Behavior and performance data of monarch butterfly caterpillars

Plants have evolved multiple defensive traits in response to herbivory; in turn, herbivore specialists evolved adaptive behaviors to avoid or tolerate such defenses. Here, we employ milkweeds (Asclepias spp.) to test two defenses, latex and trichomes, for their independent and interactive effects on behavior and performance of monarch caterpillars (Danaus plexippus). Latex exuded upon damage and the density of leaf trichomes positively correlate across milkweed species, suggesting they may have evolved together as synergistic defenses. Nonetheless, the complementary roles of these two traits have been little-studied. We focus on two behaviors: shaving, or the removal of trichomes, and chewing, which encompasses both deactivation of latex and leaf consumption. In an experiment with seven milkweed species, with and without manipulated latex flow, we found latex to be the primary determinant of reducing chewing, while both defenses positively predicted shaving behavior in the first instar. Next, we conducted a factorial experiment throughout the first three instars, manipulating latex and trichomes on a high latex, high trichome species, the woolly milkweed A. vestita. On plants with latex and trichomes intact, caterpillars spent the most time shaving and least time chewing of all treatment groups, suggesting a possible synergism. These defense-driven behavioral effects decreased later in larval development. Latex and trichomes both impacted monarch performance, additively increasing mortality and reducing growth of survivors. Thus, latex and trichomes represent two important plant defenses with effects on specialist herbivore behavior and implications for insect fitness.