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Abstract 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. 

Abstract Ant‐hemipteran mutualisms are keystone interactions that can be variously affected by warming: these mutualisms can be strengthened or weakened, or the species can transition to new mutualist partners. We examined the effects of elevated temperatures on an ant‐aphid mutualism in the subalpine zone of the Rocky Mountains in Colorado, USA. In this system, inflorescences of the host plant,Ligusticum porteriCoult. & Rose (Apiaceae), are colonized by the ant‐tended aphidAphis asclepiadisFitch or less frequently by the non‐ant tended aphidCavariella aegopodii(Scopoli) (both Hemiptera: Aphididae). Using an 8‐year observational study, we tested for two key mechanisms by which ant‐hemipteran mutualisms may be altered by climate change: shifts in species identity and phenological mismatch. Whereas the aphid species colonizing the host plant is not changing in response to year‐to‐year variation in temperature, we found evidence that a phenological mismatch between ants and aphids could occur. In warmer years, colonization of host plant inflorescences by ants is decreased, whereas forA. asclepiadisaphids, host plant colonization is mostly responsive to date of snowmelt. We also experimentally establishedA. asclepiadiscolonies on replicate host plants at ambient and elevated temperatures. Ant abundance did not differ between aphid colonies at ambient vs. elevated temperatures, but ants were less likely to engage in tending behaviors on aphid colonies at elevated temperatures. Sugar composition of aphid honeydew was also altered by experimental warming. Despite reduced tending by ants, aphid colonies at elevated temperatures had fewer intraguild predators. Altogether, our results suggest that higher temperatures may disrupt this ant‐aphid mutualism through both phenological mismatch and by altering benefits exchanged in the interaction.