Abstract
Plant chemical defenses can negatively affect insect herbivore fitness, but they can also decrease
herbivore palatability to predators or decrease parasitoid fitness, potentially changing
selective pressures on both plant investment in production of chemical defenses and
host feeding behavior. Larvae of the fern moth Herpetogramma theseusalis live in and feed
upon leaf shelters of their own construction, and their most abundant parasitoid Alabagrus
texanus oviposits in early instar larvae, where parasitoid larvae lay dormant for most of host
development before rapidly developing and emerging just prior to host pupation. As such,
both might be expected to live in a relatively constant chemical environment. Instead, we
find that a correlated set of phenolic compounds shows strong seasonal variation both within
shelters and in undamaged fern tissue, and the relative level of these compounds in
these two different fern tissue types switches across the summer. Using experimental feeding
treatments, in which we exposed fern moth larvae to different chemical trajectories
across their development, we show that exposure to this set of phenolic compounds reduces
the survival of larvae in early development. However, exposure to this set of compounds
just before the beginning of explosive parasitoid growth increased parasitoid
survival. Exposure during the period of rapid parasitoid growth and feeding decreased parasitoid
survival. These results highlight the spatial and temporal complexity of leaf shelter
chemistry, and demonstrate the developmental contingency of associated effects on both
host and parasitoid, implying the existence of complex selective pressures on plant investment
in chemical defenses, host feeding behavior, and parasitoid life history.
