Associational resistance and susceptibility: Having right or wrong neighbors

Associational resistance (AR) and associational susceptibility (AS) describe changes in plants due, to its proximity to another plant, that enhance or reduce resistance to herbivores relative to that observed in the absence of the association. Examples of AR and AS have been documented in managed and unmanaged habitats. However, a comprehensive review of these phenomena has yet to be undertaken. We present a synopsis of a review of AR and AS, focusing on some results of a meta-analysis. We scored a variety of plant and herbivore traits considered to be important variables determining AR and AS, including the habitat, palatability of the neighboring plant to the herbivore, degree of taxonomic similarity between neighboring and focal plants, herbivore feeding guild, herbivore type, herbivore host range, plot size, and plot arrangement. The results of the analyses depended on whether researchers used herbivore abundance or plant damage as predictor variables. We found that herbivore host range did not significantly affect the likelihood of detecting AR or AS. AR was found when the neighboring plant was unpalatable to the focal herbivore and AS when the neighboring plant was palatable. The influence of degree of taxonomic similarity was variable. Other traits are also discussed. We also focused on issues of particular relevance to agroecosystems. For example, AR and AS are equally likely in fields where neighboring plants are in the field or in areas adjacent to the field. Further, AR due to vegetational diversification is more evident in smaller plots than in larger plots.