HYDRILLA CONTROL WITH SPLIT TREATMENTS OF FLURIDONE IN LAKE-HARRIS, FLORIDA

After several unsuccessful management efforts, a split treatment of fl uridone 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) t o two 3 m deep, hydrilla-infested bays. Fluridone concentrations in th e water were sampled following the June treatment. Average fluridone c oncentrations were 2.1 mu g 1(-1) prior to this second application: an d a maximum concentration of 30.2 mu g 1(-1) was detected in the treat ed area on the day following application. Fluridone residues dissipate d out of the plot quickly due to dilution but concentrations declined lake-wide more slowly, following a logarithmic model, with an estimate d fluridone half-life of 97 days. Control of hydrilla in Lake Harris r esulted from the long exposure (over 25 weeks due to the split applica tion) to fluridone concentrations of 2 mu g 1(-1), well below the maxi mum labelled rate of 150 mu g 1(-1).