Glyphosate Toxicity to Native Nontarget Macrophytes Following Three Different Routes of Incidental Exposure

Abstract

Abstract A major goal of invasive plant management is the restoration of native biodiversity, but effective methods for invasive plant control can be harmful to native plants. Informed application of control methods is required to reach restoration goals. The herbicide glyphosate, commonly applied in invasive plant management, can be toxic to native macrophytes. Our study assessed the response of 2 macrophytes that are endangered in our study area (Ammannia robusta and Sida hermaphrodita) to glyphosate concentrations that mimic incidental exposure from nearby invasive plant control: spray drift of 4 × 10−7% to 5% glyphosate; pulse and continuous immersion in water containing 2 to 41 µg/L glyphosate; and rhizosphere contact with 5%-glyphosate-wicked invasive plants. We assessed macrophyte sensitivity at 14-d postexposure, and quantified abundance of arbuscular mycorrhizal fungi. Glyphosate spray concentrations as low as 0.1% reduced macrophyte growth. Ammannia was more sensitive overall to glyphosate spray than Sida, although sensitivity varied among measured endpoints. Conversely, macrophytes were not affected by immersion in low concentrations of glyphosate or rhizosphere contact with a glyphosate-wicked plant. Likewise, arbuscular mycorrhizal fungi abundance in roots was similar among glyphosate-sprayed and control plants. Based on our results, we recommend that invasive plant managers reduce risks to native nontarget plants through implementing measures that limit off-target spray drift, and consider the feasibility of more targeted applications, such as with wick equipment. Integr Environ Assess Manag 2021;17:597–613. © 2020 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC) KEY POINTS We assessed the response of nontarget, endangered macrophytes and associated mycorrhizal fungi to glyphosate exposures that mimicked realistic scenarios encountered during invasive plant control. Glyphosate toxicity to macrophytes varied with the exposure pathway, where spray at low concentrations reduced native macrophyte growth, but no effects were detected from low-level surface water residues or adjacent wick-application. Mycorrhizal fungi in macrophyte roots were not affected by glyphosate spray. We recommend that glyphosate-based invasive plant management implements measures to limit off-target spray drift, and that risk assessment procedures for nontarget macrophytes consider adding spray exposure tests with emergent species to their repertoire.

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