Early snowmelt reduces aphid abundance (Aphis asclepiadis) by creating water-stressed host plants (Ligusticum porteri) and altering interactions with ants

Declining snow cover is reshaping ecological communities. Early loss of snow cover initiates changes in key interactions that mediate herbivore abundance, i.e., top-down and bottom-up effects. In this study, we used a field experiment to test the effects of host plant water stress and phenology on the multitrophic interactions that determine aphid abundance. The aphid, Aphis asclepiadis, in our study system colonizes the flowering stalks of the host plant Ligusticum porteri and relies on a protection mutualism with ants. We added snow and water to replicate host plants and tested for a variety of phenological and physiological responses to these treatments. Relative to host plants in ambient conditions, both water and snow addition reduced key signals of water stress (senescence and abscisic acid levels) and increased seed set. While aphid colonies were generally larger with reduced host plant water stress, the ant–aphid mutualism interacted with plant quality in complex ways. Without ant tending, we did not detect differences in aphid colony growth with host plant treatment. When tended by ants, aphid colony growth was greatest on host plants with snow addition. Host plant quality also altered the benefits exchanged in this mutualism. Ant-tended colonies hosted by plants with snow addition produced honeydew enriched in trehalose, which may have decreased both ant and natural enemy abundance. Our results suggest that early loss of snow reduces aphid abundance by creating low-quality, water-stressed host plants, and this effect may be exacerbated by natural enemies and the costs of ant attendance.