A major limitation of current reports on insect declines is the lack of standardized, long-term, and taxonomically broad time series. Here, we demonstrate the utility of environmental DNA from archived leaf material to characterize plant-associated arthropod communities. We base our work on several multi-decadal leaf time series from tree canopies in four land use types, which were sampled as part of a long-term environmental monitoring program across Germany. Using these highly standardized and well-preserved samples, we analyze temporal changes of diversity and relative biomass in communities of several thousand arthropod species belonging to 23 orders using multilocus metabarcoding and quantitative PCR. Our data do not support widespread declines of α-diversity or genetic diversity within species. Instead, we find a gradual community turnover, which results in temporal and spatial biotic homogenization. Colonization by novel widespread taxa and loss of range-restricted taxa cause spatial diversity loss. At the same time, increasingly similar species occurrences between consecutive sampling years cause a temporal decline of diversity. These effects are independent of land use and observed for all arthropod orders. Our results suggest that insect decline is more complex than mere α-diversity loss, but can be driven by β-diversity decay across space and time.