No evidence of fine-scale local adaptation of winter moths to variable tree phenology.

Spatial variation in plant phenology can impose strong selective pressures on herbivorous insects whose fitness relies on synchrony with host plants, promoting local adaptation to host timing. Winter moths (Operophtera brumata) have been shown to synchronize egg hatching with host budburst, but whether this reflects local adaptation remains unclear. We used three complementary approaches to assess small-scale local adaptation of winter moths to oak phenology in Wytham Woods, UK, a 385-hectare woodland with repeatable variation in individual oak budburst phenology. We experimentally investigated whether host tree phenology predicts hatch timing using common gardens across multiple temperatures, evaluated fitness benefits of synchrony using translocations, and assessed population structure and gene-environment associations using whole-genome sequencing. We found no support for local adaptation to individual trees. Common garden experiments revealed systematic differences in hatch timing which were unrelated to host budburst, while translocations indicated no fitness consequences of asynchrony. Genetic analyses showed no detectable population structure or association with budburst timing. Local adaptation to host phenology therefore appears not to arise on individual trees but may instead occur at broader spatial scales. Understanding the scale of local adaptation is essential for predicting how insect-plant synchrony will respond to environmental change across heterogeneous landscapes.