Résumé
Since the deployment of genetic resistant cultivars, several cases of breakdown have occurred rising the question of durability. Quantitative resistances are considered broad-spectrum and more durable than qualitative resistances but this has been poorly tested. We studied selection pressures exerted by qualitative and quantitative plant resistances in fields in the aim to understand how local adaptation of pest populations cou Id occur in agrosystems and to develop strategies for the management of resistances. Field trials were set up in three areas with different melon resistance combinations in a homogeneous genetic background, associating a major gene (the Vat gene) and quantitative trait loci (QTLs) known to affect plant acceptance in two clones of Aphis gossypii. Effects of the Vat gene and the QTLson the density and the genetic diversity of A. gossypii field populations were analyzed. Among the 138 multilocus genotypes (MLGs) identified in 2772 aphids collected, only 11 developed colonies. The Vat gene reduced density of aphid populations on melon crops in ail areas and resulted in changes of the clonai diversity and the genetic structure of aphid populations; the clonai richness was reduced to 6 MLGs in colonies collected on Vat-plants. Conversely, the QTLs did not affect aphid density and slightly affected genetic diversity. Along with selection, competition and local extinction probably condition emergence of certain MLGs on Vat-plants. Because of their narrow-spectrum, the QTLs tested did not reduce the risk of emergence of MLGs adapted ta the Vat gene when these MLGs were associated ta the Vat gene.