Changing phenological cues can lead to trophic mismatch for plants and herbivores, and this often shifts herbivore feeding to plant stages of lower quality. Temperature can also mediate how herbivores respond to plant quality, leading to temperature-by-phenology interactions. We examined how both temperature and host plant phenology impact aphid abundance and their mutualism with ants. Our study system was composed of aphids (Aphis asclepiadis) that colonize flowering stalks of the host plant, Ligusticum porteri. Abundance of this aphid species is dependent on mutualism with several ant species. To understand how host plant phenology and temperature affect aphid abundance, we experimentally accelerated snow melt date by two weeks, which correspondingly advanced flowering phenology. Then, we factorially combined this phenology treatment with open top warming chambers surrounding aphid colonies. We tracked aphid colony growth and interactions with ants, and results showed the greatest colony growth at cooler, ambient temperatures on host plants without accelerated phenology. These colonies also showed the highest levels of honeydew deposition relative to their overall size. Our findings show that trophic mismatch decreases aphid abundance, and changes to the ant-aphid mutualism exacerbate this effect.

In October of 2018, we marked eight, 16 m² plots of L. porteri in a subalpine meadow near RMBL with PVC poles. Each plot was randomly assigned to either the ambient or early snow melt treatment. In April of 2019, we then placed shade cloth (EasyShade 50% Black Bulk Shade Cloth UV Resistant) across plots assigned to the early snow melt treatment. We removed the shade cloth on May 16th, 2019. Individual plants were marked with stake flags, and we tracked flowering phenology for all host plants with flowering stalks (N=81). On July 26^th, 2019, we created experimental aphid colonies by adding 10 field-collected apterous (wingless) aphids to the terminal inflorescences of 6 plants in each plot (n=48). We did not include host plants whose flowering stalks were broken, spontaneously colonized by aphids, or senescent. During colony establishment, we excluded ants and flying predators by enclosing the aphids in a fine-mesh bag and creating a stem barrier below each colony (Tree Tanglefoot, Contech Enterprises, Marysville, OH). Half of the colonies in each plot (n=24)  were randomly assigned to experimental warming, and the colonies were surrounded by an open-top warming chamber that elevated temperatures by 1.3°C. We removed mesh bags on July 29^th and counted the number of aphids and ants every two to three days until August 5^th, 2019. Another set of colonies remained bagged with ants excluded. On August 1^st, we quantified honeydew production from six randomly selected aphid colonies in each treatment combination (N=24). We placed 100 cm² squares of aluminum foil around the host plant stem of each colony. The foil squares remained in place for 24 hrs, during which time ants were excluded as described above for colony establishment. We counted the number of honeydew droplets from digital images of the foil squares using a dissection microscope.

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