Methods:
Plants were grown in the UMBS chamber array under elevated (800 ppm) or ambient (400 ppm) CO2. Aphid colonies were established from a singular individual collected near Ann Arbor, MI. Cardenolide analysis was conducted using ultra high performance liquid chromatography (UPLC, Waters Inc., Milford, MA, USA) on a Phenonenex Luna C-18 column. Foliar carbon, nitrogen ratios were determined using an elemental analyzer; foliar phosphorus was determined using coloriometric assay through an acid digestion method.