Tree species composition rather than diversity triggers associational resistance to the pine processionary moth

The reduction of insect herbivory is one of the services provided by tree diversity in forest ecosystems. While it is increasingly acknowledged that the compositional characteristics of tree species assemblages play a major role in triggering associational resistance to herbivores, underlying mechanisms are less well known. We addressed this question in the ORPHEE experiment by assessing pine processionary moth infestations (Thaumetopoea phyocampa) across a tree diversity gradient from pine monocultures to rive species mixtures. We showed that tree species richness per se had no effect on the probability of attack by this pest. By contrast, the infestation rate was strongly dependent on plot composition. Mixtures of pines (P/flits pinaster) and birches (Betula pendula) were less prone to T pityocampa infestations, whereas mixtures of pines and oaks (Quercus spp.) were more often attacked than pine monocultures. By taking into account the relative height of pines and associated broadleaved species, this effect could be explained by pine apparency. Pines were on average 343 + 5 cm height. Birches, as fast growing trees, were slightly taller than pines (363 6cm), while oak trees were significantly smaller (74 I. cm). Host trees of T pityocampa were then partly bidden in mixtures of pines and birches but more apparent in mixtures with oaks. We suggest that reduced pine apparency disrupted visual cues used by female moths to select host trees prior to oviposition. This study highlights the need to take into account tree traits such as growth rate when selecting the tree species that have to be associated in order to improve forest resistance to pest insects. (C) 2014 Gesellschaft fur okologie. Published by Elsevier GmbH. All rights reserved