Tree defoliation by winter moth Operophtera brumata L. during an outbreak affected by structure of forest landscape

Abstract

Tree defoliation by winter moth Operophtera brumata L. (Geometridae, Lepidoptera) caterpillars was quantified in the Białowieża Forest (E Poland) during the recent outbreak (2002). To assess effects of forestry management on defoliation, comparisons were made between mixed species ( Tilio– Carpinetum) old-growth stands of natural origin in strictly protected primeval and managed fragments. This forest type was continuous in the primeval part, but highly fragmented in the managed part. The old-growth remnants in the latter part were usually surrounded by younger tree stands (often coniferous plantations). We predicted the winter moth densities would be reduced in fragmented stands due to dispersal mortality incurred by the early stage larvae. As predicted, all species of host trees suffered greater defoliation in the primeval forest. Hornbeam Carpinus betulus L. was the heaviest defoliated host. Other trees, which developed leaves synchronously with hornbeam – maple Acer platanoides L., lime Tilia cordata Mill., and pedunculate oak Quercus robur L. – were strongly affected as well. While we do not show a causal mechanism, greater dispersal mortality in fragmented stands is a most plausible explanation for the observed patterns of defoliation.