COS 62-2 - Phenological shifts and the indirect effects of simulated herbivory on Asclepias incarnata and its pollinators

Abstract: Climate change can cause the phenology of insect herbivores and plants to advance asynchronously, resulting in phenological mismatches. Insects emerging at different times and shifts in peaks of insect abundance can alter both the timing on onset and peak intensity of herbivory. Herbivory can lead to changes in both floral and non-floral traits, making plants less attractive to pollinators. While previous studies have examined the effects of the timing and intensity of herbivory on plants and pollinators separately, our research seeks to address the gap in knowledge by exploring the interaction between the two. In this experiment, we manipulated the timing and intensity of herbivory on swamp milkweed at the University of Michigan Biological Station (UMBS) from May to August, 2022. We grew 90 plants in a randomized complete block design with 15 blocks. Each block contained six treatments: 1) early and low herbivory, 2) late and low herbivory, 3) early and high herbivory, 4) late and high herbivory, 5) no herbivory (control), and 6) natural herbivory. Plants in the early or late timing treatment were damaged on June 22nd or July 6th, respectively. Plants in the low or high intensity treatment had 5% or 50% of each leaf manually removed, respectively. We measured various floral and non-floral traits, the number and the identity of pollinator visitors. Our main findings demonstrate that (1) when the onset is early, high-intensity of herbivory results in the most open flowers; however, when the onset is late, low-intensity of herbivory results in the most open flowers, and (2) more open flowers increase pollinator visitation frequency and richness. Additionally, plants exposed to low herbivory produced the smallest flowers. This experiment serves as an example of how changes in herbivory patterns can scale up to affect plant and pollinator communities. Further studies are needed to assess responses in other traits, systems, and natural populations.