Decades of historical outbreak cycles in a multivoltine insect reveal a plastic response in developmental phenology 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 over years, we expect a delay in the onset of winter. For organisms where temperature is the primary driver of development rate, warming environments mean faster development. If temperature-dependence in the development rate of individuals does not also change, this will cause a mismatch between seasonal timing of the overwintering stage and the onset of winter. Ectotherms can adapt their phenology in the face of climate change to maintain correct timing by: i) evolution of the development thermal reaction norm, and ii) plastic response in their development to multiple environmental cues. Here we use high resolution records of the smaller tea tortrix (Adoxophyes honmai) over multiple decades across 9 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 1C across Japan, and our analyses show that populations slowed their developmental duration by as much as 20%. Growth rates measured from a common garden experiment reveal that the change is likely not due to evolution. Our results suggest that plasticity in developmental thermal reaction norms may explain how the phenology of some ectotherms will respond to climate warming in natural systems.