The role of natural enemies on the population dynamics of the rosy apple aphid, Dysaphis plantaginea Passerini (Hemiptera: Aphididae) in organic apple orchards in south-eastern France

Efficient pest control using conservation strategies requires sound knowledge of the dynamics of the pest and its natural enemies (NE). But the role of natural regulation under natural conditions is not easy to prove. The rosy apple aphid (RAA) Dysaphis plantaginea is the most detrimental aphid in organic apple orchards. Our aim was to determine whether and to which point the abundance and composition of arthropod communities associated with aphid colonies affects RAA population dynamics. The present study, based on weekly observations of marked and infested apple shoots was carried out from April to June in 2008 and 2009 in one experimental insecticide-free apple orchard and four organic commercial orchards located in south-eastern France. NE populations were the most diverse (7.3 taxonomic groups for 50 shoots) and abundant (1.44 individuals per shoot) in the experimental orchard compared to organic orchards (3.8 groups for 50 shoots and 0.7 individuals per shoot). The large RAA populations per shoot (more by 1.7-fold) observed in two of the five orchards were presumably related to a lower number of NE (less by 1.9-fold) and a high abundance of ants (more by 6.2-fold). Among the main NE groups, syrphids arrived first, followed by coccinellids and earwigs. Parasitism rates, mainly due to Ephedrus sp. (Braconidae), were between 0.66% and 6.93%. Based on their precocity and abundance, syrphids appeared to be the most efficient group of NE under the study conditions. By surveying the same shoots weekly we could demonstrate that RAA population dynamics were strongly affected by the presence of NE. This predatory effect was however not sufficient to prevent damage to the apple trees. New management practices aimed at favoring natural regulation, especially augmentative approaches, need to be developed.