Variation in thermal sensitivity of diapause development among individuals and over time drives life history timing patterns in an insect pest
Physiological time is important for understanding the development and seasonal timing of ectothermic animals, but has largely been applied to developmental processes that occur during spring and summer such as morphogenesis. There is a substantial knowledge gap in the relationship between temperature and development during winter, a season that is increasingly impacted by climate change. Most temperate insects overwinter in diapause, a developmental process with little obvious morphological change. We used principles from the physiological time literature to measure and model the thermal sensitivity of diapause development rate in the apple maggot fly Rhagoletis pomonella, a univoltine fly whose diapause duration varies substantially within and among populations. We show that diapause duration can be predicted by modeling a relationship between temperature and development rate that is shifted towards lower temperatures compared to typical models of morphogenic, non-diapause development. However, incorporating interindividual variation and ontogenetic variation in the temperature-to-development-rate relationship was critical for accurately predicting fly emergence, as diapause development proceeded more quickly at high temperatures later in diapause. We conclude that the conceptual framework may be flexibly applied to other insects and discuss possible mechanisms of diapause timers and implications for phenology with warming winters.