Hydrilla control with split treatments of fluridone in Lake Harris, Florida

After several unsuccessful management efforts, a split treatment of fluridone was applied to the 6700 ha Lake Harris in March and June 1987, at a rate of 3.4 kg ha^–1 (680 and 340 kg fluridone, respectively) to two 3 m deep, hydrilla-infested bays. Fluridone concentrations in the water were sampled following the June treatment. Average fluridone concentrations were 2.1 µg l^–1 prior to this second application, and a maximum concentration of 30.2 µg l^–1 was detected in the treated area on the day following application. Fluridone residues dissipated out of the plot quickly due to dilution but concentrations declined lake-wide more slowly, following a logarithmic model, with an estimated fluridone half-life of 97 days. Control of hydrilla in Lake Harris resulted from the long exposure (over 25 weeks due to the split application) to fluridone concentrations of 2 µg l^–1, well below the maximum labelled rate of 150 µg l^–1.