Exploring the potential of root-associated bacteria to control an outbreak weed

Aims The spread of invasive weeds threatens biodiversity and stability of ecosystems. Jacobaea vulgaris is an invasive weed in some countries and an outbreak species in its native European range. Although biological control using specialist herbivores is available, controlling with soil microorganisms remains far less explored.Methods Twenty bacteria strains isolated from roots of J. vulgaris were used to examine bacterial effects on seed germination, root morphology and early plant growth. Moreover, we tested direct effects of the bacteria on a specialist herbivore of J. vulgaris, the leaf chewing caterpillar (Tyria jacobaeae), commonly used in biocontrol. We also tested indirect effects of bacteria, via the plant, on the performance of T. jacobaeae and the aphid species Aphis jacobaeae. Lastly, we examined the host specificity of two tested bacteria on three other forbs.Results Two Gammaproteobacteria, Pseudomonas brassicacearum and Serratia plymuthica, significantly reduced root growth of seedlings in-vitro, while seed germination was unaffected. However, these negative effects were observed across other forb species as well. Bacillus spp. injection led to the highest T. jacobaeae caterpillar mortality, while ingestion had no effect. Inoculation of the plants with bacteria did not affect aphid performance, but significantly affected T. jacobaeae preference. Specifically, P. syringae and one Bacillus sp. strain significantly increased T. jacobaeae preference.Conclusions Our results show that two root-associated bacteria inhibit J. vulgaris growth, but their lack of host specificity restricts their potential for biocontrol. Our study also highlights that belowground microorganisms can hamper or enhance the performance of aboveground insects.