Efficacy of experimental and recently registered herbicides on hygrophila

Management of hygrophila, a submersed non-native aquatic weed, is particularly challenging because few registered aquatic herbicides effectively control the species, which most commonly occurs in slow to moderately flowing canals, streams and rivers. Seven experimental or newly registered aquatic herbicides were evaluated in 10-wk static exposure studies to determine their activity on hygrophila. Only two of the four ALS-inhibiting herbicides under evaluation reduced hygrophila biomass by 90% after treatment at experimental rates; these were penoxsulam and trifloxysulfuron, with EC90 values (EC90: the effective concentration that reduces growth by 90% compared to untreated controls) of 240 and 274 lg ai L , respectively. The EC90 values for the HPPD inhibitor topramezone and the synthetic auxin mimic quinclorac were 372 and 951 lg ai L , respectively. The EC90 values for all of these herbicides were several times higher than the current or likely maximum application concentrations on the labels. However, the EC90 value for the PPO inhibitor flumioxazin was 158 lg ai L 1 (95% CI 132–195 lg ai L ). The ALS products tested in these experiments—imazamox, bispyribac, penoxsulam and trifloxysulfuron—did not provide adequate (. 90%) control of hygrophila at maximum label rates, whereas the experimental herbicides topramezone and quinclorac had very slow activity and are unlikely to provide acceptable control of hygrophila under operational conditions. In contrast, flumioxazin, which was registered for aquatic use in Fall 2010 and has a maximum application concentration of 400 lg ai L , appears to offer effective control of hygrophila at less than half the maximum label rate, at least under static conditions.