Evaluation of two ALS inhibiting herbicides on nontarget native aquatic and wetland plants

The emergence of fluridone-resistant hydrilla (Hydrilla verticillata) populations in Florida lakes during the late 1990s prompted the search for alternative herbicides, leading to the evaluation of ALS-inhibiting herbicides penoxsulam and bispyribac-sodium. Initially tested in controlled laboratory and greenhouse studies before advancing to field trials under experimental permits, these herbicides were registered for aquatic use by the U.S. EPA as Galleon SC (penoxsulam, 2007) and Tradewind (bispyribac-sodium, 2012). Although both belong to Weed Science Society of America Group 2 (ALS inhibitors), they derive from different chemical families—penoxsulam (triazolopyrimidine) and bispyribac-sodium (pyrimidinyl(thio)benzoate)—and were originally developed for rice cultivation, targeting grasses, sedges, and broadleaf weeds at application rates of 10–50 g ai ha⁻¹ and 20–40 g ai ha⁻¹, respectively. Their aquatic labels permit use against submersed, floating, and emergent weeds, with recommended in-water concentrations of 25–75 ppb for penoxsulam and 20–45 ppb for bispyribac-sodium, though lower rates are often preferred to protect native flora that provide critical ecosystem services such as energy dissipation, substrate stabilization, and wildlife habitat. Unlike agricultural weed control, which seeks near-total weed elimination, invasive species management in natural systems aims to suppress target invasives while preserving native plants to maintain ecological balance. Efficacy requires prolonged exposure (>45 days for both herbicides, with penoxsulam’s label recommending 60 days), yet their half-lives—approximately 10 days in flooded rice fields but potentially exceeding 30 days in deeper waters—pose challenges for sustained effective concentrations without retreatment. Given the need for selective control in natural habitats, this study assessed the effects of penoxsulam and bispyribac-sodium on 13 native aquatic and wetland plants prevalent in the southern U.S., addressing the broader goal of invasive species management: minimizing non-target impacts to sustain ecosystem functionality.