Species-rich plant communities may be more resistant to invasive herbivores because of reduced host-plant accessibility and increased natural enemy diversity and abundance. We tested these hypotheses in Corsica, a Mediterranean island recently invaded by the maritime pine bast scale, Matsucoccus feytaudi Duc., which causes widespread tree mortality in Pinus pinaster Ait. The endemic Matsucoccus pini Green infests Corsican pine, Pinus nigra laricio Poiret, where it is controlled by the native predatory bug Elatophilus nigricornis Zetterstedt. As revealed by kairomone trapping, E. nigricornis was most abundant in pure Corsican pine in areas not yet colonized by M. feytaudi, and in pure maritime pine its density decreased with the distance from the nearest Corsican pine forest. The abundance of M. feytaudi was compared in five pairs of pure maritime pine and mixed maritime and Corsican pine stands. It was consistently higher in pure than in mixed maritime pine stands, whereas E. nigricornis showed the opposite pattern, and relative differences were correlated with the proportion of Corsican pine in the mixture. The predation by E. nigricornis was manipulated in pure maritime pine stands using synthetic attractants of the predator. Matsucoccus feytaudi density was significantly reduced in maritime pines baited with kairomone dispensers.

a b s t r a c tThe 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.

A major unknown in the context of current climate change is the extent to which populations of slowly migrating species, such as trees, will track shifting climates. Niche modelling generally predicts substantial northward shifts of suitable habitats. There is therefore an urgent need for field-based forest observations to corroborate these extensive model simulations. We used forest inventory data providing presence/absence information from just over a century (1880–2010) for a Mediterranean species (Quercus ilex) in forests located at the northern edge of its distribution. The main goals of the study were (i) to investigate whether this species has actually spread into new areas during the Anthropocene period and (ii) to provide a direct estimation of tree migration rate. We show that Q. ilex has colonised substantial new areas over the last century. However, the maximum rate of colonisation by this species (22 to 57 m/year) was much slower than predicted by the models and necessary to follow changes in habitat suitability since 1880. Our results suggest that the rates of tree dispersion and establishment may also be too low to track shifts in bioclimatic envelopes in the future. The inclusion of contemporary, rather than historical, migration rates into models should improve our understanding of the response of species to climate change.