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  • Unmanned Aircraft Systems and Tracer Dyes Potential for Monitoring Herbicide Spray Distribution

    Purpose: Chemical control of nuisance aquatic vegetation has long been the most widely utilized management tool due to its high level of efficacy, limited environmental impacts, and relatively low cost. However, unprecise application of herbicides can lead to uncontrolled invasive plants and unintended management costs. Therefore, precision herbicide delivery techniques are being developed to improve invasive plant control and minimize impacts to non-target plants. These technological advancements have the potential to enhance aquatic ecosystem protection from invasive species while reducing associated management costs. Despite the benefits of using registered herbicides for aquatic plant control in efforts to restore aquatic habitats, their use is often misunderstood and opposed by public stakeholders. This can lead to significant challenges related to chemical control of nuisance aquatic vegetation. Thus, US Army Corps of Engineers (USACE) Districts seek improved methods to monitor and quantify the distribution (i.e., amount of herbicide retained on plant foliage compared to those deposited into the water column) of herbicides applied in aquatic systems. Monitoring herbicide movement in aquatic systems can be tedious and costly using standard analytical methods. However, since the inert fluorescent tracer dye Rhodamine WT (RWT) closely mimics product movement in the aquatic environment it has been used as a cost-effective surrogate for herbicides tracing.
  • Small Plot Applications of Florpyrauxifen –Benzyl (Procellacor SC™) for Control of Monoecious Hydrilla in Roanoke Rapids Lake, NC

    Abstract: Four demonstration plots were selected at Roanoke Rapids Lake, NC to evaluate water exchange and aqueous herbicide residues in stands of submersed aquatic vegetation (SAV) following treatment with rhodamine wt dye and florpyrauxifen-benzyl to control monecious hydrilla. Florpyrauxifen-benzyl (Procellacor™ SC) was applied in combination with Rhodamine WT (RWT) at two of the plots. Dye measurements and herbicide residue samples were collected at specific time intervals to draw comparisons between herbicide and RWT dye dissipation. The two additional plots served as reference plots to the treatment plots. Pre- and post-treatment vegetation surveys were conducted to evaluate monoecious hydrilla control and non-target species response. RWT dye and herbicide residue data indicated rapid water exchange was occurring with each treatment plot. As a result, florpyrauxifen-benzyl concentration and exposure times (CETs) towards monoecious hydrilla were not sufficient to achieve adequate control by 4 weeks after treatment (WAT). To reduce the impact of hydraulic complexity and improve herbicide efficacy, treatments should coincide with minimal reservoir discharge events to extend herbicide CET relationships. Evaluations of florpyrauxifen-benzyl on late season, mature plants may have impacted herbicide efficacy. Evaluations should be conducted earlier in the growing season, on young, actively growing plants, to discern potential differences in efficacy due to treatment timing and phenology. More information on herbicide concentration and exposure time relationships for monoecious hydrilla should be developed in growth chamber and mesocosm settings to improve species selective management of monoecious hydrilla in hydrodynamic reservoirs.
  • Flowering Rush Control in Hydrodynamic Systems: Part 1: Water Exchange Processes

    Abstract: In 2018, field trials evaluated water-exchange processes using rhodamine WT dye to provide guidance on the effective management of flowering rush (Butomus umbellatus L.) at McNary Dam and Reservoir (Wallula Lake, 15,700 ha). Additional evaluations determined the effectiveness of BubbleTubing (hereafter referred to as bubble curtain) at reducing water exchange within potential flowering rush treatment areas. Dye readings were collected from multiple sampling points at specific time intervals until a dye half-life could be determined. Whole-plot dye half-lives at sites without bubble curtain ranged 0.56–6.7 h. In slower water-exchange sites (≥2.6 h dye half-life), the herbicide diquat should have a sufficient contact time to significantly reduce flowering rush aboveground biomass. Other sites demonstrated very rapid water exchange (<1.5 h dye half-life), likely too rapid to effectively control flowering rush using chemical treatments without the use of a barrier or curtain to slow water exchange. At one site, the use of the bubble curtain increased the dye half-life from 3.8 h with no curtain to 7.6 and 7.1 h with a bubble curtain. The bubble curtain’s ability to slow water exchange will provide improved chemical control and increase the potential for other chemical products to be effectively used.
  • Efficacy of Florpyrauxifen-benzyl on Dioecious Hydrilla and Hybrid Water Milfoil - Concentration and Exposure Time Requirements

    Abstract: This study conducted small-scale trials under various concentration and exposure time (CET) scenarios to determine florpyrauxifen-benzyl activity on dioecious hydrilla and hybrid watermilfoil and determine impact on water stargrass and elodea. Hydrilla treated with 12, 24, or 36 μg active ingredient (a.i.) L⁻¹ florpyrauxifen-benzyl and exposed for 12, 24, or 48 hr under outdoor mesocosm conditions was reduced in biomass by 30-75% at 8 weeks after treatment (WAT). An additional hydrilla trial at the same herbicide concentrations, but under longer exposures (24, 72, or 168 hr), resulted in 33–85% plant control. Under indoor conditions, hybrid watermilfoil dry weight decreased 98–100% with subsurface applications of florpyrauxifen-benzyl under CET scenarios of 3–12 μg a.i. L⁻¹ at 3–24 hr exposure times in a growth chamber trial. Under shorter exposure periods (0.5–4 hr) in a follow-up trial, low doses (3–9 μg a.i. L⁻¹) achieved 50–100% control of hybrid watermilfoil. In the same trial, the nontarget species water stargrass and elodea proved relatively tolerant to the florpyrauxifen-benzyl at doses up to 6 μg a.i. L⁻¹ (4 hr exposure) and 9 μg a.i. L⁻¹ (1 hr exposure). These small-scale trials demonstrate florpyrauxifen-benzyl’s potential to selectively manage invasive species.
  • Evaluation of New Endothall and Florpyrauxifen-benzyl Use Patterns for Controlling Crested Floating Heart and Giant Salvinia

    Purpose: The purpose of this research was to (1) evaluate concentration exposure time (CET) relationships for florpyrauxifen-benzyl (ProcellaCOR) for control of the floating leaved plant crested floating heart (Nymphoides cristata, CFH) and (2) evaluate foliar applications of endothall (Aquathol K) for control of CFH and the floating fern giant salvinia (Salvinia molesta).