Monoecious hydrilla (Hydrilla verticillata) response to intermittent herbicide exposures

Authors: Beets, Jens; HAUG, ERIKA - North Carolina Department Of Environmental Quality; SPERRY, BENJAMIN - Us Army Engineer Research And Dvelopment Center; LEON, RAMON - North Carolina State University; COPE, WILLIAM - North Carolina State University; EVERMAN, WESLEY - Iowa State University; RICHARDSON, ROBERT - North Carolina State University

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2026
Publication Date: 2/18/2026
Citation: Beets, J.P., Haug, E.J., Sperry, B.P., Leon, R.G., Cope, W.G., Everman, W.J., Richardson, R.J. 2026. Monoecious hydrilla response to intermittent herbicide exposures. Invasive Plant Science and Management. 19(e7). https://doi.org/10.1017/inp.2026.10040.
DOI: https://doi.org/10.1017/inp.2026.10040

Interpretive Summary: Aquatic weed infestations are increasingly problematic in lentic and lotic systems. With relatively few management options, new techniques are needed to effectively and efficiently control the spread of these noxious weeds. Submersed aquatic herbicide applications rely on concentration exposure time for efficacy, commonly performed in a single, continuous herbicide application. This study documented that separating single applications of herbicides into three applications with a non-treatment interval of no herbicide exposure between each exposure period does not negatively affect efficacy of florpyrauxifen-benzyl, endothall, or fluridone on monoecious hydrilla. Minimal differences were observed between herbicide concentrations tested, indicating lower concentrations may be used when required to meet water use restrictions or desired selectivity levels. These results also indicate that exposure times may be broken up or interrupted without negative consequences on efficacy with these herbicides, which may be particularly of relevance to high water flow systems where exposure time is limited. Further investigation is needed to identify optimal non-treatment intervals, herbicide concentrations, and field verification.

Technical Abstract: Hydrilla 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 hydrilla 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 hydrilla response to intermittent exposures of florpyrauxifen-benzyl, endothall, and fluridone at multiple concentrations and non-treatment intervals were evaluated in greenhouse-scale mesocosm experiments. Decreased efficacy was not observed with any tested herbicide when non-treatment intervals between exposures were introduced. Hydrilla treated with 30 µg ai L-1 florpyrauxifen-benzyl and a 6-day non-treatment interval resulted in 90% reduction in aboveground biomass. Endothall treatments of 3 mg ai L-1 with a 16 or 40-hour non-treatment interval resulted in similar efficacy to all treatments with 4 mg L-1 endothall. All treatments with fluridone, except for 5 µg ai L-1 with a 12-day non-treatment interval resulted in greater than 80% aboveground biomass reduction. This research suggests the need for additional refinement and operational verification of intermittent herbicide exposures for monoecious hydrilla control to increase cost-effectiveness and efficiency in public hydrilla control programs.