Efficacy of Diquat on Submersed Plants Treated Under Simulated Flowing Water Conditions

The contact aquatic herbicide, diquat (6,7-dihydrodipyrido[1,2- α :2’,1’-c]pyrazinediium ion) was evaluated under simulated flowing water conditions in an outdoor mesocosm facility for efficacy on five submersed aquatic plants: hydrilla ( Hydrilla verticillata (L.f. Royle), Eurasian watermilfoil ( Myriophyllum spicatum L.), sago pondweed ( Stuckenia pectinata (L.) Boerner) , American pondweed ( Potamogeton nodosus Poiret) , and egeria ( Egeria densa Planchon). Diquat was applied at concentrations of 0.37 mg/L ai and 0.185 mg/L ai (cation) under flow-through conditions to provide theoretical 3 and 6 hr herbicide half-lives that produced observed herbicide half-lives of 2.5 and 4.5 hr, respectively. An additional treatment included 0.37 mg/L ai applied under static conditions (no water exchange). Results showed that diquat applications significantly inhibited shoot biomass production from 42 to 100 percent at all application concentrations and exposure times for all species, except hydrilla. Diquat resulted in no measurable control of hydrilla, except under static conditions. Results suggest that Eurasian watermilfoil, egeria, and sago pondweed are highly susceptible to diquat even in areas where herbicide dilution may occur in less than three hours.