Slime from the invasive slug Arion subfuscus induces herbivore defense in Brassica nigra

Plants that detect cues indicative of danger can initiate a variety of defenses. Plants typically induce defense after contact with a threat (i.e., a bite from an herbivore). Because of their small size, seeds and seedlings cannot afford to wait until after they are attacked to respond to a threat. We hypothesized that seeds can detect non-damage-related cues (kairomones) from seed-eating herbivores and respond by germinating faster to escape their vulnerable state by inducing chemical defense. We analyzed whether seed germination in a native herb, Brassica nigra, is affected by slime from the invasive slug Arion subfuscus. We conducted five replicate experiments comparing germination time and rate of Brassica nigra seeds exposed to slug slime with that of seeds exposed to water only (control). Seeds were placed in petri plates containing filter paper moistened with a water+slime mixture or water only and checked every 6-12 hours until seed germination ceased. Slug slime significantly increased germination time in four of five experiments; across all experiments, B. nigra seeds exposed to slug slime germinated 8% faster (range 2%-16%) than did control seeds. These results demonstrate that B. nigra seeds can detect and respond to slug kairomones prior to any actual contact with the herbivore. Future research will explore the chemical response in the seeds using plant secondary metabolite analyses, and the efficacy of these chemical defenses in preventing herbivory using choice bioassays.