Comparative herbicide response of three US Hydrilla verticillata biotypes

Abstract

Over 50 years of research activities have documented efficacy of chemical controls for monoecious and dioecious biotypes of hydrilla ( Hydrilla verticillata L.f. Royle) in the United States. Recently, a genetically dissimilar biotype of hydrilla (“clade C”) was discovered in the Northeastern US. The response of three hydrilla biotypes to eight aquatic herbicides with distinct active ingredients was simultaneously evaluated through a greenhouse-based herbicide trial. Planted apical fragments of monoecious, dioecious, and clade C hydrilla biotypes were exposed to two concentrations of the following treatments: endothall, copper, diquat, florpyrauxifen-benzyl, and flumioxazin with 24 or 72 hours of exposure, and penoxsulam, fluridone, and topramazone with 1080 hours (45 days) of static exposure. Post-treatment plant symptomology was visually evaluated and aboveground biomass was harvested six weeks after the completed herbicide exposure period. Data were analyzed using mixed-effects models comparing response by treatment, biotype, and their interaction. Estimated visual injury of hydrilla differed by treatment (p<0.0001), and a significant treatment x biotype interaction (p<0.0001) revealed some variation in visual treatment response among biotypes. Where differences in visual response to treatment by biotype occurred, clade C hydrilla generally exhibited more injury than other biotypes. The reduction of hydrilla biomass differed by treatment (p < 0.0001) but did not differ between biotype. Highly effective treatments (>90% reduction) against hydrilla included endothall [2500 µg a.i. L ^-1 (72 hr); 5000 µg a.i. L ^-1 (24 and 72 hr)], diquat [370 µg a.i. L ^-1 ppm (24 and 72 hr)] and flumioxazin [200 µg a.i. L ^-1 (24 and 72 hr)]. This research demonstrates that all three US hydrilla biotypes respond similarly to herbicides when tested on a small scale with minimal environmental influence.