Variation among years in seasonal weather patterns can cause variation in the timing of phenological events of species. Co-occurring species oftentimes differ in their phenological responses to year-specific conditions, which can alter the ontogenetic stage at which these species interact. However, we do not have a good understanding of how these phenological shifts affect the outcome of interactions. In this study, we performed an experiment to determine how shifts in the seasonal timing of herbivory affect plant traits. Specifically, we manipulated the timing at which a plant (Narrow-leaf Milkweed, Asclepias fascicularis) was attacked by one of its specialist herbivores (Monarch caterpillar, Danaus plexippus) to determine effects on plant growth and reproduction.
We found that shifts in seasonal timing of monarch herbivory had a variety of effects on the milkweed host plants. Differences in plant size remained detectable even at the end of the growing season, months after the herbivory had occurred. We found effects of herbivory timing on some flowering traits (umbel number, umbel size, flowering duration) but not others (first flowering, mean flowering), and no effects on fruiting traits. Therefore, it appears that the effects of phenological shifts in this plant-herbivore interaction fade as the phenological stage of the plant becomes increasingly removed from the herbivory. While these results represent an important step toward identifying the qualitative and quantitative effects of phenological shifts on plant-herbivore interactions, more studies are needed to determine the generality of these findings. Determining the consequences of phenological shifts for species interactions is important for understanding the dynamics of seasonal communities as well as how these dynamics will be affected by climate change.