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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.

International audience; Lombardy poplar (Populus nigra cv ‘Italica’) has been planted all over Europe, mostly as single trees or in rows along roads or around squares. This clone represents a unique opportunity to study local effects (e.g. associated community diversity) on a single genotype. We investigated the effect of local environmental conditions (biogeographical species pools, site conditions, surrounding vegetation) and clines in environmental conditions (climate, latitude) on the diversity of associated organisms, notably insect species. Pairs of trees were selected on at least five sampling sites (mixture of rural and urban) in each of the following countries: France, Germany, Hungary, Italy and Poland. Fifty leaves per tree were selected randomly once a month from May until September and evaluated for damage by six functional groups of insect: chewers, skeletonizers, leaf miners, sucking and sap feedings, gall makers and leaf rollers. Insect species were also determined for leaf miners and gall makers. There was no significant difference in damage levels between pairs of trees at each site, but significant differences were found among sites and among countries for the means of leaves damaged by each functional group. The same leaf miner species were found in the five counties but at different frequencies. The dominant gall-maker species was the aphid, Pemphigus spyrothecae, in all countries; however both species composition and dominance varied strongly among sites. Remarkably, species diversity (Shannon-Wiener Index) of gall makers and leaf miners was higher on urban sites than on rural sites.

National audience; Des recherches récentes ont montré que la diversité génétique des arbres peut conditionner l’assemblage des communautés associées et ainsi façonner l’évolution des écosystèmes. Les arbres peuvent héberger une importante communauté d’insectes et certains de leurs traits fonctionnels (débourrement végétatif, surface foliaire...) peuvent avoir une influence majeure sur la consommation des insectes. L’objectif de cette étude est d’évaluer comment l’herbivorie interagit avec la diversité génétique intraspécifique de huit clones de peuplier noir du bassin de la Loire. Nous avons installé en avril 2009 un dispositif expérimental en milieu naturel au sein de la ripisylve à peuplier noir de l’ISS Loire (intensive study site, http://www.evoltree.eu/), comprenant 120 parcelles unitaires de 6 plants, en combinaison variable de 1 à 6 génotypes. Des dispositifs similaires ont été installés en Hongrie, Italie et Pologne dans le cadre du réseau d’excellence européen Evoltree. L’herbivorie a été mesurée par estimation visuelle des dommages foliaires de six guildes fonctionnelles d’insectes : brouteurs, squeletteurs, mineuses, enrouleurs, galligènes, piqueurs-suceurs. La réponse des insectes a été analysée en tenant compte du niveau de diversité et de la composition des parcelles. Les premiers résultats du dispositif de Loire montrent un effet positif de la diversité, principalement expliqué par une plus forte sensibilité des parcelles les plus diversifiées génétiquement.