Macroevolutionary Trends in the Defense of Milkweeds against Monarchs Latex, Cardenolides, and Tolerance of Herbivory

Theory predicts that plants will increase their defenses against herbivores over time; however, several milkweed defenses have declined as the genus Asclepias has diversified. We review the evolutionary history of milkweed defense and provide new findings from an experiment on 53 Asclepias species. As predicted, monarch performance increased on more recently evolved species, and latex was primarily responsible for this effect. We next conducted focused analyses of eight Asclepias species, spanning early diverging and recently evolved species. We hypothesized that a decline in latex and cardenolides may be favored if (1) chemical defenses lose effectiveness due to sequestration, and (2) milkweeds evolve tolerance to herbivory in place of resistance traits. Monarch sequestration did not decline on derived Asclepias because they concentrated cardenolides from midto low cardenolide plants; nonetheless, derived milkweed species were more tolerant of herbivory because of enhanced investment in roots and the potential for clonal growth. The costs of producing latex and cardenolides, costs of sequestration for monarchs, and overall herbivore attack rates likely contributed to these macroevolutionary patterns.