Environmental stressors can have unintended negative effects on beneficial organisms. Testing environmental effects of pharmaceutical residues is therefore often mandated in form of single-species ecotoxicological laboratory tests, yet differential sensitivity of species can ultimately disturb the entire community. A more realistic approach is to directly assess toxicity in the field to account for spatio-temporal variation in climate and habitat. We conducted the first field study at the landscape scale, replicated at 24 Swiss farms north of the Alps, to test the impact of a realistically low concentration (6.6 μg/kg fresh dung) of the popular parasiticide ivermectin on the beneficial cow dung insect community over the season. We found a graded negative impact of ivermectin on many but not all species, with significant reductions in ca. one third of 32 evaluated fly (Diptera) and wasp (Hymenoptera) taxa. No taxon benefited from ivermectin presence. Small hymenopteran parasitoids particularly decreased in numbers (62%), with large dipteran predators (38%) being reduced more than primary coprophagous dung consumers (23%). This supports theoretical expectations that the top trophic levels of a community are particularly vulnerable to environmental disturbance because their numbers tend to be low and their body size large, but also because pollutants can biomagnify in the food chain. Sensitivity to pollutants may further exhibit a phylogenetic signal. Ivermectin residues thus generally reduced the biodiversity of the dung insect community, but impact varied among farms, years and over the season, being lowest in autumn.We conclude that spatio-temporal replication of such higher-tier field studies is advised to capture the full extent of effects in realistic natural settings, as even low concentrations of a pharmaceutical can have varying impact on the different organisms comprising an ecosystem.