Field Experiments on Common Milkweed Support Hypothesized Synergies Between Plant Defense Traits

Photo 1. Freshly hatched swamp milkweed beetle (Labidomera clivicollis; pictured here) or monarch caterpillar (Danaus plexippus; not shown) larva were placed on common milkweed plants (Asclepias syriaca) in the field to assess the effect of defense traits on herbivore performance. Photo credit: Collin B. Edwards. Photo 2. For each A. syriaca plant used, we measured a variety of defense traits, including the quantity of latex exuded from severed leaf tips (left), and the density of trichomes visible under a microscope (right). Photo credits: Collin B. Edwards. Photo 3. We evaluated the effects of defense traits on herbivore survival and growth. Top, a D. plexippus larva has been glued to the leaf with latex and later died; bottom, a different larva survived and began to consume its host plant. Our study found evidence that D. plexippus mortality was increased by a synergy between latex production and trichome density, and between latex and concentrations of toxic cardenolide compounds. Photo credits: Collin B. Edwards. Photo 4. Many other arthropods use A. syriaca as a food source or habitat, including from left to right, top to bottom: (A) ants (Formica sp.), (B) sweat bees (Augochlora pura), (C) wolf spiders (Lycosidae), (D) red milkweed beetles (Tetraopes tetrophthalmus), and (E) lady beetles (Cocinellidae) and oleander aphids (Aphis nerii). Defense against multiple herbivores is another possible benefit of expressing multiple defense traits. Photo credits: Collin B. Edwards. These photographs illustrate the article “Plant defense synergies and antagonisms affect performance of specialist herbivores of common milkweed” by Collin B. Edwards, Stephen P. Ellner, and Anurag A. Agrawal published in Ecology. https://doi.org/10.1002/ecy.3915