Intercropping Alters Phytochemical Defenses Against Insect Herbivory

Given the multiple possible mechanisms for interspecific chemical interaction between adjacent heterospecific plants, phytochemical defenses of pest-susceptible crop species could potentially be enhanced or altered by intercropping with phytochemically diverse neighbors. We assessed the influence of intercropping between phytochemically diverse plants on aerial volatile organic compound (VOC) emission profiles by intercropping Melilotus alba and Triticum aestivum with Silphium integrifolium in AMF-inoculated soil. We also assessed the impact of intercropping on induced plant defenses by conducting an in-situ, no-choice bioassay with Spodoptera frugiperda. Of eight compound classes we identified across the three plant species, prenol lipids (terpenoids) were upregulated in silflower plants when monocropped with wheat and when herbivory was induced. Carboxylic acids and organooxygen compounds were reduced in sweetclover when intercropped with silflower, but increased under herbivory. Uninfested wheat plants emitted more organooxygen compounds and fatty acyls than infested plants when intercropped with silflower, but not when monocropped. This study showed that VOC emissions of plants from three diverse taxa are altered by both intercropping and herbivory in ways that may impact their resistance to insect herbivory. Further research into the role of intercropping on pest resistance in agroecological systems could help farmers to design intercropping systems that optimize natural plant herbivory defenses, thus improving agricultural sustainability.