From leaves to defenders: How the amount and dispersion of leaf damage affect extrafloral nectar production and ant‐mediated protection in wild cotton

Abstract Extrafloral nectar (EFN) drives mutualistic interactions between plants and ants. However, EFN production is costly, and its induction is predicted to vary with herbivore‐related factors. We examine how the amount and within‐plant damage spatial uniformity (i.e., dispersed and concentrated damage) affect EFN production and ant‐mediated defence in wild cotton ( Gossypium hirsutum ). Previous work suggests that severe and concentrated damage is costliest to plants, leading us to hypothesise that such damage types would induce greater EFN production and, consequently, stronger ant attraction and protection. We conducted a greenhouse experiment in which plants were subjected to one of the following mechanical damage treatments: control (no damage), low damage (two damaged leaves, 30% of area removed each), high concentrated damage (two damaged leaves, 60% each) and high dispersed damage (four damaged leaves, 30% each; that is, same total area than concentrated damage). The day after establishing damage treatments, we measured the EFN volume and sugar content from one nectary per plant (a per‐nectary EFN response) and the number of secreting nectaries per plant (a proxy for whole‐plant level EFN response). Subsequently, we performed a field experiment applying the same treatments to another set of plants to evaluate EFN‐mediated ant recruitment and ant‐provided protection by placing caterpillars on plants. Both high‐damage treatments increased EFN volume per nectary, whereas low damage did not. However, dispersed damage was the only treatment that increased the number of secreting nectaries. Consistently, once placed in the field, plants with dispersed damage recruited more ants and caterpillars on them were more likely to be attacked by ants. Contrary to expectations, plants subjected to dispersed damage exhibited a greater total investment in EFN, presumably via local induction of more nectaries per plant, and thereby enhanced ant‐mediated defence. This study highlights the importance of herbivory intensity and within‐plant spatial uniformity in shaping ant–plant interactions.