Phyllosphere bacterial communities in milkweeds: composition diverge as the season progresses, and links to cardenolides and arthropods depend on host identity

Leaf bacterial communities shape plant defense and interactions with herbivores, yet how host filtering and stochastic processes assemble them remains unclear. We predicted that host identity, chemistry, and arthropods structure them, and that stochastic and deterministic contributions shift seasonally. Across one growing season we sampled leaf bacteria monthly and arthropods weekly on four Asclepias species (Apocynaceae: A. curassavica, A. incarnata, A. syriaca, A. tuberosa) spanning a cardenolide gradient. We sequenced 16S rRNA genes and quantified selection and dispersal contributions to turnover using null models. Host identity shaped bacterial richness and composition, with A. tuberosa hosting the richest communities, and both richness and phylogenetic diversity rose through the season. Early on, homogenizing dispersal made communities more similar among plants; by mid-season no single process dominated turnover, and by late season dispersal limitation prevailed. Homogeneous selection was episodic, not sustained, and arthropod associations were host-specific, negatively so on A. syriaca. Phyllosphere assembly thus shifts from convergence to divergence in one season: young leaves recruit from a shared pool delivered by wind, rain, and arthropods, whereas exchange among ageing plants declines and communities drift apart. Herbivores arriving late meet plant-specific microbial environments, so microbial mediation of herbivory should be host-specific and seasonally contingent.