Too hot, too tough, too crowded: Abiotic and biotic constraints on Niphograpta albiguttalis establishment on water hyacinth (Pontederia crassipes) in southeastern United States

• Field study identifies limits of Niphograpta albiguttalis establishment in SE USA. • Cooler waters with higher nitrate levels favor insect presence and density. • Soft, nitrogen-rich host-plants in interior mats support N. albiguttalis presence. • Feeding damage by other biocontrol agents lowers N. albiguttalis density. • Findings inform future biocontrol releases in other climates and regions. The water hyacinth moth Niphograpta albiguttalis was introduced from Argentina to the United States in the 1970 s for the biological control of water hyacinth. While this moth successfully established in southeastern states, it failed to persist in other regions. To identify factors constraining establishment, we tested three hypotheses: establishment may be limited by (I) abiotic factors, (II) host-plant traits, or (III) biotic interactions with other herbivores. We surveyed 22 sites across Louisiana, Mississippi, Alabama, Texas, and Florida, spanning 8° latitude. At each site, we measured water physicochemical properties, plant traits (petiole toughness, tissue nitrogen, C:N ratio, growth form, position in the mat), and herbivore damage (from Neochetina spp. and Orthogalumna mites). Niphograpta albiguttalis was found at only 6 of 22 sites (27%), which were characterized by softer petioles (1.72 ± 0.79 vs. 2.57 ± 1.20 N), and higher tissue nitrogen (2.74 ± 0.76% vs. 1.94 ± 0.62%), as well as lower feeding damage from mites (6.70 ± 5.20% vs. 30.60 ± 3.60% petioles damaged) and Neochetina weevils (9.53 ± 10.15 vs. 16.61 ± 15.40 scars per leaf). At the most abundant sites, insect densities averaged 0.10 ± 0.01 larvae per petiole. Generalized models for presence and density identified consistent drivers: both metrics increased in cooler, nitrate-rich waters and on softer plants within mat interiors and declined with greater feeding damage from co-occurring herbivores. Our findings indicate that establishment of N. albiguttalis is constrained by the interplay of abiotic factors, host-plant traits, and biotic interactions, informing future releases.