Agricultural intensification shapes community composition and parasite prevalence of native but not introduced bumble bees

Bumble bees are important pollinators of crops and native plants, yet face widespread declines linked to agricultural intensification and parasite pressure. Here, we investigate how landscape-scale agricultural management and bumble bee community composition shape microparasite prevalence across an intensification gradient in southwestern British Columbia, where the commercially reared Bombus impatiens has recently become naturalized. We characterized landscape-scale agricultural management, surveyed bumble bee communities, and assessed the prevalence of four parasite genera ( Apicystis spp., Ascosphaera spp., Crithidia spp , and Vairimorpha spp (previously known as Nosema )) in foraging bumble bees. Bumble bee species richness was higher in landscapes with greater edge density, while richness, native bee abundance, and native bee parasite prevalence all declined with increasing high-bush blueberry cultivation. In contrast, B. impatiens abundance and parasite prevalence were insensitive to landscape characteristics, although higher B. impatiens abundance was associated with decreased Crithidia prevalence in co-occurring native bumble bees. We propose several non-mutually exclusive mechanisms to explain these patterns. Decreased parasite prevalence in blueberry-dominated landscapes may reflect landscape-scale dilution of foraging activity across flowers, reducing transmission at shared floral contact sites, and/or post-bloom nutritional stress reducing host tolerance and increasing mortality of infected foragers. Decreased Crithidia prevalence in native bees at sites with more B. impatiens may be the result of dilution driven by a numerically dominant, less competent host: if B. impatiens acquires and transmits Crithidia less readily than native species, its dominance of floral interaction networks may reduce per-visit transmission risk for native bees. Alternatively, resource competition with abundant B. impatiens may generate nutritional deficits that increase parasite-induced mortality in native bees, reducing observed prevalence. Together, these results highlight asymmetric vulnerability to landscape change across the bumble bee community, with agricultural intensification shaping the composition and disease dynamics of native species in particular.