Monoecious hydrilla ( Hydrilla verticillata ) response to intermittent herbicide exposures

Abstract Hydrilla [ Hydrilla verticillata (L.F.) Royle] is a known, persistent aquatic weed in many lakes and reservoirs across the United States. Recently, focus has been placed on the management of invasions of H. verticillata into flowing systems due to increased difficulty in achieving effective control. In flowing environments, increased flow and water exchange rates commonly result in shorter herbicide exposure times using traditional application techniques. This research evaluated the concept of intermittent herbicide exposure as a potential management strategy to overcome operational challenges in high water exchange environments. Monoecious H. verticillata response to intermittent exposure to florpyrauxifen-benzyl, endothall, and fluridone at multiple concentrations and nontreatment intervals was evaluated in greenhouse-scale mesocosm experiments. Decreased efficacy was not observed when nontreatment intervals between exposures were introduced in florpyrauxifen-benzyl treatments. Monoecious H. verticillata treated with 30 µg ai L −1 florpyrauxifen-benzyl and a 6-d nontreatment interval resulted in 89% reduction in aboveground biomass. Nontreatment intervals did not influence endothall efficacy and biomass reduction increased with herbicide concentration (66% to 93%). Fluridone treatments at 5 µg L −1 resulted in 82% biomass reduction, while treatments at 10 and 15 µg L −1 resulted in 89% biomass reduction. Biomass reduction was slightly lower in fluridone applications with a 12-d nontreatment interval (83%) compared with other nontreatment intervals (85% to 91%). The high biomass reduction and similar response between treatments observed suggests the need for additional refinement and operational verification of intermittent herbicide exposures for monoecious H. verticillata control to increase cost-effectiveness and efficiency in public H. verticillata control programs.