Decades of historical outbreak cycles in a multivoltine insect reveal a plastic phenological response to climate change.

Many organisms overwinter in a specific life stage, which means their phenology must be well-timed with seasonal changes in the environment. As environments warm, we expect a delay in the onset of winter. For organisms where temperature is the primary driver of development rate, warming environments also mean faster development. If temperature dependence in the development rate of individuals does not also change, this will cause a mismatch between the seasonal timing of the critical overwintering stage and the onset of winter. There are two biological mechanisms by which ectotherms can adjust their phenology in the face of climate change to maintain correct timing: (1) organisms undergo evolution of the development thermal reaction norm and (2) organisms have a plastic response in their development to multiple environmental cues. Here, we use high-resolution records of densities of the smaller tea tortrix (Adoxophyes honmai) over multiple decades across nine locations in Japan to infer temperature-dependent changes in development rates over both time and space. The pest insect displays regular single-generation limit cycles, which provides a unique opportunity to infer changes in developmental rates directly from historical records of natural populations. The last half century has seen a temperature increase of about 1°C across Japan, and our analyses show that populations slowed development on average by 16% to maintain the correct timing of the overwintering stage. Development rates measured from common garden experiments reveal that the change is not due to evolution. Our results build on recent laboratory studies to suggest that there is substantial plasticity in developmental thermal reaction norms that may explain how the phenology of ectotherms could respond to climate warming in natural systems.