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Abstract Anthropogenic pollution affects environments differently depending on proximity to pollution source, exposure route, and species ecology. Thus, understanding organism’s ecological role and exposure route to contaminants is central to assessing pollution impact. Treated municipal wastewater releases contaminants into waterways and alters microbial communities. Plants absorb contaminants and expose animals through foraging and nest-building activities. Nesting ecology differences of ground vs wood cavity-nesting bees offers insight into niche-specific susceptibility to pollution. Because contaminants bind to soil strongly, ground-nesting bees near wastewater are likely most impacted, while wood cavity-nesting bees likely less impacted since plants’ ability to uptake contaminants are species dependent. We compared gut microbiomes of directly exposed soil-nestingHalictus ligatusand indirectly exposed wood-nestingCeratinaspp. upstream/downstream of wastewater. We collected bees, flowers, and soil, and analyzed their bacteria microbiomes (16S rRNA). Wastewater altered ground-nestingH. ligatusmicrobiome >18 times greater than wood cavity-nestingCeratinaadults.Ceratinalarvae and pollen provisions showed significant but smaller shifts. Conversely, soil and flower microbiomes remained stable, indicating higher resilience. These results demonstrate that exposure routes drive contaminants susceptibility, with animal-associated microbes most vulnerable. Because bees are important pollinators and biodiversity contributors, these disruptions point to broader ecological risks in increasingly contaminated landscapes. Abstract Figure 

Summary High temperatures (e.g., fever) and gut microbiota can both influence host resistance to infection. However, effects of temperature‐driven changes in gut microbiota on resistance to parasites remain unexplored. We examined the temperature dependence of infection and gut bacterial communities in bumble bees infected with the trypanosomatid parasiteCrithidia bombi. Infection intensity decreased by over 80% between 21 and 37°C. Temperatures of peak infection were lower than predicted based on parasite growthin vitro, consistent with mismatches in thermal performance curves of hosts, parasites and gut symbionts. Gut bacterial community size and composition exhibited slight but significant, non‐linear, and taxon‐specific responses to temperature. Abundance of total gut bacteria and of Orbaceae, both negatively correlated with infection in previous studies, were positively correlated with infection here. Prevalence of the bee pathogen‐containing family Enterobacteriaceae declined with temperature, suggesting that high temperature may confer protection against diverse gut pathogens. Our results indicate that resistance to infection reflects not only the temperature dependence of host and parasite performance, but also temperature‐dependent activity of gut bacteria. The thermal ecology of gut parasite‐symbiont interactions may be broadly relevant to infectious disease, both in ectothermic organisms that inhabit changing climates, and in endotherms that exhibit fever‐based immunity.