Age-dependent benefits of developmental plasticity: Thermal acclimation benefits erode rapidly upon adult emergence

Environmental temperature strongly influences insect development, survival, and population dynamics, yet the extent to which developmental temperature affects adult performance, and whether such effects are population-specific, reversible, or persistent, remains unclear. We examined the interactive effects of population origin, developmental thermal acclimation and adult age on adult thermal tolerance and its persistence in three populations of Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) originating from distinct climatic zones (Greece: Crete and Volos; Croatia: Zaton). Immature stages completed development at 20, 25, or 30°C, after which newly emerged adults were transferred to a common environment (25°C). Cold (CTmin) and heat (CTmax) tolerance of both sexes were assessed on days 1 and 6 of adult life. Developmental acclimation and population origin significantly affected adult CTmin and CTmax. Older adults were generally more tolerant to both thermal extremes than younger ones, irrespective of developmental acclimation; however, age-related responses varied among populations, particularly for CTmin, highlighting age as a critical factor in evaluating developmental acclimation effects of adult medflies. Thermal tolerance was positively correlated with developmental temperature at emergence, especially for CTmin, but variation disappeared in 6-days-old flies, indicating that developmental effects are transient rather than canalized. The results suggest a potential trade-off between plasticity and basal tolerance across adult ages, with cold tolerance being more plastic than heat tolerance. Developmental acclimation affected both sexes similarly and may confer advantages when developmental and adult thermal environments match, potentially enhancing seasonality and invasion success.