Winter moth populations are isolated on co‐occurring tree species with contrasting budburst‐phenology

Abstract

Abstract Differences between neighbouring tree species in phenology could isolate populations of host plant generalists that depend on matching the phenology of their host. We studied the relationship between the budburst phenology of two co‐occurring tree species with early (hornbeams) and late (oaks) budburst and the egg hatching date of associated winter moths (Operophtera brumata) during two seasons (autumns starting in 2020 and 2021). A previous study in spring 2019 had found no winter moth larvae on the focal oaks, while we found them mainly on oaks with hornbeam neighbours in 2022. Congruently, adult winter moths were rarely encountered during the autumns of 2018 and 2019 and sparsely in 2020 and 2021, and then mainly near hornbeams. The vast majority of winter moths had early hatching eggs when both hornbeams and oaks were present, matching the hornbeams rather than the oaks. Where hornbeam was absent in the neighbourhood, the few winter moths had late hatching eggs. These results suggest that winter moth populations on hornbeam and oak are to some degree isolated from each other despite spatial proximity, so that recolonization of oaks by populations synchronized with budburst of hornbeam is slow. Therefore, trees would benefit from differing from their neighbours in budburst phenology with respect to herbivore damage.