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 À PR Ò ) in laboratory toxicity tests. The early life stages of L. siliquoidea were also exposed to a formulation of the dipotassium salt of endothall (Aquathol À K Ò ) in separate tests. Juveniles of the freshwater gastropod snail, Somatogyrus viriginicus (Lithoglyphidae), were exposed (96 h) to the Sonar À Genesis Ò 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 mg/L (95% CI, 729À1,026 mg/L) for glochidia (24 h), 511 mg/L (309À843 mg/L) for juvenile L. siliquoidea (96 h), and 500 mg/L (452À553 mg/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 D 0.06) or siphoning behavior (p D 0.08). The 24-h LC50 for glochidia exposed to the dipotassium salt of endothall was 31.2 mg/L (30.3À32.2 mg/L) and the 96-h LC50 for juvenile mussels was 34.4 mg/L (29.3À40.5 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 (5À15 mg/L and 1À5 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 mg/L), indicating minimal risk of short-term exposure effects.

Toxicity Of Sodium Dodecyl Sulfate tO Federally Threatened and Petitioned Freshwater Mollusk Species

“…If no movement was observed after five-minutes, the unionid was classified as dead. Gastropods were classified as dead if no movement was detected within a five-minute observation period (Archambault et al 2015) or after the ''tickle'' test, performed by touching the organisms with a soft pick to provoke a stimulus response. An eyelash stick was used to prevent any excess pressure being placed on the foot and observing a false reaction.…”

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

“…Toxicological and other studies on freshwater mollusks (mainly freshwater mussels) have increased over the past~20 years (Cope et al 2008;FMCS 2016), but they still number far fewer than studies of other taxa (e.g., fishes, insects, and other invertebrates). Gastropods -especially gill-breathing species in the clades Caenogastropoda and Neritimorpha (formerly known from the subclass Prosobranchia) -are represented by just a few recent studies (Besser et al 2009(Besser et al , 2016Archambault et al 2015;Poznanska et al 2015;Gibson et al 2016) despite their high imperilment rates and importance to the functional ecology of freshwater systems (Johnson et al 2013).…”

“…For example, researchers have long worked to identify chemicals to combat invasive mollusks (e.g., Zebra Mussels (Dreissena polymorpha)) without harming non-target species, including native mussels (e.g., Waller et al 1993;Cope et al 1997;Meehan et al 2014). The effects of herbicides used to combat invasive aquatic plants, such as hydrilla (Hydrilla verticillata, Hydrocharitaceae), on nontarget organisms has also been investigated (Hamelink et al 1986;Keller 1993;Paul et al 1994;Yi et al 2011), including the most recent study on freshwater mussels and snails (Archambault et al 2015). Hydrilla is an aquatic invasive weed non-native to the United States (US), and is included on the Federal Noxious Weed List (USDA APHIS 2012).…”

“…The most common hydrilla control methods include application of aquatic herbicides, introduction of non-native (to the US) Grass Carp (Ctenopharyngodon idella), and mechanical removal (Langeland 1996). Fluridone (market name Sonar t ; CAS number 59756-60-4), typically prescribed for one to four months, and the dipotassium salt of endothall (market name Aquathol t ; CAS number 2164-07-0), typically prescribed two to three times during the growing season, each for a period of days, are among the most commonly used aquatic herbicides for control of hydrilla (Archambault et al 2015). The impetus for this study was the persistence of hydrilla in the Eno River, located in the Piedmont region (Durham and Orange Counties), North Carolina, USA -a river with high biodiversity, high rates of endemism, and the presence of threatened and endangered species (Smith et al 2002;NCWRC 2015;LeGrand et al 2013; NatureServe 2013) -where the targeted use of herbicides has been recommended as the most appropriate hydrilla control method.…”

“…Prior to the recent study by Archambault et al (2015) on the effects fluridone and the dipotassium endothall on freshwater mussels and juvenile S. virginicus, peer-reviewed toxicity data were limited to only a few studies of other freshwater invertebrates and fishes (Crosby and Tucker 1966;Hamelink et al 1986;Paul et al 1994;Yi et al 2011). The toxicity thresholds for freshwater mollusks ranked among the lowest (i.e., most sensitive) compared to fishes and other invertebrates, but concentrations associated with acute toxicity were still greater than the concentrations typically prescribed for controlling invasive aquatic weeds (~10 to 100 times greater; Archambault et al 2015). The potential risks of such aquatic herbicides to freshwater mollusks should be assessed and balanced appropriately against the significant biological threat posed by invasive aquatic weeds like hydrilla.…”