John G. Skogerboe and Kurt D. Getsinger

ABSTRACT

Species selectivity of the aquatic herbicide endothall as the formulation Aquathol¨ K was evaluated on a variety of plant species commonly found in southern latitude United States aquatic plant communities. Submersed species included hydrilla (Hydrilla verticillata (L.f.) Royle.), wild celery (Vallisneria americana L.), American pondweed (Potamogeton nodosus Poiret), southern naiad [Najas guadalupensis (Sprengel) Magnus], watershield (Brasenia schreberi J. F. Gmelin), water stargrass (Heteranthera dubia (Jacq.) MacM.), and Illinois pondweed (Potamogeton illinoensis Morong.). Emergent species included soft-stem bulrush (Scirpus validus Vahl) and arrowhead (Sagittaria latifolia Willd.), and floating-leaf species included spatterdock (Nuphar luteum (L.) Sibth. & Sm.), and fragrant waterlily (Nymphaea odorata Aiton). The study was conducted in outdoor mesocosm systems (860 to 7000 L tanks) at the U.S. Army Engineer Research and Development Center, Waterways Experiment Station, Lewisville Aquatic Ecosystem Research Facility (LAERF) in Texas. The selectivity evaluations of submersed species were conducted in mesocosm tanks, and treatment rates included 0, 1, 2, and 5 mg/L active ingredient (ai) endothall with a 24 hour water flow-through half-life. Emergent species treatment rates included 0, 1, 2 and 5 mg/L ai endothall and a static water-flow exposure period of 120 hours. Floating-leaf treatment rates included 0, 1, 2, and 5 mg/L ai endothall and a static water-flow exposure period of 120 hours. Endothall was effective at controlling hydrilla at the 2 and 5 mg/L application rates, reducing the biomass by more than 90%, 6 weeks after treatment (WAT), with no regrowth observed. Wild celery and Illinois pondweed biomass were also significantly reduced following the endothall application at 1, 2 and 5 mg/L, but healthy regrowth was observed by 6 WAT. American pondweed and southern naiad biomass was significantly reduced by more than 90% by 6 WAT, and no regrowth was measured. Watershield and water stargrass showed no effects from endothall application at 1, 2, and 5 mg/L, and growth was actually enhanced by reduced competition from invading annual species. Soft-stemmed bulrush also showed no effects from treatment with endothall. Biomass of arrowhead, spatterdock, and fragrant waterlily was significantly reduced at the 2 mg/L application rate, and plants were controlled at the 5 mg/L application rate. Because the emergent and floating-leaf evaluations were conducted under static water-flow conditions, these evaluations represented a worst case scenario, which may not be indicative of endothall use patterns under field conditions. These evaluations demonstrate the potential of using endothall to selectively control hydrilla in mixed-plant communities.