Keys to preimaginal stages of the five species of aphidophagous syrphids more frequenly found indifferents crops and pastures in Argentina: Allograpta exótica (Wied.), Syrphus phaeostigma Wied., Platycheirus stegnus Say, Pseudodorus clavatus (Fabricius) and Ocyptamus argentinus (Curran), were made fora quick identification of these species in ecological studies. It was not possible to make keys to eggs sincethese are not easily separated with a naked eye. Both larvae and pupae present stable color patternscharacteristics of each species and in the case of larvae, characteristics patterns of each instar inside eachspecies. The following natural enemies were found in the surveys: one entomopathogen (Entomophtoramuscae), four parasitoids (Diplazon laetatorius (Fabricius), Callaspidia sp., Encyrtus sp. and an eggparasitoid) and a predator spider, Thomisus sp. (Araneae, Thomisidae).