Spore production decreases for non-farmers when mixed with farmers carrying their host-associated bacteria. We performed 10 sets of pairwise farmer:non-farmer mixes and found that non-farmers produce fewer spores per cell as the proportion of farmer in the mixture increases. Spore production for farmers is the same in all mixture conditions. Error bars equal s.e.m.; n=19. Significant differences found between spore outputs are designated by different letters as indicated by a post hoc Tukey’s HSD test.