Shifting shapes, shifting risks: density-driven leaf plasticity in Salvinia auriculata (Salviniaceae) and its effects on herbivory by Samea multiplicalis (Lepidoptera, Pyralidae)

Aim This study investigated whether phenotypic plasticity induced by different plant density conditions in Salvinia auriculata affects herbivory and the abundance of Samea multiplicalis larvae. Methods The study consisted of two stages: (i) In October 2022, four weekly samplings were conducted, during which ~30 clonal structures of S. auriculata were collected from two ponds differing in plant density. From each pond, 30 ramets were randomly selected to measure floating leaf width and length. Larval presence and the frequency of herbivory on floating leaves were also recorded; (ii) An experiment was conducted using 30 ramets from each density condition, placed individually in 500 mL containers. Five larvae were introduced per container, and after seven days, the number of partially and fully consumed leaves was recorded. Results S. auriculata displayed density-dependent plasticity: plants from high-density conditions developed longer, wider, and more vertically oriented leaves. Despite this, leaf consumption did not differ between density conditions, although both larval abundance and herbivory marks were higher under high-density conditions. Conclusions Our findings suggest that the high-density phenotype of S. auriculata may facilitate the establishment and herbivory of S. multiplicalis, potentially by offering structural refuge against predators and environmental stressors.