Enhancement of Diaretiella rapae by banker plants to control cabbage aphid | Role des plantes relais pour renforcer l'action de Diaeretiella rapae, principal parasitoide du puceron cendre du chou

Parasitoid action is studied in a cauliflower plot in the central Valais. The cauliflower main crop field is crossed in the center or bordered by beds of Savoy cabbage and turnip serving as banker plants for aphids and their parasitoids. The densities of healthy and parasitized (full mummies) cabbage aphid Brevicoryne brassicae and "green aphids", the green peach aphid Myzus persicae and the potato aphid Macrosiphum euphorbiae, were estimated using visual monitoring or plant sampling. The collected mummies were transferred to emerging cages to identify parasitoids and hyperparasitoids. The parasitoids all belong to the sub-family of Aphidiinae: Diaeretiella rapae, Aphidius matricariae, Aphidius ervi and Praon sp. and the hyperparasitoids to the families of Charipidae: Alloxysta sp. or Pteromalidae: Asaphes suspensus and Pachyneuron aphidis. The turnip revealed poor banker plant quality. The Savoy cabbage carried strong aphid populations which were already at the beginning of the season hosts of parasitoids. Until June parasitoids are made available for the surrounding crops. Later, this system becomes also a source of hyperparasitoids which are undesirable in biological control. Because of its specificity for the cabbage aphid D. rapae clearly dominated the parasitoids with generally more than 90/ of the individuals. Its action however is strongly affected by the hyperparasitoids, in particular by Alloxysta sp. The density of the hyperparasitoids can become so high that D. rapae is locally exterminated. In order not to facilitate the overwintering of the hyperparasitoids, banker plants and remainders of the main crop after its harvest should be destroyed. As D. rapae has good faculties of host prospection when those are still scattered and as its zone of host prospection is larger than that of Alloxysta sp., early sowing of banker plants allows D. rapae to establish on new aphid colonies before the arrival of the hyperparasitoids and to visit neighboring new crops. Parasitism was high enough to get heads of cauliflower free from cabbage aphids, provided that they were planted from mid May to beginning of June with a cropping period under 70 days, while D. rapae is still increasing whereas the cabbage aphid is already decreasing.