Effect of host tree density and apparency on the probability of attack by the pine processionary moth

The pine processionary moth (PPM, Thaumetopoea pityocampa) is the main defoliator of pines in the Mediterranean area, necessitating constant surveillance and regular pest management. A sound understanding of the spatial distribution of infested trees, both within and between stands, is required to increase management efficiency. We hypothesized that both host tree density at stand scale and tree apparency at individual tree scale were responsible for between- and within-stand patterns of PPM infestation. We tested these hypotheses on a sample of 171 maritime pine stands in the Landes de Gascogne, the largest plantation forest in Europe. We showed that PPM infestation (percentage of infested trees) decreased significantly with stand density, and was therefore greater in older than in younger stands. The probability of a pine tree being attacked increased significantly with tree height and proximity to the edge of the stand. Mortality rates of exposed sentinel egg batches did not differ with distance from the stand edge. We discuss three likely explanations for higher infestation of taller trees at stand edges: better survival of larvae on sun-exposed trees, and random interception vs. active host selection by gravid females. Our findings suggest that stand management could be adapted in order to decrease the risk of damage by the pine processionary moth, and that predictive tools for infestation dynamics can be based on forest growth models