Herbivory and Nectar Traits in Asclepias: Effects of Leaf Damage on Pollen Movement

Pollinators place plants under selective pressures, changing variables associated with attraction such as nectar. However, herbivory may also have consequences for floral traits, which could then affect pollination services. Flowers offer nectar to pollinators as a reward for visiting the plant, so nectar is presumably under selective pressures by those pollinators. Those differences in nectar variables can impact pollinator behaviors and overall reproductive success of the plant. In this study, we examined whether simulated herbivory altered nectar traits and pollen movement in two milkweeds, Asclepias exaltata and Asclepias syriaca. Several nectar traits, such as volume produced and overall sugar concentration, were measured and we compared across species and treatments. This study found that A. syriaca had significantly more insertions than A. exaltata. Significantly more nectar was produced in plants that had been herbivorized, and the volume of nectar produced per flower differed significantly by species and by date. There were no significant differences in nectar sugar concentration. These results show little change in nectar traits and pollen movement under stressful conditions, which is contrary to what we predicted. Understanding how variation in nectar affects pollinator behavior is critical to understanding pollination, a key ecosystem process