Predation Experiment
For the predation trials, I deployed four different treatments of
artificial larvae in a 2 x 2 factorial design: eyespotted in leaf rolls,
eyespotted on open leaves, non-eyespotted in leaf rolls, and
non-eyespotted on open leaves. I affixed prey to individual host plants,
interspersed by both treatment and host plant species (S. albidumor L. tulipifera ). I generated unique treatment assignments for
each trial. During deployment, I selected the artificial prey’s location
on the plant haphazardly and measured its height (in cm) from the
ground. Prey were placed 11.0-281.0 cm high on plants (mean: 83.5, SD:
44.1cm), comparable to the heights of live P. troilus prey I
observed in the field (25-164cm, mean: 83.8, SD: 40.5cm; supplemental
table 1).
For the open leaf treatment group, I attached artificial prey to the
adaxial side of fully expanded host plant leaves by poking the loose
wire-ends of each prey through the leaf, then twisting them tightly
around the midrib. For the prey in leaf rolls, I attached them to the
leaf in the same way, then folded the leaf over the artificial prey and
secured it shut with a strip of Scotch double-sided tape (Fig. 1e). I
positioned all prey with the “head” pointed up towards the leaf
petiole, which reflects this species’ typical resting position (Fig. 1a,
1f). During Trial 2, I also included a fifth treatment group of
eyespotted prey in leaf rolls, oriented down away from the petiole, to
test the effect of eyespot orientation on predator perception. However,
as I was unable to confidently determine whether predators perceived
this treatment group as eyespotted or not, I excluded data on these prey
from the final analysis. Before the start of the trial, I took a photo
of each artificial caterpillar in place.
I collected artificial prey after approximately 5 days of exposure
(mean: 121 hours, SD: 8 hours). At the end of each trial, I visually
inspected prey for evidence of predation, using the pre-trial
photographs as a baseline for non-attacked prey. Avian and mammalian
attacks are clearly distinguishable by the shape of the bite-marks in
the clay (see supplemental figure 3). I recorded any missing prey items
(that could not be found after carefully scouring a
1m2 area around the original location) as attacked by
an unknown predator. I photographed all recoverable prey with visible
attack marks. In a few cases (n = 9), either the artificial caterpillar
or the entire leaf roll fell from the plant with no sign of predator
damage; these prey were excluded from analysis. I also excluded one
artificial caterpillar in which a live P. troilus caterpillar had
crawled into a leaf roll, and three artificial prey where the plant
could not be found (and thus the clay caterpillar could not be
recovered). In total I deployed 809 artificial caterpillars. Of those,
data from 659 artificial caterpillars are included in the analysis,
excluding prey attacked by non-avian predators. Because birds are common
visual predators of insects (Nyffeler et al. 2018), avian
predation patterns are the most informative (relative to mammalian or
unknown predators) for the goals of this study (Hossie and Sherratt
2012, 2013). Additionally, as expected, mammalian and unknown predators
showed little variation in predation between experimental treatments
(see supplemental figure 4). As it is difficult to detect evidence of
arthropod attacks using clay caterpillars, these types of predators were
not considered in this experiment.