Effects of variability in the amount and dispersion of within-plant herbivory on resistance- and tolerance-related responses in wild cotton

Herbivory triggers complex induced defensive responses in plants that may vary depending on the amount and dispersion of damage, the latter has received much less attention. We examined how wild cotton (Gossypium hirsutum) responds to different amounts of herbivory and how it is dispersed among the leaves (concentrated versus dispersed) using the specialist caterpillar Alabama argillacea. In a greenhouse experiment, plants underwent different herbivory treatments: no damage, low herbivory (two caterpillars, one on each of two leaves), high concentrated herbivory (four caterpillars, two on each of two leaves), and high dispersed herbivory (four caterpillars, each on one of four leaves). We measured extrafloral nectar (EFN), an indirect defense trait, and phenolic production, a direct chemical defense, as well as regrowth capacity (new leaf biomass two months after herbivory) as a tolerance proxy. To distinguish between local and systemic defense induction, we measured resistance-related responses on damaged and undamaged leaves. EFN was significantly and more consistently induced in damaged leaves (i.e., stronger local induction) in response to high dispersed damage. Phenolic compounds were not affected by any of the damage treatments. Additionally, wild cotton fully compensated for damage in terms of new leaf biomass under all herbivory treatments, except after high concentrated damage, suggesting higher costs incurred by this type of herbivory. These results highlight the importance of studying plant-induced defenses under varying herbivory conditions and suggest opposing effects on resistance-related (EFN, indirect) versus tolerance-related induced responses, which are known to affect tri-trophic interactions associated with wild cotton. Main ConclusionVariability in caterpillar damage to wild cotton leaves differentially shapes resistance- and tolerance-related responses; dispersed damage induces more extrafloral nectar production, and concentrated damage more strongly drives reductions in regrowth.