Woody seeds and seedlings are unresponsive to herbivore kairomones

Herbivory is particularly threatening to young plants that lack the resources needed to survive an attack. Seeds and seedlings should thus benefit greatly from using pre-attack cues to induce defence before damage. Locomotion mucus from slugs, generalist herbivores that consume young plants, has been shown to speed germination, slow growth, and increase both chemical defences and resistance to herbivores in several herbaceous plants. Whether woody species exhibit similar responses has not been tested. Arion subfuscus, an invasive slug in the eastern USA, is a major herbivore of young sugar maples (Acer saccharum); we explored the effect of its locomotion mucus on Ac. saccharum seeds and seedlings. We exposed sugar maple seeds and seedlings to mucus and measured germination speed and rate, seed susceptibility to slugs, seedling emergence, growth, chemical defences, and foliar susceptibility to Lymantria dispar and Ar. subfuscus. Contrary to our expectations and previous findings with herbaceous species, we found that mucus had no effect on these performance or resistance traits. Habituation to repeated cue exposure or the limited coevolutionary history between Ac. saccharum and Ar. subfuscus could be the reason for the lack of an observed response by the plants. Further studies should investigate the effects of kairomones using a short-term cue exposure procedure or by using a woody plant species and native slug herbivore with a coevolutionary history. Understanding how woody plants respond to kairomones would provide insight into the risk and defence strategies used by long-lived species in crucial early life stages.