Plants respond to herbivory through sequential induction of cheaper defenses before more costly ones.

Plants encounter natural antagonist threats of varying intensity and respond by activating multiple defense traits. Due to the fitness costs associated with producing defense traits, plants are expected to activate less costly traits first, reserving more costly defenses for potentially more severe damage ("cheaper first hypothesis"), but evidence to date is scarce. Here, we tested this hypothesis by measuring six putative defense traits in the annual plant Ambrosia artemisiifolia. We found that all traits were effective against insect herbivores, but production of three of them more strongly reduced plant growth, suggesting higher growth costs. When plants were attacked by insect herbivores, less costly traits were induced first, even at the lowest levels of damage, while more costly traits were activated only after higher damage thresholds. This cost-dependent sequential pattern was consistently observed in plants when challenged by 12 different herbivore species from three insect orders. These findings demonstrate that plants can employ the "cheaper first" sequential induction defense strategy, potentially allowing them to reduce defense costs and maximize fitness. Our study provides new insights into how plants fine-tune their defense responses under variable antagonistic pressures.