Data from: Food-plant-associated predation risk and its impact on larval development and adult morphology in a specialist butterfly

Predation influences herbivore behavior, physiology, and reproductive
success through both direct consumption and non-lethal effects arising
from predation risk alone. Plant traits, including defensive traits and
architecture, can also influence herbivores both directly and indirectly,
either by affecting feeding efficiency or by affecting herbivores'
risk of predation. Yet, it remains unclear how predators and plant traits
interact to shape the developmental outcomes of larval herbivores,
particularly under field conditions. To understand larval performance
within a tritrophic framework, we investigated how predation risk and
food-plant identity influence caterpillar development and subsequent adult
morphology using the monarch butterfly (Danaus plexippus) as a focal
species. We compared responses to predation risk in monarch
caterpillars feeding on two common species of western milkweed, Asclepias
speciosa and Asclepias fascicularis, which differ in various traits,
including defensive traits and architectural complexity. Using an outdoor
common garden experiment in two growing seasons, caterpillars were
assigned to plants of each of the two species with or without a
predator-exclusion cage. Interestingly, we observed trends of slower
weight gain in caterpillars feeding on A. speciosa than in those feeding
on A. fascicularis, but only when caterpillars were exposed to predators.
Additionally, in the first year of our study, predator exposure increased
caterpillar development time on A. speciosa, but not on A. fascicularis.
Predator exposure on A. speciosa was associated with reduced larval weight
gain in the second year of our study, and because larval weight gain
strongly predicted adult wing area, this pattern suggests a potential
indirect effect of predators on wing size. In contrast, predator exposure
had no detectable effect on wing morphometrics of adults that had
developed on A. fascicularis. Our findings highlight that larval
food-plant species can modulate the magnitude of predator effects at
multiple life stages of an insect herbivore. This demonstrates that
predator-exposure induced developmental changes can depend on the identity
of the larval food-plant and can carry forward to affect adult morphology,
with potential implications for the fitness of migratory insects.