Elevated Atmospheric CO₂ Modifies Symbiont-Associated Dispersal Behavior Under a Standardized Assay in an Aphid Clone

Rising atmospheric CO₂ levels may alter behavioral dynamics that mediate species interactions, a largely overlooked dimension of ecosystem functioning. Aphids offer a tractable model for examining these modifications, as their primary protection from natural enemies relies on the alarm pheromone (E)-β-farnesene (Eβf). Facultative aphid endosymbionts like Candidatus Hamiltonella defensa complicate this dynamic by modifying parasitoid resistance and associated behaviors, yet their interaction with elevated CO₂ remains largely unexplored. Using a single clone of the pea aphid Acyrthosiphon pisum (Harris), we investigated how CO₂ levels and H. defensa D3 status, independently and interactively, altered Vicia faba L. plant size, aphid fecundity, and aphid dispersal responses to Eβf. Here we show that dry shoot weights of bean plants grown under elevated CO₂ increased, compared to those grown under ambient conditions. Irrespective of CO₂ level, however, aphids harboring H. defensa produced significantly more offspring, and dispersed at significantly higher rates in response to Eβf than symbiont-free individuals. Under the standardized Eβf assay, the difference in dispersal between H. defensa-harboring aphids and those that were symbiont-free was large under ambient CO₂ (59% vs. 9%) and smaller under elevated CO₂ (23% vs. 12%). Thus, elevated CO₂ reduced symbiont-associated differences in dispersal under standard assay conditions in this aphid clone with implications for tri-trophic interactions, biological control efficacy, and symbiont prevalence under future atmospheric conditions.