After spring, after crops: which alternative hosts for the generalist parasitoid Diaeretiella rapae (Hymenoptera: Braconidae)?

Abstract

Generalist aphid parasitoids seem to behave as specialists at the local scale. The temporal pattern of host exploitation by parasitoids is key to understand this result. We investigated the temporal dynamics during twenty-one consecutive months of host exploitation by Diaeretiella rapae (M’Intosh) (Hymenoptera: Braconidae), one of the most generalist aphid parasitoid, in cultivated and uncultivated areas. We applied two complementary approaches: molecular detection of parasitism within living aphids and rearing of aphid mummies collected in Brassicaceae crops and adjacent areas. We built a Maximum Likelihood tree to determine the influence of host plant and aphid species on the genetic structure of D. rapae. We confirmed the existence of a cryptic species developing on Hayhurstia atriplicis (L.) (Hemiptera: Aphididae), but we found no other host-related genetic variation. D. rapae exploited almost exclusively Brevicoryne brassicae (L.) (Hemiptera: Aphididae), a pest of Brassiceae and rarely other hosts. When the crop is absent, D. rapae parasitized B. brassicae on wild Brassicaceae and cover crop instead of alternative aphid host on other plants. These results show the limited role of uncultivated habitats as a reservoir for D. rapae and the interest of wild Brassiceae and cover crops to enhance the control of B. brassicae.