Habitats in agroecosystems are ephemeral, and are characterized by frequent disturbances forcing pest species to successively colonize various hosts belonging either to the cultivated or to the uncultivated part of the agricultural landscape. The role of wild habitats as reservoirs or refuges for the aphid Sitobion avenae that colonize cultivated fields was assessed by investigating the genetic structure of populations collected on both cereal crops (wheat, barley and oat) and uncultivated hosts (Yorkshire fog, cocksfoot, bulbous oatgrass and tall oatgrass) in western France.Classical genetic analyses and Bayesian clustering algorithms indicate that genetic differentiation is high between populations collected on uncultivated hosts and on crops, revealing a relatively limited gene flow between the uncultivated margins and the cultivated part of the agroecosystem. A closer genetic relatedness was observed between populations living on plants belonging to the same tribe (Triti! ceae, Poeae and Aveneae tribes) where aphid genotypes appeared not to be specialized on a single host, but rather using a group of related plant species. Causes of this ecological differentiation and its implications for integrated pest management of S. avenae as cereals pest are discussed.