Herbivore diet and plant damage-associated-compounds alter seed germination, growth, and 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. In the summer of 2024, 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 exposure to the mucus of a generalist herbivore, A. subfuscus, that have been fed either closely- or distantly-related species (B. nigra and iceberg lettuce, respectively). Seeds exposed to slug mucus germinated more quickly than control seeds; crushed B. nigra leaves only increased germination speed in combination with mucus. The same pattern was found in herbivore bioassays: Spodoptera exigua only avoided seedlings exposed to both slug slime and crushed leaves. 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.