Timing of short-term drought structures plant-herbivore dynamics

Drought is an important stressor that affects plant growth, survival and
physiology and, through plant responses, alters plant-herbivore
interactions and herbivore population dynamics. Short-term drought can
occur at different times during a growing season, affecting herbivore
populations and plants at various stages of development and growth. As
phenology influences drought response, drought timing could strongly
structure plant-herbivore interactions. We grew common milkweed plants
with or without its dominant aphid herbivore, allowing both plants and
herbivore populations to develop over time as in a typical growing season.
To determine how drought timing affects plants and aphid populations, we
applied short-term (1 week) drought at different times. Plants and aphids
were sensitive to drought timing, with a few weeks difference shifting the
effects of drought from minimal to causing massive declines in plant
growth, plant survival and herbivore population density. Aphid herbivory
added to or overrode drought consequences in plants, with different plant
fitness components responding distinctly to herbivory. Drought timing also
affected whether plant and herbivore responses were parallel or divergent,
suggesting that shifts in the seasonal timing of drought have the
potential to disrupt plant-herbivore interactions and affect the larger
ecological community. Our results show drought timing has important and
disparate consequences for herbivores and their host plants and indicates
the necessity of considering phenology and timing in assessing drought
response.