From leaves to defenders: how amount and dispersion of leaf damage affect extrafloral nectar production and ant-mediated protection in wild cotton (Gossypium hirsutum)

Extrafloral nectar (EFN) drives mutualistic interactions between plants and ants. However, EFN production is costly and its induction is predicted to vary with herbivory-related factors. This study examines how the amount and within-plant damage variability (i.e., damage uniformity within the plant canopy) 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 hypothesize that such damage 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; i.e., same total area than concentrated). The day after treatments, we measured EFN volume and sugar content from one nectary per plant (a measure of 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 using the same treatments 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. In the field, plants with dispersed damage recruited more ants and exhibited a higher probability of caterpillar attack by ants. Contrary to expectations, plants subjected to dispersed damage exhibited a greater total investment in EFN, presumably via local induction of more nectaries, enhancing ant-mediated defence. This study highlights the importance of herbivory intensity and uniformity patterns in shaping ant-plant interactions.