Insect herbivory response to Populus nigra genetic diversity

International audience; Recent research suggests that genetic diversity in tree population may shape associated species assemblages and then drive ecosystem processes. Trees can support large numbers of insect species and some functional traits of trees have been shown to have major influence on insect communities and resultant damage. Studying the response of insect herbivore populations to the genetic diversity of tree populations is therefore a relevant approach to test community genetics hypotheses. The objective of our study was to evaluate how insect herbivory is affected by genotypes of Populus nigra, a dominant native poplar species in riparian forests of Europe. In a field experiment, we planted eight native P. nigra clones in plots of increasing genetic diversity with combinations of one to six clones. Insect herbivory was measured using visual estimation of leaf damage by six functional guilds of insects: chewers, skeletonisers, leaf miners, gall makers, leaf rollers, sap suckers and sap feeders. Response of insect herbivory to increasing genotypic diversity was analysed by considering the net diversity effect which was further divided in a complementary and a selection effects. Overall we observed slightly higher insect herbivory in poplar clone mixtures than in clone monocultures. This pattern was due to two opposite mechanisms. There was a positive complementary effect, with higher associational susceptibility in more genetically diverse mixtures. On the other hand, the selection effect also increased with the genetic diversity as the more diverse clone mixtures were more likely to comprise insect resistant clones.