Attraction of the specialist parasitoid Cotesia rubecula to Arabidopsis thaliana infested by host or non-host herbivore species

In this study we investigated whether in a two-choice set-up the parasitoid Cotesia rubecula ( Marshall) ( Hymenoptera, Braconidae) distinguishes between volatiles emitted by Arabidopsis thaliana ( L.) Heynh. ( Brassicaceae) infested with its host, Pieris rapae ( L.) ( Lepidoptera: Pieridae) and Arabidopsis infested with non-host herbivores. Four non-host herbivore species were tested: the caterpillars Plutella xylostella ( L.) ( Lepidoptera: Plutellidae) and Spodoptera exigua ( H,bner) ( Lepidoptera: Noctuidae), both chewing insects, the spider mite Tetranychus urticae ( Koch) ( Acari: Tetranychidae), which punctures parenchymal cells, and the aphid Myzus persicae (Sulzer) ( Hemiptera: Aphidoidea), which is a phloem-feeder. Compared with undamaged plants, C. rubecula females were more attracted to Arabidopsis plants infested by P. rapae, P. xylostella, S. exigua, or T. urticae, but not to plants infested by M. persicae. The parasitoids preferred host-infested plants to spider mite- or aphid-infested plants, but not to plants infested with non-host caterpillars ( P. xylostella or S. exigua). The data show that when Arabidopsis plants are infested with a leaf tissue-damaging herbivore they emit a volatile blend that attracts C. rubecula females and the wasps only discriminate between a host and non-host herbivore when the type of damage is different ( chewing vs. piercing). When Arabidopsis is infested with a herbivore that hardly damages leaf tissue, C. rubecula females are not attracted. These results may be explained by differences in the amount of damage and in the relative importance of different signal-transduction pathways induced by different types of herbivores.