The effects of multiple insect attacks on herbivore-induced plant volatiles and carnivorous arthropods are increasingly studied. Phytopathogens also represent an important threat to plants, and plant defense strategies against pathogens and insects are strongly interconnected, yet the potential impact of pathogens on insect-induced volatiles has been largely overlooked, and degree of pathogenicity rarely considered. We investigated how pathogen challenge, with virulent and avirulent strains of Xanthomonas campestris either alone or with simultaneous Pieris brassicae caterpillar herbivory, affected the volatile emissions of Brassica nigra plants. The impact of these volatiles on the foraging behavior of Cotesia glomerata parasitoids was then assessed. Pathogens themselves induced volatiles that were highly attractive to parasitoids, and enhanced the attractiveness of host-infested plant volatiles. Chemical analyses revealed that virulent and avirulent strains differentially induced plant volatiles, with primarily sesquiterpene, homoterpene and green leaf volatile compounds contributing to the differences. Strong similarities were found in the blends induced by the virulent strain and caterpillar herbivory. Challenge by either virulent or avirulent pathogens has a significant impact on plant chemistry and its interactions with other community members, demonstrating the importance of integrating pathogen- and insect-based research to broaden our knowledge of plant defenses under conditions of increasing complexity.

Wasp choice assays in windtunnel dataThe three data sheets show the data for the three types of comparisons that were made: (1) wasp choice when offered acaterpillar infested plant and a caterpillar + pathogen infected plant (2) wasp choice when offered a healthy plant against a singleattacker infected/infected plant and (3) wasp choice when offered a caterpillar infested plant against a pathogen infected plant.Wasp windtunnel data.xlsxVolatile dataThis table shows the corrected peak areas by fresh plant mass for each sample. For the X.campestris pv. campestris infected plant samples, disease severity is also noted.