Temperature variability homogenized thermal responses in an ectotherm community along a European longitudinal gradient

Climate change, through rising temperatures, greater variability, and more frequent extremes, is reshaping insect phenology and thermal niches, with profound effects for pest outbreaks. Predicting these impacts requires a clear understanding of species and communities' responses across geographic gradients. We assessed thermal tolerance (CTmax, CTmin, CCRT) of three cereal aphid species Sitobion avenae , Rhopalosiphum padi , Metopolophium dirhodum from 30 populations along a 1200 km longitudinal gradient in Europe, comparing autumn and spring collections. We measured guild‐level functional diversity to assess thermal trait patterns along the longitudinal gradient. We tested whether 1) eastern populations experiencing greater seasonality and harsher winters exhibited broader thermal ranges, 2) autumn aphids were more cold‐tolerant and less heat‐tolerant than spring aphids, and 3) stronger seasonality facilitated thermal trait convergence within the guild. Across the longitudinal gradient, autumn populations in eastern Europe exhibited broader thermal ranges, supporting the climatic variability hypothesis (CVH). In contrast, spring populations displayed a counter‐gradient pattern, with stronger cold tolerance in the milder western winters, likely reflecting differences in overwintering strategies (active adults in the west versus diapausing eggs in the east). Additionally, autumn aphids were less heat‐tolerant than spring individuals. Eastern communities exhibited trait convergence driven by large intraspecific variation, whereas western communities showed interspecific divergence, indicating differential environmental filtering. Increasing climate variability may drive thermal traits homogenization (negative standardized effect size of functional diversity for all traits) in ectotherm communities. Climate change may intensify season‐dependent physiological changes, shift eastward the geographic range of aphids overwintering as active adults, and homogenize thermal niches, potentially altering pest dynamics and diminishing the effectiveness of integrated pest management strategies. Our study underscores the importance of integrating seasonal dynamics and intraspecific trait variations when predicting climate change responses, highlighting how changes in temperature variability – not just warming – may reshape ectotherm communities during the growing season.