Elevated Ozone Modulates Herbivore-Induced Volatile Emissions of Brassica nigra and Alters a Tritrophic Interaction.

Plants damaged by herbivores emit volatile organic compounds (VOCs) that are used by parasitoids for host location. In nature, however, plants are exposed to multiple abiotic and biotic stresses of varying intensities, which may affect tritrophic interactions. Here, we studied the effects of ozone exposure and feeding by Pieris brassicae larvae on the VOCs emitted by Brassica nigra and the effects on oriented flight of the parasitoid Cotesia glomerata. We also investigated the oriented flight of C. glomerata in a wind-tunnel with elevated ozone levels. Herbivore-feeding induced the emission of several VOCs, while ozone alone had no significant effect. However, exposure to 120 ppb ozone, followed by 24 hr of herbivore-feeding, induced higher emissions of all VOCs as compared to herbivore-feeding alone. In accordance, herbivore-damaged plants elicited more oriented flights than undamaged plants, whereas plants exposed to 120 ppb ozone and 24 hr of herbivore-feeding elicited more oriented flights than plants subjected to herbivore-feeding alone. Ozone enrichment of the wind-tunnel air appeared to negatively affect orientation of parasitoids at 70 ppb, but not at 120 ppb. These results suggest that the combination of ozone and P. brassicae-feeding modulates VOC emissions, which significantly influence foraging efficiency of C. glomerata.