USE OF FLURIDONE FOR HYDRILLA MANAGEMENT IN THE WITHLACOOCHEE RIVER, FLORIDA

Fluridone at a theoretical sustained concentration of 10 to 12 mu g l( -1) was applied over a period of 13 weeks to several lakes along a sec tion of the Withlacoochee river in central Florida, to control hydrill a. Residue analyses indicated that although the theoretical concentrat ion of 10 to 12 mu g l(-1) fluridone was not sustained for 13 weeks, h ydrilla was susceptible to lower concentrations over the long exposure periods that resulted from low water discharges that occurred in 1990 . The combined fluridone concentrations and exposure times at a given site were expressed in Fluridone Exposure Days (FEDs), and these value s were related to water flow data, particularly the influence of sprin g-fed tributaries. Hydrilla was controlled for 10 to 12 months in the upstream half of the target area, where fluridone exposure was in exce ss of 500 FEDs, but in do downstream areas subject to 250 FEDs, hydril la control lasted for only six to nine months. This treatment was repe ated in 1991 under increased flow conditions, with only a 10-week appl ication period. Effective weed control was achieved despite lower FEDs possibly because of the increased susceptibility of the actively grow ing plants and the stresses caused by high water levels.