Delphacodes kuscheli Fennah is the most efficient vector of Mal de Río Cuarto virus (MRCV) maize disease in Argentina, which is caused by a persistently transmitted fijivirus. Delphacodes kuscheli is abundant in a wide area of Central Argentina (32–35uS), affecting oat, wheat and maize crops, and weeds. All five immature instars of D. kuscheli are described, keyed and illustrated. A key is provided to highlight the main features for identification of the fifth-instar nymphs of three other Delphacid species on maize. Adults of D. kuscheli were reared Ander controlled conditions in individual glass cages containing oat plants. The description of each instar was based on 1-day-old nymphs. Coloration pattern, shape of frons, length of the rostrum segments, and length, shape and number of spur teeth are useful features for identification of the fifth nymphal instars of these species.Fil: Marino, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Entomología; ArgentinaFil: Brentassi, Maria Eugenia. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Entomología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Toledo, Andrea Vanesa. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Estudios Parasitológicos y de Vectores. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Estudios Parasitológicos y de Vectores; Argentin
