Measuring the importance of neighbors: Using a model system to understand how neighboring plants and herbovire density influence herbivore damage

Neighboring plants of a different species or genotype can influence a focal plant’s vulnerability to herbivory. Such "associational effects” occur when herbivory on a focal plant, at a particular density, is a function of neighbor composition.  These effects underlie many pest management strategies including push-pull systems and companion planting. Nevertheless, our understanding of associational effects is limited by experiments which confound focal plant and neighbor plant densities and which do not consider herbivore density. Teasing apart these three factors—focal plant, neighbor plant, and herbivore densities—however, is necessary to better predict how neighbors contribute to yield, population dynamics, and evolutionary outcomes. Here we present results from a model system in which two seeds, black-eyed peas (Vigna unguiculata) and mung beans (V. radiata), share an herbivore, the cowpea weevil (Callosobruchus maculatus), to ask: (1) how do associational effects vary across focal plant and neighboring plant densities and (2) how does herbivore density influence the strength of associational effects? In these experiments we found that the presence of neighbors reduced the focal seed’s vulnerability to damage and that these effects diminished with increasing focal seed frequency.  Surprisingly, we also found that neighbors reduced the focal seed’s vulnerability to herbivory more when herbivore density was high. These results suggest that the effects of neighbors depend on the relative frequencies of neighbor and focal plants, as well as on the density of the herbivore. Benefits of mixed-crop pest management strategies, therefore, may be likely to vary as widely as the herbivore populations themselves.