When living in close proximity, competing organisms can also indirectly interact with each other through shared enemies. While such interactions have often been documented in the field, the role of trait plasticity in generating or moderating associational effects is unknown. Plasticity may be particularly important in indirect interactions between plants, which modify their phenotypes in response to both competitors (shade avoidance) and herbivores (induced resistance). Two native wetland species, spotted Joe Pye weed (Eutrochium maculatum) and common boneset (Eupatorium perfoliatum), share a common suite of herbivores including the specialist leaf beetle Ophraella notata, but differ in defenses. Using two summers of observational work across wetlands in upstate New York, I explored patterns of indirect interactions between Joe Pye weed, boneset, and O. notata. I also manipulated neighbor proximity and plant defenses in a common garden to evaluate the relative effects of of association and plasticity on herbivore abundance and distribution.
Results/Conclusions
Over two seasons of field surveys, Joe Pye weed consistently experienced associational susceptibility to O. notata oviposition when near boneset. Boneset appeared to experience associational resistance near Joe Pye weed in 2016, but this effect was not observed in 2017 surveys. Similarly, in a common garden Joe Pye weed experienced associational susceptibility when planted near boneset, but Joe Pye weed only conferred resistance to boneset in certain spatial contexts. To test the hypothesis that induction of plant defenses could mitigate associational susceptibility and enhance associational resistance, jasmonic acid was applied to focal plants. Induced defenses only reduced O.notata oviposition on boneset in mixed-species environments. On the other hand, jasmonic acid induction led to increased numbers of larvae across all neighborhoods, demonstrating conflicting roles of induced plant responses for herbivore preference and performance. Results from the common garden suggested that toxic neighbors and induced defenses may produce similar patterns of resistance, and that associational effects for preferred hosts are likely to depend on the spatial context of mixed-species assemblages.