Herbivore populations are regulated by plant defenses and natural
enemies. While plant defense can suppress herbivore populations, these
defenses adversely affect natural enemies thereby releasing herbivores
from top-down control. Over their lifespans, herbivores and their
natural enemies may experience substantial variation in plant defense.
Defense variability can suppress the growth of herbivores, but the
impacts of defense variability on natural enemies and top-down control
of herbivores are unknown. We independently manipulated the mean and
variation of a plant toxin experienced by individual Trichoplusia
ni caterpillars and its parasitoid Copidosoma floridanum.
Increases in the mean toxin concentration, but not its variance,
experienced by individual T. ni and C. floridanum
decreased the fitness of C. floridanum, whereas both mean and
variance impacted T. ni fitness. Thus, increased defense
variability for individual herbivores suppressed herbivore fitness with
no perceptible cost to top-down control. However, impacts of variability
depend heavily on scale of variability.