To Grow or Defend? Effects of Competition and Herbivory on Common Milkweed, Asclepias syriaca

To gain a better understanding of the drivers of escalation in defenses in common milkweed, Asclepias syriaca, we tested two hypotheses: (1) increased plant density results in competition which lowers plant defenses to herbivory and (2) Natural and simulated herbivory should induce the same defensive responses in milkweed as long as the simulated herbivory is done in the same relative amount and to the same plant tissue as natural herbivory. Our study consisted of common milkweed grown at low and high densities, which were then subjected to natural herbivory by monarch caterpillars or simulated herbivory done by ripping of foliar tissue. Plant size and defenses were sampled three times within a 72 hour timeframe. Our density treatments showed that competition has significant effects on all measures of size. As expected, plants in the low-density treatments were consistently larger than those in the high-density treatment. For example, leaf area in the low-density was 44.47± 2.84 cm2 versus the high-density which was 27.81± 2.57 cm2 (P = 0.003). Height and basal stem diameter showed similar results. Plants in the high-density treatment had higher cardenolide concentrations than those in the low-density treatment after 24 and 48 hours, but this pattern reversed at 72 hours (P = 0.04). We also found that simulated herbivory induced significantly higher cardenolide concentrations than natural herbivory across both density treatments after 72 hours: Simulated = 7.20 ± 0.31, Caterpillar = 6.44 ± 0.31 μg mg-1 (P = 0.04)