Herbivore-specific plant defense responses to a pre-attack risk cue.

Cues of potential herbivory, such as the locomotion mucus from gastropod herbivores, may influence germination and growth. Because it remains unclear whether use of gastropod cues to initiate defense is a widespread phenomenon in plants, we examined how mucus from multiple gastropod herbivores (Arion subfuscus, Novisuccinea ovalis, Ambigolimax valentianus, Cornu aspersum, Deroceras laeve, and Deroceras reticulatum) affects Brassica nigra germination and seedling biomass. In a two-year replicated experiment, germination timing exhibited species-specific responses and varied across years. Exposure to Ar. subfuscus mucus consistently accelerated germination timing relative to the water-only control, while Am. valentianus and C. aspersum mucus increased germination speed in one but not both years. Mucus from D. laeve, D. reticulatum and N. ovalis did not affect germination speed. Mucus exposure did not affect the proportion of seeds germinating in either year. Seedling biomass responses to mucus exposure were limited and varied between years: only C. aspersum affected biomass in both years, reducing biomass in the first year and increasing it in the second relative to the control. Exposure to D. laeve mucus reduced seedling biomass by 29% in the first year, whereas mucus from its congener D. reticulatum, tested in the second year, had no effect. Mucus from the remaining species did not affect seedling growth. Together, these results indicate that exposure to some gastropod mucus treatments can alter B. nigra germination timing and early seedling growth, with responses varying among gastropod species and between years. The lack of consistent responses across several gastropod species suggests that plants may perceive or respond to herbivore cues unevenly, potentially reflecting differences in mucus chemistry among species or environmental conditions influencing plant sensitivity to these cues. Our findings highlight the complexity of gastropod-plant interactions and suggest that environmental or physiological conditions play an important role in shaping plant responses to herbivore-associated chemical cues.