Weak and inconsistent associations between birch-feeding insects and pollution levels around two industrial towns during substantial emission decline (1988-2025).

Industrial emissions have historically altered terrestrial ecosystems, but their long-term effects on insect herbivores remain insufficiently understood, particularly during ecosystem recovery following emission decline. We analysed a dataset on the densities of 24 Recognisable Taxonomic Units (RTUs) of birch-feeding insects collected from 1988 to 2025 around two industrial towns in northwestern Russia previously identified as pollution hotspots. Volkhov originally hosted an aluminium smelter, and Syasstroy was home to a pulp and paper mill. Industrial emissions from these two towns formed pollution gradients that differed in pollutant composition but were originally comparable in their environmental effects, as indicated by Norway spruce vigour. A small subset of RTUs exhibited higher densities near both polluters during the high-emission years (1988-1996), and these patterns persisted until 2025, despite the subsequent reduction in emissions and apparent ecosystem recovery. Insect herbivore diversity was independent of pollution level during both high- and low-emission periods. At the same time, birch leaf quality for autumnal moth, Epirrita autumnata increased near polluters, whereas foliar damage by insects decreased. From 1988 to 2025, the densities of monitored insects showed no significant temporal trend in sites with any pollution levels. Our results demonstrate that moderate industrial pollution has limited and taxon- or guild-specific effects on insect herbivore communities, which often show inconsistencies between polluters and among the study years. Furthermore, substantial emission reductions do not produce long-term trends in the abundance of birch-feeding insects. These findings challenge the putative role of industrial pollution as a major driver of insect decline.