Herbivore cues and plant damage-associated compounds jointly alter seed germination and seedling herbivory.

While plant defense against herbivory is primarily thought to occur following attack, there is also evidence that plants can detect and respond to pre-attack cues. These cues include chemicals released from damaged conspecifics and kairomones, non-attack-related substances emitted by an herbivore that plants can detect and use to their benefit. It is unknown, however, whether or how plants react to the interaction of these pre-attack cues. We measured germination, growth, and herbivore susceptibility of B. nigra seedlings in an experiment that crossed the presence/absence of crushed B. nigra leaves with the presence/absence of mucus of a generalist herbivore, A. subfuscus. Seeds exposed to both crushed leaves and slug mucus germinated 8% more quickly than control seeds; neither risk cue increased germination speed when tested individually. The same pattern was found in herbivore bioassays: Spodoptera exigua ate almost 10 × more foliage from control seedlings than from seedlings exposed to both crushed leaves and slug mucus. There was no difference in the final biomass of mature plants, suggesting that plants exposed to herbivore cues early in their development can increase defense without a measurable cost in size at maturity.