Summary Insect responses to recent climate change are well documented, but the role of resource specialization in determining species vulnerability remains poorly understood. Uncovering local ecological effects of temperature change with high‐quality, standardized data provides an important first opportunity for predictions about responses of resource specialists, and long‐term time series are essential in revealing these responses. Here, we investigate temperature‐related changes in local insect communities, using a sampling site with more than a quarter‐million records from two decades (1992–2009) of full‐season, quantitative light trapping of 1543 species of moths and beetles. We investigated annual as well as long‐term changes in fauna composition, abundance and phenology in a climate‐related context using species temperature affinities and local temperature data. Finally, we explored these local changes in the context of dietary specialization. Across both moths and beetles, temperature affinity of specialists increased through net gain of hot‐dwelling species and net loss of cold‐dwelling species. The climate‐related composition of generalists remained constant over time. We observed an increase in species richness of both groups. Furthermore, we observed divergent phenological responses between cold‐ and hot‐dwelling species, advancing and delaying their relative abundance, respectively. Phenological advances were particularly pronounced in cold‐adapted specialists. Our results suggest an important role of resource specialization in explaining the compositional and phenological responses of insect communities to local temperature increases. We propose that resource specialists in particular are affected by local temperature increase, leading to the distinct temperature‐mediated turnover seen for this group. We suggest that the observed increase in species number could have been facilitated by dissimilar utilization of an expanded growing season by cold‐ and hot‐adapted species, as indicated by their oppositely directed phenological responses. An especially pronounced advancement of cold‐adapted specialists suggests that such phenological advances might help minimize further temperature‐induced loss of resource specialists. Although limited to a single study site, our results suggest several local changes in the insect fauna in concordance with expected change of larger‐scale temperature increases.