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.