores. The outcomes of associational effects span the spectrum of ways in which neighboring plants can affect insect herbivory on a focal plant, from positive (i.e., associational resistance) to negative (i.e., associational susceptibility), but we lack a framework for understanding when positive or negative associational effects should arise and the mechanisms underlying these processes.

Phil Field Exp

Constructing herbivore-exclusion cages in South Carolina, USA.

My past work focused on two key mechanisms underpinning associational effects: insect herbivore abundance and insect foraging behavior. Specifically, I found that herbivory on a palatable plant species was related to the amount and type of neighboring plants, whereas insect abundance had only weak effects on herbivore damage to focal plants. These findings suggest that changes in plant cover, which were driven by land-use and fire history, can modify damage rates by altering foraging behavior rather than insect abundance (Hahn and Orrock 2015 Oikos). Moreover, in a subsequent study I coupled a field experiment with a behavioral lab assay to show that two mechanisms were responsible for providing associational resistance to a palatable plant when it was rare relative to an unpalatable plant: frequency-dependent foraging by grasshoppers on the less common (and more palatable plant) and reduced foraging activity in unpalatable neighborhoods (Hahn and Orrock 2016 Ecology). Together, this work has provided novel insight into the role of insect foraging behavior in generating associational effects, which can contribute substantially to the variation observed in insect-plant interactions.