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

This is the first published study to conduct a laboratory trial examining the effects of dipotassium salt of endothall and fluridone, both commonly used herbicides to treat aquatic invasive Hydrilla, on freshwater molluscs. Lethality due to acute exposure was examined for glochidia and juveniles of the freshwater mussel Lampsilis siliquoidea, adults of the freshwater mussel Lampsilis fullerkati, and juveniles of the freshwater gastropod Somatogyrus virginicus. Behavioral effects on siphoning behavior and foot protrusion were also examined for the adult mussels.

Overall, environmentally relevant levels of fluridone and endothall salts were not acutely toxic to the molluscs tested; and chronic exposure (28 days) of fluridone to adult mussels was not lethal and no significant effects on adult mussel behavior were observed. Lethal fluridone concentrations for glochidia and juvenile mussels and juvenile snails were 30 to 60 times greater than the highest recommended concentrations for field use, and lethal concentrations of the dipotassium salts of endothall on mussel glochidia and juveniles were on average 7 times greater than the highest recommended concentration for field use. At the time of this study, glochidia and juvenile mussels were shown to be more acutely sensitive to fluridone and endothall salts than most other aquatic organisms that have been tested. While overall the lethal doses were higher than recommended application levels, the margin for error is still not large, and there could be other unstudied sublethal effects of these herbicides on freshwater molluscs.