Functional role of hedgerows and landscape composition in controlling a forest insect pest

Mixed forests exhibit greater resistance to pests compared to monospecific forests. However, stand-level diversification is challenging for managers. An alternative approach is to enhance tree diversity at the landscape scale. Building on the agricultural landscapes with hedgerows, we hypothesized that in a pine plantation landscape broadleaved hedgerows along pine stands locally increase tree diversity, while broadleaved stands increase landscape level diversity. According to the associational resistance concept, both methods should reduce pest damage. We tested these hypotheses in a pine plantation landscape (dominated by Pinus pinaster), by counting nests of the pine processionary moth (Thaumetopoea pityocampa, hereafter “PPM”) along 36 pine edges adjacent or not to broadleaved hedgerows, in landscapes with low or high broadleaved cover. PPM bird and bat predators activities were assessed using acoustic devices. At the local level, PPM nests were 42% less abundant in pine edges adjacent to taller hedgerows. This reduction was attributed to the ability of hedgerow to disrupt PPM host tree location and to the predation by great tit (Parus major), which was more active in pine edges adjacent to hedgerows. At the landscape level, higher broadleaved cover resulted in 72% reduction of PPM infestation. Although broadleaved cover did not significantly affect the activity of PPM predators along pine edges, PPM nest abundance decreased with increasing species richness of bird predators consistently with the natural enemies hypothesis. Planting broadleaved hedgerows, combined with the conservation of broadleaved remnants, represents an effective management strategy for mitigating PPM infestation in pine plantation landscapes.