A parsimonious Adaptive Dynamics model of insect partial bivoltinism and phenology

Understanding selective pressures on life history strategies is crucial for predicting insect responses to climate change. In seasonal environments, insects must synchronize reproduction and development with favourable seasons and induce overwintering diapause to survive harsher conditions. With changing climates, precise tuning of seasonal life history strategies, such as diapause timing and the optimal number of annual generations, becomes essential. This study proposes a baseline lifecycle model to theoretically examine the evolution of life history timing and generational transitions. Using the Adaptive Dynamics (AD) framework, we explore the feedback between population dynamics and life history strategy evolution. The AD model reveals diverse evolutionary scenarios based on environmental conditions and demographic parameters. Adaptive responses in life history strategies, such as the fraction of first-generation individuals reproducing in the current season, and phenological strategies, such as developmental duration and reproduction timing, describe the patterns of univoltine, bivoltine, and partial bivoltinism in insect populations. Insects in less conducive environments with low maximum temperatures evolve an exclusively univoltine lifecycle. As temperatures and season lengths increase, the univoltine lifecycle shifts to partial bivoltinism, and eventually to full bivoltinism. Notably, full bivoltinism may also occur under short growing seasons if maximum temperatures are sufficiently high. Demographic performance, particularly fertility and within-season survival rate, influences the selection direction on voltinism. The model indicates a smooth transition between uni- and bi-voltinism, with partial bivoltinism occurring across a broad range of parameter conditions. Ongoing environmental changes may thus lead to increased multi-voltinism and consequently outbreaks of insect pests and disease vectors. These results provide a theoretical baseline for understanding the adaptive evolution of insect life-history strategies in seasonal environments, consistent with previous studies on generational patterns. However, the model relies on strong simplifying assumptions, such as deterministic seasonality and the absence of stochasticity, and has not been empirically validated; therefore, the conclusions are exploratory rather than predictive and will require future empirical testing to assess their real-world applicability.