Interactions between plants and herbivores occur within rich ecological communities. This can lead to associational effects - increased or decreased herbivore pressure in the presence of certain neighbors. In Chapter 1, I hypothesized that inducible defenses offer a cost-saving response to the unpredictable nature of neighbor effects. To test whether inducible defenses could mitigate the effects of bad neighbors, I manipulated neighbor presence and induced defenses in a common garden, after documenting a broad pattern of neighbor effects amongst two plant species (Eupatorium perfoliatum and Eutrochium maculatum). My experiment showed inducible defenses effectively eliminated the dominant pattern of negative neighbor effects for one of these plants, suggesting that this ‘plastic’ phenotype may be an important mechanism to cope with spatial heterogeneity in enemy attack. In plants, close proximity to neighbors also corresponds with competition for space and abiotic resources. To test whether resource limitation affected plant susceptibility to enemies, in Chapter 2 I manipulated belowground competition for resources and observed beetle preference in different neighbor treatments. I found that when plants had greater access to resources, “bad neighbor” effects were exacerbated, with insects moving from one species (the primary host) to the other more quickly. Increased nutrient availability had surprising effects, causing greater insect attraction to both plant species despite an accompanied increase in physical defenses (trichomes). While a single neighboring species or genotype can have significant impacts on plant susceptibility to insect parasites, so too can the broader diversity of surrounding species. Over nineteen meta-analyses have evaluated plant diversity impacts on insect herbivores and their predators, from native plants to crops. In Chapter 3, I review this literature, identifying important predictors of variation in insect responses such as plant spatial arrangement, feedbacks between trophic levels, and insect diet breadth. I also explore remaining gaps after three decades of meta-analysis. Improving our understanding of what generates variation in plant diversity effects is of fundamental importance for evolutionary ecology, as well as for the management of plants that serve as important resources for society.