Trait-based approaches provide robust tools to understand how spatial and temporal environmental variability shapes community assembly and functional diversity. Here, we investigated the thermal tolerance of three cereal aphid species (Sitobion avenae, Rhopalosiphum padi, and Metopolophium dirhodum) across a 1200 km longitudinal gradient from western to eastern Europe, comparing autumn and spring populations. We tested whether: 1) eastern populations (experiencing larger seasonality and harsher winters) exhibited broader thermal tolerance ranges than western ones; 2) autumn aphids were more tolerant to cold and less tolerant to heat comparing with spring aphids; and (3) larger seasonality drove trait convergence within the aphid guild. Our results revealed that thermal tolerance varied across the longitudinal gradient, with autumn populations in eastern Europe with larger temperature variability displaying broader thermal ranges, supporting the Climatic Variability Hypothesis. However, spring populations exhibited a counter-gradient pattern, where aphids from regions with milder winter (western Europe) had higher cold tolerance than those from regions with harsher winter (eastern Europe), likely explained by different overwintering strategies between western (as active adults) and eastern Europe population (as diapausing eggs). Seasonal differences were pronounced: autumn aphids were less heat-tolerant than spring individuals. At the guild level, eastern populations exhibited trait convergence driven by large intraspecific variation, while western populations showed interspecific divergence, suggesting environmental filtering shaped thermal traits differently across climates. These findings highlighted the importance of seasonal and geographic context in thermal adaptation. Increasing climate variability may drive functional homogenization in ectotherm communities, potentially stabilizing populations but reducing long-term resilience. Our study underscored the need to integrate seasonal dynamics and intraspecific variation when predicting species responses to climate change, emphasizing how temperature variability - not just warming - reshaped insect communities.