Life Stage Sensitivity of a Freshwater Snail to Herbicides Used in Invasive Aquatic Weed Control

Abstract

Invasive aquatic plants like hydrilla (Hydrilla verticillata) threaten native species in many ways, ultimately degrading overall habitat quality and quantity. Aquatic herbicides are often chosen as a control and management strategy, but few peer-reviewed studies address their effects on non-target organisms, especially native freshwater mussels and snails. The aim of this study was to assess the life stage sensitivity of a rare snail, Somatogyrus virginicus (Lithoglyphidae), to two aquatic herbicides (dipotassium salt of endothall and fluridone). We collected adult snails, cultured their eggs on a vinyl card substrate, exposed adults and eggs in 96-h static-renewal experiments, and monitored eggs through hatching. Because fluridone is typically applied for ≥ 60 d, an additional treatment was exposed in staticrenewal through hatching (30 d total) to improve environmental relevance. Eggs present on the shells of adult snails were also monitored. Endpoints were adult survival and egg hatching success. Fluridone did not affect adult snail survival at concentrations up to 1500 µg/L, and in the test with eggs on vinyl cards, fluridone did not significantly delay (p = 0.12) or influence overall hatching success (p = 0.22), including in the 30-d exposure (Dunnett's p = 0.09). However, fluridone significantly delayed hatching of eggs on adult shells (p < 0.01) and reduced their overall hatching success (p < 0.01). The 96-h median effect concentration (EC50) for fluridone on hatching success of eggs on adults was 1334 µg/L (95% CI, 1215 – 1466 µg/L). For endothall, the adult 96-h median lethal concentration (LC50) was 223 mg/L (157 – 318 mg/L). Endothall negatively affected hatching success in both egg tests by delaying hatching (p < 0.01 in both tests) and by reducing overall hatching success (p = 0.04 for eggs on cards, and p < 0.01 for eggs on adults). The endothall 96-h EC50s for egg hatching success were 54.1 mg/L (95% CI, 35.6 – 82.2 mg/L; eggs on adults) and 83.4 mg/L (95% CI, 60.4 – 115.2 mg/L; eggs on cards). Neither herbicide had toxic effects to either life stage at concentrations typically prescribed for control of hydrilla (5 – 15 µg/L fluridone and 1 – 5 mg/L endothall). However, applying the minimum amount of herbicide needed for effective weed control is recommended for ensuring safety of non-target organisms.