Plant variation in susceptibility to herbivores is a long-standing puzzle in community and evolutionary ecology. Even when herbivores are abundant, plants in a population may vary widely in damage as a result of spatial variation in plant community composition. In close proximity, certain neighbors may attract or repel herbivores to a focal plant (“associational effects”), contributing to variation in herbivore susceptibility. However, nearby neighbors also interact with focal plants through sharing resources. Competition for resources is known to change plant phenotype in ways that affect herbivore susceptibility, yet the role of neighbor competition in associational effects is poorly understood. Using two wetland asters, spotted Joe Pye weed (Eutrochium maculatum) and common boneset (Eupatorium perfoliatum), and a shared herbivore (Ophraella notata), we investigated the contribution of resource competition to a pattern of associational susceptibility previously documented for Joe Pye weed growing near boneset in field populations. To do so, we manipulated belowground competition for resources (fertilizer) and observed beetle preference for Joe Pye weed growing near boneset, both in an open field experimental population and in contained greenhouse cage trials.
Results/Conclusions
In greenhouse and field trials, we repeatedly found that beetles laid more eggs, and performed more damage, on Joe Pye weed growing in the absence of belowground competition with boneset. Thus, reduced competition for nutrients led to greater susceptibility to herbivores. However, leaf protein did not differ between competition treatments, suggesting that beetles did not prefer plants due to a higher nutritional value. We expect that additional physiological changes, such as a reduction in plant defenses, may have played a role in greater herbivore preference for plants in less competitive environments. Broadly, our results suggest that competition for resources can modify the strength of associational effects, and is likely an important component of neighbor-mediated spatial heterogeneity in plant-herbivore interactions.