Sensitivity of freshwater molluscs to hydrilla-targeting herbicides: providing context for invasive aquatic weed control in diverse ecosystems

Abstract

Hydrilla (Hydrilla verticillata) is an invasive aquatic weed that has spread rapidly throughout the USA, especially in the southeast. A common control method is the application of aquatic herbicides, such as fluridone and endothall. However, there is limited documentation on the effects of herbicides commonly used to control hydrilla and other aquatic weeds on many non-target freshwater species and no published information exists on the toxicity of these herbicides to freshwater molluscs. We exposed juveniles (96 h) and glochidia (48 h) of the unionid mussel Lampsilis siliquoidea and adults (28 d) of Lampsilis fullerkati to a formulation of fluridone (Sonar – PR®) in laboratory toxicity tests. The early life stages of L. siliquoidea were also exposed to a formulation of the dipotassium salt of endothall (Aquathol – K®) in separate tests. Juveniles of the freshwater gastropod snail, Somatogyrus viriginicus (Lithoglyphidae), were exposed (96 h) to the Sonar – Genesis® fluridone formulation. Endpoints were survival (all species and life stages) as well as siphoning behavior and foot protrusion (adult mussels). Median lethal fluridone concentrations (LC50s) were 865 μg/L (95% CI, 729–1,026 μg/L) for glochidia (24 h), 511 μg/L (309–843 μg/L) for juvenile L. siliquoidea (96 h), and 500 μg/L (452–553 μg/L) for juvenile S. viriginicus (96 h). No mortality occurred in the 28-d exposure of adult L. fullerkati and we found no statistically significant effect of fluridone concentration on foot protrusion (p = 0.06) or siphoning behavior (p = 0.08). The 24-h LC50 for glochidia exposed to the dipotassium salt of endothall was 31.2 mg/L (30.3–32.2 mg/L) and the 96-h LC50 for juvenile mussels was 34.4 mg/L (29.3–40.5 mg/L). Freshwater molluscs were more sensitive to fluridone and endothall than most other species previously tested. Fluridone and endothall concentrations typically recommended for hydrilla treatment (5–15 μg/L and 1–5 mg/L, respectively) were not acutely toxic to the molluscs we tested and a 28-d exposure to fluridone was not lethal to adult mussels even at the highest concentration (300 μg/L), indicating minimal risk of short-term exposure effects.